Discussion Paper No. 1051

A MULTI-FACTOR UZAWAGROWTH THEOREM AND
ENDOGENOUS CAPITAL-AUGMENTINGTECHNOLOGICAL CHANGE

Gregory Casey
Ryo Horii

March 2019

The Institute of Social and Economic Research
Osaka University
6-1 Mihogaoka, Ibaraki, Osaka 567-0047, JapanA Multi-factor Uzawa Growth Theorem andEndogenous Capital-Augmenting Technological Change∗
Gregory Casey, Williams College
Ryo Horii, Osaka University†
This version: March 28, 2019

Abstract
We prove a generalized, multi-factor version of the Uzawa steady-state growth theorem. In
the two-factor case, the theorem implies that a neoclassical growth model cannot be simultaneously consistent with empirical evidence on both capital-augmenting technical change andthe elasticity of substitution between labor and reproducible capital. In the multi-factor case,balanced growth with capital-augmenting technical change is possible as long as capital has aunitary elasticity of substitution with any single non-reproducible factor, increasing the likelihood that neoclassical models can be consistent with empirical findings. To illustrate theimportance of this result, we also build a three-factor growth model with endogenous and directed technical change and show that is has a stable balanced growth path with a strictly
positive rate of capital-augmenting technical change.
Keywords: Endogenous Growth, Balanced Growth Path (BGP), Uzawa Steady-State Growth
Theorem, Direction of Technological Change
JEL Classification Codes E13, E22, O33, O41∗
The authors are grateful to Been-Lon Chen, Oded Galor, Andreas Irmen, Cecilia Garcia-Peñalosa Alain Venditti,
Ping Wang, David Weil, and seminar participants at Aix-Marseille School of Economics, Brown University, Chulalongkorn University, GRIPS, Kobe University, Shiga University, OSIPP, Tongji University, for their helpful comments
and suggestions. This study was financially supported by the JSPS Grant-in-Aid for Scientific Research (15H03329,
15H05729, 15H05728, 16K13353, 17K03788). Any remaining errors are our own.
†
Correspondence: Institute of Social and Economic Research, 6-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan.

11Introduction

The neoclassical growth model (NGM) was developed to explain a set of stylized macroeconomic
facts that can be classified under the umbrella of balanced growth (Solow, 1956, 1994). As conventionally understood, the Uzawa (1961) steady state growth theorem says that on the balanced growth path (BGP) of a neoclassical growth model, all technological change must be laboraugmenting, unless the production function is Cobb-Douglas (Jones, 2005; Acemoglu, 2008; Jonesand Scrimgeour, 2008). This creates a significant problem for the NGM, because data from theUnited States strongly suggest that (i) there is capital-augmenting technical change on the BGP
and (ii) the aggregate production function in not Cobb-Douglas (see, e.g., Klump et al., 2007;
Oberfield and Raval, 2014). The evidence for capital-augmenting technical change comes from thefalling relative price of capital goods (Greenwood et al., 1997; Grossman et al., 2017).1
The standard NGM assumes that there are only two factors of production, labor and reproducible capital. In reality, there are many other factors of production, including various types ofland, energy, and materials. These factors do not fit well the notion of capital in the NGM inthat they cannot be readily accumulated (or reproduced) through savings. In this paper, we examine whether incorporating more factors of production makes it possible for neoclassical models
to be consistent with the empirical regularities mentioned above. We prove a generalized versionof the Uzawa (1961) steady state growth theorem that incorporates an arbitrary number of factors. We also prove several related propositions and discuss the implications of the theorem indetail, highlighting the difference between the true underlying production function and alternate
representations of that function which are the focus of the theorem. We also formulate and analyze an endogenous growth model with three factors of production and directed technical change,highlighting how the theoretical results can be implemented in a tractable framework.
Our results suggest that adding more factor of production may make it possible for neoclassical growth models to incorporate K-augmenting technical change on the BGP, while still being
consistent with evidence on the elasticity of substitution between capital and labor. In somewhat
colloquial terms, the theorem says that it is possible to have capital-augmenting (K-augmenting)technical change on the BGP as long as there is one non-reproducible factor that has a CobbDouglas relationship with reproducible capital. Given the vast number of additional factors thatexist in reality, it seems plausible that at least one has this desirable property of unit-elastic substitution with capital. To have K-augmenting technology on the BGP, the generalized Uzawa theorem
implies that there must be a specific log-linear relationship between the growth rates of the factors of
production and the factor-augmenting technologies. We show that this is the endogenous outcome
in a model of directed technical change calibrated to U.S. data. Thus, this log-linear relationshipdoes not impose any extra restrictions on the underlying parameter space that is consistent with1

See Section 2 for more detail.

2the data. We discuss our results with greater precision below.

1.1

Preview of Results

Consider a standard neoclassical environment with an arbitrary number of factors of production
and capital that is accumulated linearly from saved output. Suppose that there is a time-varying
function that maps aggregate inputs to aggregate output (i.e., an aggregate production function
exists). Then, on any BGP, it is possible to represent the relationship between aggregate inputs
and aggregate output using a time-invariant function that captures all productivity improvements
through factor-augmenting terms on factors other than reproducible capital (Proposition 1). We
call this the Uzawa Representation of the BGP. In addition, we demonstrate that it has the same
derivatives as the true production function on the BGP as long as factor shares are constant (Proposition 2), which guarantees that the Uzawa representation can be used as a local approximation of
the original production function.2 In the two-factor case, this result has given rise to the conventional wisdom that the Uzawa theorem rules out capital-augmenting technical change on the BGP.
The Uzawa theorem, however, is not informative about the actual underlying production function,and it does not establish the uniqueness of the Uzawa Representation. Thus, it does not actually
preclude the existence of capital-augmenting technical change on the BGP.
After proving this generalized version of the Uzawa steady state growth theorem, we examineconditions under which it is possible to represent the relationship between inputs and output via
a function that has capital-augmenting technical change on the BGP. As explained above, this isnecessary if we want to find a function that is able to match data on the relative price of capital
goods. We focus on production functions where all technological improvement is captured byfactor-augmenting terms. We call such mappings Factor-Augmenting representations. We find that
if there is any single factor of production that can be combined with reproducible capital withunit elasticity, then there are a continuum of different Factor-Augmenting Representations of the
mapping from inputs to outputs that have positive growth rates of capital-augmenting technicalchange (Proposition 3). Moreover, all of these representations have the same derivatives as the true
production function as long as factor shares are constant (Proposition 4).
In the two factor case, this result simply says that capital-augmenting technical change ispossible with Cobb-Douglas production, because capital- and labor-augmenting technical change
are essentially equivalent. This is not very helpful when it comes to matching data, however,
because empirical studies reject the existence of two-factor Cobb-Douglas production functions.
The multi-factor case, however, allows much wider possibility. In reality, there are many factorsof production besides labor and reproducible capital. For example, there are many types of land,
energy, and materials used in production. If any of these has a Cobb-Douglas relationship with2

We are the first to prove these results when there are more than two factors of production. See, Acemoglu (2008)
for a similar result in the two-factor case.

3

reproducible capital, then it is possible to represent output via a function that is consistent withdata on both (i) the non-unitary elasticity of substitution between capital and labor and (ii) the
falling relative price of capital goods.
In all of the Factor-Augmenting Representations discussed above, there is a specific log-linear
relationship between different types of technology that must hold on the BGP. In a world withexogenous technical change, this log-linear relationship between technologies would pose an extra
restriction on the nature of production. Our generalized Uzawa theorem, however, is broad enoughto incorporate endogenous technical change. To show that the log-linear technology relationship is
not actually a restrictive condition for growth models to incorporate capital-augmenting technicalchange on the BGP, we build a three-factor endogenous growth model with directed technicalchange and demonstrate that the log-linear technology relationship is the endogenous long-run
outcome. We accomplish this goal in two steps. First, we present and analyze an analytic model
where the BGP has a positive rate of capital-augmenting technical change. The dynamical system
has three state variables. To examine the stability of the system, we calibrate the model to longterm macroeconomic data from the U.S. We then demonstrate that the calibrated model is bothglobally and locally stable for a wide range of parameter values. This result guarantees that the
log-linear technological relationship is satisfied in the long run regardless of the initial state.
Our results have broad implications for the study of economic growth. It is obviously beneficial
in many settings to have a growth model that is consistent with the data on technological progress
and the elasticity of substitution between capital and labor. Moreover, incorporating two (or more)types of technology will allow growth-, development-, and business-cycle accounting exercises toprovide a richer description of economy, while still being consistent with balanced growth. For
example, our numerical analysis suggests that it takes much longer time than capital for the relative
productivity of different technologies to converge to the BGP value. It can provide a possible
explanation why most developing countries are not catching up the developed countries as quickly
as a standard neoclassical growth model predicts.

1.2

Contribution and Related LiteratureThis paper is related to a long literature on balanced growth and the Uzawa steady state growththeorem. Although the theorem is well known, Uzawa (1961) does not provide a clear statement of
proof of the theorem. A simple and intuitive proof was proposed by Schlicht (2006) and updated byJones and Scrimgeour (2008), Acemoglu (2008), and Irmen (2016). We contribute to this literatureby extending the theorem to multiple factors of production and a wider class of neoclassical models.
We also provide a more detailed discussion of the theorem and its implication.
The growth literature has long considered the Uzawa theorem to be overly restrictive (Jones

4and Scrimgeour, 2008). As a result, a second strain of the literature has examined how endogenous
technical change might lead an economy to conform to the conditions of the theorem. For example,
Acemoglu (2003) and Irmen and Tabaković (2017) present models of directed technical change
where the economy endogenously converges to a steady state with only labor-augmenting technicalchange. Similarly, Jones (2005) and Leon-Ledesma and Satchi (2019) demonstrate conditions underwhich Cobb-Douglas production is the endogenous outcome of firms choosing between technologies.
We build on this work and construct a directed technical change model with where technologicalprogress endogenously conforms to the restrictions generated by the multi-factor Uzawa theorem.
Unlike existing work, our model is consistent with data in that it has capital-augmenting technicalchange on the BGP and an elasticity of substitution between capital and labor that is different
than one.
To the best of our knowledge, Grossman et al. (2017) provide the only other attempt to square
the Uzawa steady state growth theorem with data on the elasticity of substitution between capitaland labor and the capital-augmenting technical change. They consider a neoclassical model with a
specific form of complementarity between physical and human capital. We will explain that theirresults can be understood as a particular case of the 2-factor Uzawa theorem.3 Our results indicatethat there is a much wider scope for ways in which the NGM can be made to be consistent withthe data on elasticities of substitution and capital-augmenting technical change.
Roadmap – The remainder of the paper proceeds as follows. Section 2 presents the data motivating
this study. Motivated by the evidence, Section 3 proves a generalized, multi-factor version of the
Uzawa steady state growth theorem. In Section 4, we prove that neoclassical models can havea positive rate of capital-augmenting on the BGP, as long as there is a single non-reproducible
factor that has a unitary elasticity of substitution with reproducible capital. Section 5 summarizes
and explain these results, focusing on simple cases and existing literature. Sections 6 presents theendogenous growth model, Section 7 and demonstrates that the the model endogenously converges
to a BGP with positive K-augmenting technical change. Section 8 discusses the broader implicationsof our findings, and Section 9 concludes.

2

Empirical Motivation

In this section, we discuss the empirical facts that motivate this study. In particular, we quickly
review facts on balanced growth and evidence for the existence of capital-augmenting technicalchange. The Uzawa (1961) steady state growth theorem suggests that these two sets of facts cannot
be reconciled in a standard two-factor growth model. So, we conclude by presenting evidence that
other factors, such as land and energy are important in the production process.
3

See subsection 5.2.

5

120
100

10000

Index (1950=100)

Billions of $2012 (log scale)

100000

1000
100
10
I

1
1950

60
40
20

GDP
1960

80

C

1970

K

1980

1990

2000

0
1950

2010

(a) BGPAll

1960

Non-Residential

1970

1980

Equipment

1990

2000

2010

(b) K-Augmenting Technology

Figure 1: Balanced Growth with capital-augmenting technical change. This figures presents some of the mainfeatures of balanced growth in the United States. Panel (a) demonstrates the real output, investment, consumption
and the capital stock have grown at roughly constant rates over long periods of time. These empirical patterns
summarize the notion of balanced growth. Panel (b) demonstrates that the relative price of investment goods, andequipment in particular, having been falling in the United States. It suggests that capital-augmenting technicalchange is happening along the BGP. See appendix section A.2 for details on data sources.

The neoclassical growth model, first developed by Solow (1956) and Swan (1956), is the central
building block of much contemporary research in economic growth. Such models are designed to
explain a set of stylized facts, known as ‘balanced’ or ‘steady’ growth (Jones, 2016). The main
stylized is that income per capita has grown at a constant rate over long periods of time. Panel(a) in figure 1 presents U.S. data from 1950-2012, which clearly demonstrates this pattern.4 It also
demonstrates that other macroeconomics aggregates have grown at similar rates to GDP, capturing
the notion of balance.5
To explain these facts, the neoclassical growth model focuses on an aggregate production function that has constant returns to scale in two factors, capital and labor. Intuitively, the key toexplaining balanced growth is that capital is reproducible (i.e., it is accumulated from saved output). Thus, capital ‘inherits’ the constant growth rate of output, implying that the capital-output
ratio will be constant in the long-run (Jones and Scrimgeour, 2008). Their joint growth rate is thendetermined by population growth and technological progress.
The ability of the neoclassical growth model to provide a simple explanation for these facts
has led to its widespread adoption (Jones and Romer, 2010). The model, however, relies on somestrong assumptions, including those described by the Uzawa (1961) steady state theorem. As
4

See Papell and Prodan (2014), Jones (2016), and others for longer time series and data on other countries.
As shown in Section 3, the formal definition of balanced growth in the steady state growth theorem only relieson the notion that various macroeconomic aggregates grow at (possibly different) constant rates. In this sense, theconstant labor share and capital-output ratio need not be part of our formal definition of balanced growth. Being
able to recreate these facts, however, was an important part of the motivation for the original neoclassical model. As
we will discuss, they are also important for understanding the implications of theorem.
56

conventionally stated, the Uzawa steady state growth theorem is as follows: on a balanced growthpath, all technological progress must be labor-augmenting, unless the aggregate production function
is Cobb-Douglas. As discussed above, on a balanced growth path with constant factor shares,
effective capital and effective labor must grow at the same rate, which must also be the growth rateof output. This is a result of constant returns to scale. Yet, reproducible capital ‘inherits’ the growthrate of output, which implies that there is no ‘space’ left for capital-augmenting technical change.
Cobb-Douglas production is as exception, because capital- and labor-augmenting technologicalchange are essentially equivalent in this special case.
Given the restrictive nature of these conditions for balanced growth, it is natural to ask whether
they are consistent with data. A long literature has estimated the elasticity between capital andlabor in a two-factor production function and rejected the Cobb-Douglas specification. Most of
papers in the literature argue that the elasticity is less than one (Chirinko, 2008). For example,
Oberfield and Raval (2014) estimate the macro elasticity of around 0.7 using firm-level micro data,
and Antras et al. (2004) estimates an elasticity of 0.6 directly from macro time-series data.6
Panel (b) of Figure 1 demonstrates that the relative price of investment goods has been fallingin the United States. As discussed in Grossman et al. (2017), this is a type of capital-augmenting
technical change. In a setting with perfect competition, decreases in the relative price of capital
goods reflect improvement in the efficiency of the investment goods sector. In other words, each
unit of forgone consumption is better able to produce output in the future. When we measurecapital goods in terms of their value relative to total output, this technological improvement isreflected in the increasing productivity of a measured unit of capital. Section 3 formalizes thisintuition, which is also discussed in Grossman et al. (2017). A long literature demonstrates thatdeclining investment prices are a quantitatively important source of growth in the United States(e.g., Greenwood et al., 1997; Krusell, 1998; Krusell et al., 2000). As a result, there is broadconsensus that capital-augmenting technical change has been pervasive in the United States overat least the last half a century, even as the economy exhibited signs of balanced growth.
These findings create a puzzle. Given that the elasticity of substitution between capital andlabor is not equal to one, the Uzawa theorem implies that any two-factor neoclassical growthmodel that is consistent with balanced growth is necessarily at odds with evidence on capitalaugmenting technical change. Put differently, the standard neoclassical growth model cannot explain the broader set of stylized growth facts that we observe in the United States.
In this paper, we argue that the inconsistency created by the Uzawa theorem can be rectified byconsidering additional factors of production, beyond reproducible capital and labor. It obvious that
other factors, such as land, energy, and materials, exist in the production process. To highlight theirimportance, table 1 collects some evidence on the importance of some of these factors in the United6

See also, Klump et al. (2007) and Chirinko et al. (2011). Using cross-country data, Karabarbounis and Neiman(2014) find an elasticity greater than one.

7

FactorNatural Resources
Land
Energy

Share
8%
5%
8.5%

SourceCaselli and Feyrer (2007)
Valentinyi and Herrendorf (2008)
Energy Information Administration

Table 1: This table presents some estimates of U.S. factor shares for inputs other than reproducible capital andlabor. Definitions and methodologies vary. See Appendix Section A.2 for details.

States.7 Broadly speaking, estimates suggest that non-reproducible factor other than labor accountfor about 10% of total factor payments. The rest of this paper examines how our understandingof balanced growth and the Uzawa theorem can be improved by explicitly considering these other
non-trivial factors in production.

3

A Multi-factor Uzawa Theorem

The aim of this section is to show that the steady-state growth theorem by (Uzawa, 1961, hereafter
the Uzawa theorem) extends to multi-factor environments that explicitly consider inputs beyond
labor and capital. We will then discuss the implications of the theorem and its limitations. The
section concludes with a new proposition that clarifies the condition under which the functional
form implied by the Uzawa theorem can be used in economic analysis as a first-order approximationof the original production function.
We start with a description of a neoclassical model, which is defined as broadly as possible inorder to incorporate a majority of dynamic macroeconomic models. For readability and consistency
with the following sections, we consider a discrete time settings, where t = 0, 1, 2..., but it isstraightforward to consider the continuous-time equivalents of the following results.
Definition 1. A multi-factor neoclassical growth model is an economic environment thatsatisfies:
1. Output, Yt , is produced from capital, Kt , and J ≥ 1 kinds of other inputs, {Xj,t }Jj=1 :Yt = F (Kt , X1,t , ..., XJ,t ; t).

(1)

The shape of production function F (·; t) can change with time t. In any t ≥ 0, it has constant returns to scale (CRS) in all inputs, Kt , X1,t , ..., XJ,t , and each input has positive anddiminishing marginal products.8
7

Estimating factor shares for inputs other than labor is notoriously difficult and often requires structural assumptions. Our intention is not to endorse any particular estimate. Instead, we simply note that there is ample evidence
that factors other than labor and reproducible capital play a non-negligible role in production.
8
If we allow J = 0, the only constant-returns-to-scale production function is in the form of Yt = AK,t Kt . Although
it cannot satisfy decreasing marginal products of its input (Kt ), this AK functional form obviously satisfies the Uzawa
theorem.

8

2. The amount of capital, Kt , evolves according toKt+1 = Yt − Ct − Rt + (1 − δ)Kt , K0 > 0,(2)where Ct > 0 is consumption, Rt ≥ 0 is expenditure other than capital investment or consumption (e.g., R&D inputs), and δ ∈ [0, 1] is the depreciation rate.
Here, Yt − Ct − Rt in (2) represents physical capital investment. There are a number of points
to note regarding Definition 1. First, if J equals 1 and X1,t is interpreted as labor, Lt , equation (1)reduces to a familiar two-factor neoclassical production function, Yt = F (Kt , Lt ; t). In addition,
if we assume Lt grows exogenously, Definition 1 essentially coincides with the definition of a neoclassical growth model in Schlicht (2006) and Jones and Scrimgeour (2008), who provide a simple
statement and proof of the two-factor Uzawa theorem.9
Second, the only reason why capital, Kt , is distinguished from other production factors X1,t , ..., XJ,t
is that we explicitly specify its accumulation process as in (2), which guarantees that Kt can beaccumulated linearly with the output.10 We will show that the Uzawa theorem holds regardless ofthe evolution process for other inputs.
Third, we allow for the term Rt term in (2). If we set Rt = 0, equation (2) is in line with theprevious definitions of the Uzawa theorem. This term is not essential for the proof of the Uzawa
Theorem itself, but this generalization accommodates the possibility of endogenous growth, as we
examine in later sections. In production function (1), any technological change is captured by thelast t term. If we think technology can be affected by the R&D expenditure, then such expenditure
would be included in Rt in (2). Similarly, the evolution of other factors Xj,t (including population
Lt ) can be either exogenous or dependent on particular types of expenditure, such as child-raisingcosts. Such costs are also be included in Rt .
Fourth, we measure the amount of capital by its value in terms of final output, not by its
productivity. Specifically, equation (2) implicitly normalizes the unit of period t + 1 capital so that
period t final output to can be converted to the same units of period t + 1 capital. However, there
are models where the amount of capital is measured by productivity. In particular, Greenwood
et al. (1997) and Grossman et al. (2017) assumed that qt+1 > 0 units of period t + 1 capital can beproduced from period t output, where qt increases over time by investment-specific technological
9

Grossman et al. (2017) considered a production function with three inputs: capital, labor, and schooling. However, as we discuss in 5.2, it is a particular case of two-factor neoclassical model (J = 1) because the production
function has constant returns to scale only in capital and labor.
10
Referring to this property, Jones and Scrimgeour (2008) noted “capital inherits the trend of output.” Grossman
et al. (2017) make the more precise statement “the value of physical capital that is produced from final goods inherits
the trend in output” which holds even when the accumulation technology changes through time (although it should
be linear at each point in time). From the theoretical viewpoint, Kt needs not to be limited to physical capital. it canbe any combination of factors that can be accumulated linearly with the output. For example, in the pre-industrial
Malthusian economy where population was proportional to output (e.g., Galor and Weil, 2000; Galor, 2011; Ashraf
and Galor, 2011), labor should be included in Kt , not in Xj,t . See also Li et al. (2016).

9change. Definition 1 can accommodate such an extension by a change of variables. Suppose that,instead of (2), physical capital accumulates according toK̆t+1 = (Yt − Ct − Rt )qt+1 + (1 − δ̆)K̆t ,(3)

and the production function is given by Yt = F̆ (K̆t , ·; t). Suppose that the growth factor ofinvestment-specific technology qt+1 /qt , is constant at gq > 0.11 Then, by defining Kt ≡ K̆t /qt andδ = (δ̆ + gq − 1)/gq , equation (3) turns out to be identical with equation (2).12 Also, the production
technology can be interpreted in the form of (1) by defining F (Kt , ·; t) ≡ F̆ (qt−1 Kt , ·; t) = F̆ (K̆t , ·; t).
In this definition of F (Kt , ·; t), the effect of investment-specific technological change qt−1 is capturedby the time dependence of F (Kt , ·; t) (i.e., as a part of technological change).
We now define the balanced growth path.
Definition 2. A balanced growth path (BGP) in a multi-factor neoclassical growth model isa path along which all quantities, {Yt , Kt , X1,t , ..., XJ,t , Ct , Rt }, grow at constant exponential rates
for all t ≥ 0. On the BGP, we denote the growth factor of output by g ≡ Yt /Yt−1 , and the growth
factors of any variable Zt ∈ {Kt , X1,t , ..., XJ,t , Ct , Rt } by gZ ≡ Zt /Zt−1 .13
Definition 3. A non-degenerate balanced growth path is a BGP with gK > 1 − δ.
From (2), condition gK > 1 − δ means that physical capital investment Yt − Ct − Rt is strictly
positive along the balanced growth path. The rest of the paper focuses on this non-trivial case. We
call it a non-degenerate BGP and simply mention it as a BGP when there is no risk of confusion.
Note that, while a BGP requires variables to grow at constant rates, it does not require them togrow at the same rate. Still, the following lemma confirms that capital and consumption need to
grow at the same speed as output to maintain a BGP.
Lemma 1. On any non-degenerate BGP in a multi-factor neoclassical growth model, capital-output
ratio Kt /Yt and consumption-output ratio Ct /Yt are constant and strictly positive.
t+1
t − R gt +Proof. Using the notation in Definition 2, (2) can be written as K0 gK= Y0 g t − C0 gC
0 R
t . Dividing all terms by g t and rearranging them gives(1 − δ)K0 gK

Y0 = C0 (gC /g)t + R0 (gR /g)t + K0 (gK + δ − 1)(gK /g)t .
11(4)

While, for simplicity, we assume qt+1 /qt to be constant, it only needs to be constant on the BGP for the purposeof proving the Uzawa theorem.
12
Note that δ = (δ̆ + gq − 1)/gq means 1 − δ̆ = (1 − δ)qt+1 /qt . Using this, Kt+1 = K̆t+1 /qt+1 = (Yt − Ct − Rt ) +(1 − δ̆)K̆t /qt+1 = (Yt − Ct − Rt ) + (1 − δ)Kt , which coincides with (2). In this representation, Kt ≡ K̆t /qt represents
the total value of period t capital in terms of the previous period’s final goods. If gq > 1 (positive investment-specifictechnological change), the depreciation rate δ should be higher than δ̆ because older capital experiences obsolescencein addition to physical depreciation.
13
For simplicity of exposition, we use the growth factor, rather than the growth rate. The conventional period
growth rate is obtained by subtracting 1 from the growth factor.

10Because all three terms on the right hand side (RHS) of (4) are non-negative exponential functionsof t, every one of them needs to be constant for the sum of all the terms to become constant (Y0 ).
For the first term C0 (gC /g)t to be constant, gC = g must hold since C0 > 0 from Definition 1. Thismeans Ct /Yt = C0 /Y0 > 0. For the third term (gK + δ − 1)(gK /g)t to be constant, gK = g must
hold since K0 > 0 and gK > 1 − δ. This implies Kt /Yt = K0 /Y0 > 0.14
Based on this lemma, we are ready to present a multi-factor version of the Uzawa Theorem.
Proposition 1. (a Multi-Factor Uzawa Theorem) Consider a non-degenerate BGP in a multieXj,t ≡ (g/gXj )t where j = 1, ..., J. Then, on the BGP,
factor neoclassical growth model, and define A
eX1,t X1,t , ..., A
eXJ,t XJ,t ) holds for all t ≥ 0,
Yt = Fe(Kt , A(5)where Fe(·) ≡ F (·; 0).
eXj,t ≡ (g/gXj )t , the growth factor of A
eXj,t Xj,t is g for all j. The
Proof. From the definition of A
growth factor of Kt is also g from Lemma 1. Therefore, all the arguments in function Fe(·) are
multiplied by g each period. This means that the RHS of (5) is multiplied by g each period since
Fe(·) ≡ F (·; 0) has constant returns to scale. Note that in period 0, equation (5) holds true because
it is identical with (1). Therefore, (5) holds for all t ≥ 0, where the both sides are multiplied by gin every period.
This proposition states that the conventionally-known Uzawa theorem applies to multi-factor
environments. While the Uzawa theorem applies to quite a wide array of macroeconomic models,
it is important to understand what the theorem does and does not imply. Recall that the neoclassical production function F (·; t) in (1) is a time-varying function that potentially depends ont in complex ways. However, if a BGP exists, the Uzawa theorem says that there should be asimple representation of this dependence of function F (·; t) on t, which holds at least along thisparticular BGP. We call this representation, given by (5), the Uzawa representation, which consists
eXj,t terms.
of a time-invariant function Fe(·) along with exponentially growing A
Caution is needed when interpreting Fe(·) as a production function, because (5) is not a functional relationship. Proposition 1 only guarantees that the value of Fe(·) coincides with that of theoriginal production function F (·; t) on a particular BGP. As is clear from the proof of the proposition, function Fe(·) contains no information about what will happen when inputs deviate evenslightly from the BGP. When the amount of one of the factors is changed from the BGP value,equation (5) does not hold in general. In this sense, the Uzawa Theorem does NOT say that Fe(·)
in (5) is a production function.
14

Similarly, we can show gR = g if R0 > 0.

11

Moreover, there is no guarantee that the derivatives of function Fe(·), even on the BGP, are equal
to the derivatives of the production function F (·; t), apart from time t = 0. This is inconvenient
because it implies that the Uzawa representation is unusable for any marginal analysis. Thus,
we now provide a condition under which the Uzawa representation has the “right” first-orderderivatives.
Proposition 2. (Derivatives of the Uzawa representation) Let FZ (·; t) denote the partial
derivative of function F (·; t) with respect to its argument Z ∈ {Kt , X1,t , ..., XJ,t }.15 If the share offactor Z, i.e., sZ = FZ (·; t)Zt , is constant on a non-degenerate BGP of a multi-factor neoclassicalgrowth model, then the following holds on the BGP:16∂ e
eX1,t X1,t , ..., A
eXJ,t XJ,t ) = FZ (Kt , X1,t , ..., XJ,t ; t) for all t ≥ 0.
F (Kt , A
∂Zt

(6)

Proof. See appendix section A.1.
If the factor shares are constant on the BGP, as implied by Kaldor (1961) Facts17 in the case of
two factors, (6) says that Fe(·) has the same derivatives as the original production function F (·; t)on the BGP, justifying its use in marginal analyses.18
Equally importantly, Propositions 1 and 2 jointly imply that the Uzawa representation is a
first-order approximation of the original production function around the BGP if factor shares on
the BGP are constant. This result legitimizes the use of production functions in the form of the
Uzawa representation in many applications, such as the business cycle research, where variables
fluctuate along a BGP. 19

4

Factor Augmenting Representations

The particular form of the Uzawa representation invites us to interpret the time dependence ofeXj,t ’s in
production function in terms of factor-augmenting technological change. By viewing the A
15

More precisely, FK (·; t) denotes the partial derivative of function F (·; t) with respect to its first argument,
whereas FXj (·; t) denotes the partial derivative of F (·; t) with respect to its 1 + jth argument.
16
Following the convention in economics, ∂X∂j,t Fe(·) represents the partial derivative with respect to variable Xj,t ,which is the marginal product of factor Xj,t given Fe(·) is the production function. This should not be confusedwith FeXj (·), which represents the partial derivative of function FeXj (·) with respect to its (1 + j)th argument. They
differ from each other when the argument of the function is not just a variable. For example, using the chain rule,∂
eX1,t X1,t , ..., A
eXJ,t XJ,t ) = A
eXj,t FeX (Kt , A
eX1,t X1,t , ..., A
eXJ,t XJ,t ).
Fe(Kt , A
j
∂Xj,t
17

See also the discussion in Jones and Romer (2010).
See Acemoglu (2008) for a related results in the two-factor case.
19
In the previous studies on the Uzawa theorem, there seems to be no consensus whether the constancy of factor
shares was a requirement for the Uzawa theorem or not. The original Uzawa (1961) paper assumed constant factor
shares, but others such as Schlicht (2006) did not require them. Proposition 1 and 2 clearly show that constant factor
shares are not required for the conventionally-known part of the Uzawa theorem, but they are needed for interpreting
the Uzawa presentation as an approximated production function.
18

12

the Uzawa representation (5) as the factor Xj,t -augmenting technology terms, Proposition 1 implies
that it is always possible to interpret the time variation of the original production function F (·; t)on the BGP in terms of exponential augmentation of production factors. Then, it is tempting toconclude that there should be no technological change that enhances the productivity of capital on
eK,t term in (5).
the BGP, because there is apparently no A
However, this reasoning is insufficient, because Proposition 1 does not establish uniqueness.
As a result, it does not rule out the existence of factor-augmenting representations of the original
production function other than the Uzawa representation. In this section, we explore the possibility
that the original production function has multiple factor-augmenting representations, where the
Uzawa representation is just one possibility. Let us start by defining what constitutes a factoraugmenting representation.
Definition 4. A Factor-Augmenting Representation of the original production function (1)is a combination of a time-invariant constant-returns-to-scale function F AUG (·) and the growth
factors of factor-augmenting technologies γK > 0 and γXj > 0, j ∈ {1, ..., J}, such that the pathsof output and inputs on a BGP satisfy
Yt = F AUG (AK,t Kt , AX1,t X1,t , ..., AXJ,t XJ,t ) holds for all t ≥ 0,

(7)

where AK,t = (γK )t and AXj,t = (γXj )t .
By comparing (5) with (7), it is clear that the Uzawa representation constitutes a factoraugmenting representation. However, (5) focuses only on a certain special case where all effective
factors grow at the same rate of g, while (7) permits different growth rates among separate effective
factors. In other words, the Uzawa representation hypothesizes that there is no factor substitution
taking place when the economy grows along the BGP. The homothetic expansion of every effective
input is the simplest interpretation of a steadily growing economy, but it does not necessarily
constitute the best description of the reality.
To see this, suppose that every effective input, including effective capital, grows at the same
speed as the output. Recall that the physical capital is already growing at the same speed as
output on the BGP (from Lemma 1). Then, there is no room for additional capital-augmenting
technological progress to further augment its effectiveness. (This is a well-known property of the
Uzawa Theorem). However, as discussed in Section 2, there is clear evidence that the productivityof capital, measured in terms of output as in our model, has steadily been increasing for a longtime. Therefore, the interpretation of the BGP as being a homothetic expansion of every input is
at odds with a well-established stylized fact.
This contradiction leads us to consider a broader range possibilities in which effective inputs
grow at different constant rates. For such a possibility to constitute a BGP, output must also

13

grow at a constant rate. To illustrate this point, note that the growth rate of output in thefactor-augmenting representation (7) can approximately be written as follows:20

Yt+1 /Yt ≈ sk,t γK gK +J
X

sXj,t γXj gXj ,

(8)

j=1

where sk,t is the share of capital at time t and similarly for sXj,g . In the Uzawa representation,
γK gK = γXj gXj holds for all j. Therefore, as long as the production function is constant returnsP
to scale (which guarantees sK,t + Jj=1 sXj,g = 1), the RHS is always constant, even if factor sharesare not constant. In contrast, if the effective factors grow at different speeds, i.e., when γK gK andγXj gXj ’s are different, the RHS of (8) becomes stationary only when the factor shares, sk,t andsXj,g ’s, remain constant over time.
This raises an important question: can factor shares be constant when factor substitution
is taking place on the balanced growth path? The answer is yes, as long as some of the effective
factors are substitutable to each other with the unitary elasticity of substitution. Below, we formallyestablish this conjecture. We must first define the elasticity of substitution when there are morethan two inputs.21
Definition 5. The Elasticity of Substitution between capital Kt and input Xj in multi-factor
neoclassical production function F (K, X1 , ..., XJ ; t) in (1) is defined by
σKXj,t = −d ln(K/Xj ),d ln (FK (K, X1 , ..., XJ ; t)/FXj (K, X1 , ..., XJ ; t)) Y,X−j :const(9)

where X−j ≡ {X1 , ..., XJ }\Xj represents the inputs other than K and Xj .
Let us consider explicitly many factors of production other than capital, {X1,t , ..., XJ,t }, andsuppose that some of them (e.g., land or energy) are substitutable with capital Kt with unitaryelasticity in the period 0 production function, F (·; 0).22 In that case, without a loss of generality,
we reorder these factors so that the first j ∗ ∈ {0, ..., J} of them can be substituted with capitalwith the unitary elasticity of substitution.23 Then, production in period 0 can be rewritten in thefollowing nested form:
20

This decomposition is obtained by Taylor-expanding the RHS of (7) for t+1 with respect to every effective factor,
around the period t values for the variables, and divide the result by the RHS of (7) for t. The Taylor-expansion isexact when the variables in t and t + 1 are sufficiently close, or equivalently, in continuous time.
21
When there are more than two production factors, there are various ways to define the elasticity of technicalsubstitution (See, Stern 2011 Journal of Productivity Analysis for a concise taxonomy). The elasticity in (9) is
calculated in one of the most straightforward settings, where the output and other inputs are kept constant when Xj
and K are simultaneously changed. It is the inverse of the symmetric elasticity of complementarity (SEC), definedin Stern (2010, Economics Letters), which has a desirable property of symmetry between the two variables.
22
We only need to assume that the production function has this property at some point in time. We just normalizethat time as t = 0.
23
When there is no such input, with a slight abuse of notation, let us write j ∗ = 0.

14

Lemma 2. Suppose that σKXj,0 = 1 for j = 1, ..., j ∗ . Then, there exist parameters α > 0 andP∗ξj > 0, j ∈ {1, ..., J}, such that α + jj=1 ξj = 1 and


Yj ∗
ξj
α
b
∗
(Xj,0 ) , Xj +1,0 , ..., XJ,0 ,Y0 = F K0
j=1

(10)

where Fb(·) is a constant-returns-to-scale function, defined by
1/α
Fb(z0 , zj ∗ +1 , ..., zJ ) ≡ F (z0 , 1, ..., 1, zj ∗ +1 , ..., zJ ; 0).
| {z }(11)j∗

Proof. See appendix section A.1.
Intuitively, if K0 is substitutable with factors {X1,0 , ..., Xj ∗ ,0 } with unit elasticity, then the
production function can be expressed as if they are combined together in the Cobb-Douglas fashionQ∗
to form an intermediate input Mt ≡ (AK,t Kt )α jj=1 (AXj,t Xj,t )ξj . This virtual intermediate input,
which we call capital composite, will be placed inside a constant-returns-to-scale function Fb(·), which
has j ∗ fewer arguments than Fe(·) ≡ F (·; 0). Using this nested form, the following proposition shows
the set of the possible factor-augmenting representations of a given BGP.
Proposition 3. (Factor-Augmenting Representations of a BGP) Suppose that σKXj,0 = 1
for j = 1, ..., j ∗ . On a non-degenerate BGP, let constants γK > 0 and γXj > 0, j ∈ {1, ..., j ∗ }, be
any combination that satisfy
α
γK

Yj ∗
j=1

(γXj gXj )ξj = g 1−α .

(12)Also let γXj = g/gXj for j = j ∗ + 1, ..., J. Then, on the BGP,


Yj ∗
α
ξj
b
Yt = F (AK,t Kt )(AXj,t Xj,t ) , AX j ∗ +1,t Xj ∗ +1,t , ..., AXJ,t XJ,t for all t ≥ 0,
j=1

(13)

where AK,t = (γK )t and AXj,t = (γXj )t , j ∈ {1, ..., J}.
Proof. See appendix section A.1.
Note that (13) constitutes a factor augmenting representation because its RHS is a function ofeffective factors (AK,t Kt ), (AX1,t X1,t ), ... , (AXJ,t XJ,t ) and it has constant returns to scale in allof these J + 1 factor-augmented variables.
Proposition 3 can be seen as a generalized version of the multi-factor Uzawa theorem (Proposition 1). When there is no factor that is substitutable with capital with unit elasticity at time
0 (i.e., j ∗ = 0), then Proposition 3 becomes identical with Proposition 1.24 However, given thatP∗
If j ∗ = 0, condition α + jj=1 ξj = 1 in Lemma 2 implies α = 1. Then, condition (12) reduces to γK = 1, whichmeans AK,t = 1 for all t. Then, (13) becomes identical to (5).
2415there are many factors of production in reality, it seems highly likely that at least one of them issubstitutable with capital with unit elasticity (j ∗ ≥ 1).25 In this case, there is a continuum of combinations {γK , γXj } that satisfy condition (12), and there always exists a continuum of its subset
where γK > 1. In other words, if j ∗ ≥ 1, there are a continuum of possible factor-augmenting representations of a BGP, and many of them are with strictly positive capital-augmenting technologicalchange. The Uzawa representation, where γK = 1 and γXj = g/gXj for all j, is also included in
the set of possible factor-augmenting representations. However, this is a special case in that it only
constitutes a single point in a continuum of possible factor-augmenting representations. In principle, to distinguish between various factor-augmenting representations, we need more information
than just the paths of the output and inputs, such as observations on the productivity growth ofcapital or other inputs from macroeconomic data, as we discuss in the next section.
We finish this section by confirming that any factor-augmenting representation has the same
derivative as the original production function under a condition similar to Proposition 2, and thus
can be used as a first-order approximation in economic analyses:
Proposition 4. (Derivatives of the Factor-Augmenting Representation) Suppose that
σKXj,0 = 1 for j = 1, ..., j ∗ and that the production function at time t = 0 can be represented
in the form of (10). If the share of factor Z ∈ {Kt , X1,t , ..., XJ,t }, i.e., sZ = FZ (·; t)Zt , is constanton a non-degenerate BGP of a multi-factor neoclassical growth model, the following holds on the
BGP:


Yj ∗
∂ b
α
ξj
∗
∗
F (AK,t Kt )(AXj,t Xj,t ) , AX j +1,t Xj +1,t , ..., AXJ,t XJ,t
j=1
∂Zt

(14)

= FZ (Kt , X1,t , ..., XJ,t ; t) for all t ≥ 0.
Proof. See appendix section A.1.

5

Summary and Examples

The previous sections established both the wide applicability of the Uzawa theorem and also its
limitations, focusing on economic environments specified as loosely as possible so that most dynamic
macroeconomic models fit into our framework.
As for the wide applicability, we have shown that if the economy exhibits balanced growth, asobserved in many countries, there always exists a representation of the aggregate production func25

As discussed in Section 8, we hope that our results will motivate further empirical work to examine the elasticity
of substitution between reproducible capital and a wide range on non-reproducible factors. In the exiting literature,
there is some suggestive evidence that certain forms of land may have a unitary elasticity of substitution with capital.
For example, Epple et al. (2010) and Ahlfeldt and McMillen (2014) suggest that the elasticity of substitution between
land and structures is likely close to one for residential housing, and . These papers, however, do not estimate
aggregate elasticities and, therefore, do not measure the structural parameters of interest in our model. Further work
focusing explicitly on a macroeconomic context is necessary before meaningful conclusions can be drawn.

16

eX1,t X1,t , ..., A
eXJ,t XJ,t ), which explains the growth process by factor-augmenting
tion Yt = Fe(Kt , A
technological changes on other factors than capital (Proposition 1). This simple representation,
called the Uzawa Representation, explains the balanced growth by homothetic expansion of every
production factor. If the factor shares are constant, we have shown that the Uzawa Representation
matches the behavior of the actual (unknown) production function not only on the BGP, but also
around the BGP (Proposition 2). This means that, with the observation that the economy is on
average steadily growing and the factor share are stationary, we can always use a production function in the form of the Uzawa Representation to study long-term growth and fluctuations aroundthe BGP, as long as we believe that the economy can be described by a neoclassical environment,
potentially with many production factors.
However, the Uzawa representation fails to explain one critical aspect of growth. Because thereeK term in the Uzawa representation, it appears to say that there should be no improvement in
is no Athe productivity of capital. We have clarified that the Uzawa theorem does not say that the Uzawa
representation (without any capital-augmenting technological change) is the only representationthat explains the observed balanced growth. To the contrary, our generalized theorem (Proposition3) constructs a continuum of factor-augmenting representations of the production function. Everycandidate representation can explain the observed balanced growth, but they differ in the rates offactor-augmenting technological progress among production factors. This means that it is possible
to choose from those candidate representations so that the rate of capital-augmenting technologicalprogress in the model matches the rate observed in data. Proposition 4 guarantees that, if factor
shares are constant, the chosen representation can be used as a production function not only for
analyzing long-term growth but also for studying economic fluctuations, because the representation
constitutes a local approximation of the actual production function along the BGP.
So far, we have presented our results in as general a setting as possible. To incorporate theseresults into neoclassical models suitable for economic analysis, it is necessasry to consider specificfunctional forms for the aggregate production function. In the remainder of this section, we present
three examples that explore the simplest way to make neoclassical models consistent with aggregate
data on the relative price of capital and the elasticity of substitution between capital and labor.
In subsection 5.1, we explain why a standard neoclassical economy only with two factors cannot
simultaneously be consistent with these. Then, subsection 5.2 discusses the approach taken by
Grossman et al. (2017) as a special case of the 2-factor neoclassical environment. Finally, subsection
5.3 shows that the conflict between data and neoclassicaly models can be resolved when including
factors of production beyond labor and reproducible capital.

17

5.1

Standard 2-Factor Neoclassical Growth Model

Suppose that the original production function Yt = F (Kt , Lt ; t) uses only two kinds of inputs,
capital, Kt , and labor, Lt . Then Proposition 1 says that, on any BGP with positive investment,
this production function can always be written as: Yt = Fe(Kt , AL,t Lt ). However, if these two factors
are substitutable with unit elasticity (σKL = 1), Proposition 3 shows there are other possible factoraugmenting representations of the same production function:26
Yt = A(AK,t Kt )α (AL,t Lt )1−α , where A > 0 is a constant,(15)

eL,t Lt )1−α as a special case. Condition (12)which includes an Uzawa representation Yt = AKtα (Aimplies that the growth factor of technologies, γK > 0 and γL > 0, can take any values as long
α γ 1−α = g 1−α . By rewriting (15) as Y = A K α L1−α , where the TFP A is given by A ≡
as γK
t
t t t
t
t
L

AAαK,t A1−α
L,t , it is clear that various combinations of capital- and labor-augmenting technologicalchanges give the same rate of growth for the TFP and, therefore, output.
This result confirms the widely understood version of the Uzawa theorem: on a BGP, alltechnological progress must be labor-augmenting, unless the production function is Cobb-Douglas.
As we have seen in Section 2, this theoretical results is in contradiction with the two stylized findings:(i) the productivity of capital has been steadily increasing, and (ii) the elasticity of substitution
between capital and labor is less than unity, ruling out the Cobb-Douglas production function. Anystandard, two-factor production function cannot reconcile these two stylized facts.

5.2

Inclusion of Schooling in a Two-Factor model

Grossman et al. (2017) propose a possible solution to this contradiction by including schooling,st ≥ 0, in a standard two-factor production function. Their result can be understood intuitively
in terms of our analytical framework. While they directly started their analysis from a factoraugmenting representation, it is worthwhile to consider an underlying time-varying production
function in the form of (1):Yt = F (Kt , Lt ; t) = F s (D(st )a Kt , D(st )−b Lt ; st , t),(16)

where a > 0, b > 0, D(·) ∈ [0, 1], and D0 (·) < 0. The definition of a neoclassical growth modelin Definition 1 is general enough to include function F s (·; st , t) in (16) as a particular case of a
two-factor neoclassical production function with J = 1.27
When there are two factors (J = 1) and they are substitutable with unit elasticity (j ∗ = 1), equation (13) inProposition 3 implies that Yt = Fb (AK,t Kt )α (AL,t Lt )1−α . Because function Fb(·) has constant returns to scale andhas only one argument, we can write Fb(x) = Ax for some A > 0, which gives (15).
27
The time-varying effects of D(st ) and st itself on output Yt in (16) are captured by the t term in F (Kt , Lt ; t).
Note also that Grossman et al. (2017) considered investment-specific technological change, which can be rewritten in
26

18

When schooling, st , increases, the multiplier D(st )a on Kt shrinks, raising the marginal productof capital. The opposite holds for labor. In this way, Grossman et al. (2017) specified a certain
type of complementarity between schooling and capital. Note that st is not a production factor ina neoclassical sense, because the production function has constant returns to scale only in capitaland labor.
eL,t Lt )From Proposition 1, this production function has an Uzawa representation Yt = Fe(Kt , A
eL = (g/gL )t on a BGP, where both effective factors Kt and A
eL,t Lt grow at the same speedwith A
as output. Therefore, if we directly look at the simplest representation of the BGP, then it would
imply no capital-augmenting technological progress. Instead, Grossman et al. (2017) interpret the
production function in the following way, keeping the multiplier D(st ) term in the expression:
eL,t Lt ) = Fe(AK,t D(st )a Kt , AL,t D(st )−b Lt ).
Yt = Fe(Kt , A(17)Comparing the arguments in the right hand side and these in the middle, we immediately obtain
eL,t D(st )b . Because the multiplier D(st )a shrinks as st increases,
AK,t = D(st )−a and AL,t = A
the capital-augmenting technology AK,t must grow so as to exactly offset the shrinking D(st )aterm. Conversely, the labor-augmenting term AL,t should grow slower than that in the Uzawa
eL,t because the multiplier D(st )−b is also augmenting the labor.28
representation A
Within the limits of the two-factor Uzawa theorem, Grossman et al. (2017) propose a newinterpretation of the production function, which provides the first possible solution to the contradiction raised by the Uzawa theorem. It is important to note, however, that schooling must enter
the production function precisely in the form of (17), where exactly the same function D(st ) must
appear both before capital and labor, with the powers of opposite signs. In addition, the functional
form of D(st ) and the dynamic path st in equilibrium must be in such a way that D(st ) shrinks
exponentially over time. Future empirical work could greatly inform our understanding of long-run
economic growth by testing whether the formulation (16) is consistent with data. In this paper, we
propose a wider class of functions that are consistent with balanced growth. The next subsection
discusses a particularly simple example.

5.3

A Simple Three-Factor Model with LandAs we have seen in Section 2, a significant portion of GDP is paid to production factors that do
not fit well in either the notion of Kt or Lt . Thus, it is natural to consider production functions
the form of Definition 1 as we discussed in Section 3.
28
From these observations, the main result of (Grossman et al., 2017, proposition 2) can easily be obtained as−afollows. Taking the growth factor of the both sides of AK = D(st )−a gives γK = gD
. From this we obtain a discrete−1/atime equivalent of their proposition 2(ii): gD = γK . Note that Grossman et al. (2017) assumed Lt = D(st )Nt ,which means gL = gD gN . Because effective labor AL,t D(st )−b Lt in (17) must grow at the same rate as output,(b−1)/a−b1−bg = γL gD
gL = γL gD
gN = γL γK
gN , which is a discrete time equivalent of their proposition 2(i).

19that are beyond the limits of the two-factor Uzawa theorem.
Adding additional factors of production makes it possible to reconcile the data with a neoclassical model. While the labor cannot be substituted by capital with unitary elasticity (σKL 6= 1),
Proposition 3 only requires that there is a single production factor that is substitutable with capitalwith unitary elasticity. In this case, there exist factor-augmenting representations of the production
function that have capital-augmenting technological change (γK > 1).
Let us consider the simplest extension of the standard neoclassical production function,t
Yt = Ft (Kt , Lt , Xt ; t), where Xt = X0 gX
for all t, X0 > 0, gX > 0.

(18)

Here, we have a third production factor Xt , which is either growing (gX > 1), shrinking (gX ∈(0, 1)), or constant (gX = 1). One example of such a factor is land. In that case, gX represents thegrowth factor of the available land space. If the total area of available land asymptotes to an upperbound in the long run, then gX would be one on the BGP. Another example is natural resources.
If it is non-renewable, gX ∈ (0, 1) will likely hold, while a renewable energy source (e.g., sun light)
could have gX = 1.
Among many candidates for the third production factor, we focus on those that have a unitaryelasticity of substitution with capital in the initial period: σKX,0 = 1. For concreteness, let us callthis factor land. Then, Proposition 3 implies that, along a non-degenerated BGP, the production
function can be represented in a factor-augmenting fashion:Yt = Fb

AK,t Kt

αAX,t Xt

1−α, AL,t Lt , α ∈ (0, 1),(19)

where the growth factor of technology variables can be any combination of γK > 0, γX > 0, andα (γ g )1−α = g 1−α = γ g . Rearranging, we find that in a BGP, the growth
γL > 0 such that γK
X X
L L

factor of capital-augmenting technology must satisfy,

γK =

γL gL
γX gX

(1−α)/α.

(20)

Thus, there is a positive capital-augmenting technological change on a BGP (γK > 1), if theeffective input of the third factor AX,t Xt grows slower than effective labor AL,t Lt , which grows
proportionally to output Yt .
This finding raises an important question: even if there is a factor with σXj ,K = 1, will therates of technological change γK , γX , and γL be determined so as to satisfy the log-linear condition(20)? If their values are exogenously given, then this is a knife-edge case. If growth rates areendogenous, however, this need not pose any additional restrictions on the model. In Section 6, wedevelop a growth model with endogenous and directed technical change, where γK , γX , and γL are20

endogenously chosen. We will confirm that, on the BGP, condition (20) is satisfied. In Section 7,
we calibrate a version of the model to moments from the long-term U.S. data and show that the
BGP with positive capital-augmenting technical change is both locally and globally stable. These
two sections jointly demonstrate that regardless of the initial state of technologies, condition (20)is always satisfied in the long run.

6

A Full Model with Endogenous Directed Technological Change

So far, we discussed the implications of the extended Uzawa theorem only focusing on the production
sector, without explicitly considering R&D activities or the demand side of the economy. Now, wedevelop a complete endogenous growth model, where firms decide the direction of technologicalprogress so as to maximize their profit. We will show that the log-linear technology condition (20)is endogenously satisfied on the BGP. To keep the analysis from becoming overly complicated, webase this section on a streamlined version of the model of tasks by Irmen (2017) and Irmen andTabaković (2017) and expand it to incorporate three production factors. There are two benefits
from our specification. First, we can analyze intentional R&Ds within a perfectly competitive
economy, which fills a gap between the standard neoclassical growth model (perfectly competitive)
and the standard endogenous growth theory (usually with some imperfect competition). Second,
while standard endogenous growth models suffer from the scale effects, our model of tasks will
become scale independent, which means that we can explain the BGP even when the amount oflabor is changing.

6.1

The Economic Environment

There are non-overlapping generations of representative firms, each of them existing for only one
period. A representative firm performs two types of tasks, M-tasks and N-tasks. The number ofM-tasks, as well as that of N-tasks, determines the amount of final output. The M-tasks require
effective capital AK,t Kt and effective land AX,t Xt as inputs, where AK,t and AX,t are the representative firm’s capital-augmenting and labor-augmenting technologies (these will be explained
in detail below). Effective capital and effective land are substitutable with each other with unit
elasticity, which means when more land is used, less capital is required to complete the same task.
Specifically, if an M-task uses AX,t x units of effective land, it requires at least AK,t k ≥ (AX,t x)−ζ ,ζ > 0, units of effective capital. Then, when the representative firm uses Kt units of capital and21

Xt unites of land in total, the maximum number of M-tasks it can complete is given by29
Mt = (AK,t Kt

αAX,t Xt

1−α,

α = 1/(1 + ζ) ∈ (0, 1).

(21)

N-task uses only effective labor, AL,t Lt , where AL,t is the labor-augmenting technology of therepresentative firm. Assuming that an N-task requires at least one unit of effective labor, themaximum number of N-tasks that the representative firm can perform using Lt is simply
Nt = AL,t Lt .

(22)

By performing Mt and Nt tasks, the representative firm produces
Yt = Fb(Mt , Nt ) = Fb

AK,t Kt

αAX,t Xt

1−α, AL,t Lt(23)

units of output, where = Fb(·) is a standard neoclassical production function that has constantreturns to scale and an intensive form that satisfies the Inada conditions.
Now, we explain how the factor-specific technologies {AK,t , AX,t , AL,t } are determined. Technical knowledge can be kept within a firm only for one period, after which there are (potentially
incomplete) knowledge spillovers. Thus, any firm can freely use a part of the previous period’s average factor-augmenting technologies. In addition, the firm can improve each of factor-augmenting
technologies through R&D. Specifically, in a M-task, the capital augmenting technology for thattask is given byAK,t = AK,t−1 γ K + ΦK (ik ) ,γ K ∈ [0, 1],(24)

where we omit the subscript for each task because all M-tasks are symmetric. Here, γ K is thefraction of the previous period’s firms knowledge that is freely available to a period-t firm. Also,ΦK (ik ) represents the addition of technological knowledge that results investing ik units of final
goods in R&D. Function ΦK (·) satisfies Inada-like conditions: ΦK (0) = 0, Φ0K (0) = ∞, Φ0K (ik ) > 0
and Φ00K (ik ) < 0 for ik > 0. We also assume that ΦK (∞) > 1 − γ K , which ensures that technology
can be improved over time with sufficient amount of R&D inputs.30
It is convenient to rewrite (24) in terms of investment cost function. Let γK,t ≡ AK,t /AK,t−1
denote the growth factor of AK,t and γ K ≡ γ K + ΦK (∞) be its upper bound, which can be either
29

With unit elasticity of substitution between capital and land, it is optimal to allocate capital and land
to individual M-tasks with equal quantities. If the representative firm is to operate Mt kinds of M-tasks, itmeans k = Kt /Mt and x = Xt /Mt . Substituting these into the input requirement AK,t k ≥ (AX,t x)−ζ givesα1−αMt ≤ (AK,t Kt
AX,t Xt, where α = 1/(1 + ζ).
30
To minimize notations, limik →∞ ΦK (ik ) is written as ΦK (∞). We use similar conventions hereafter when theycause no ambiguities.

22

infinity or a finite number. Then we can define the R&D cost function as31(−1)

ik (γK,t ) ≡ ΦK

γK,t − γ K ,defined for γK,t ∈ [γ K , γ K ],

(25)

(−1)

where ΦK (·) denotes the inverse function of ΦK (·). From properties of ΦK (·), it can be seenthat R&D cost function iK (γK ) satisfies iK (γ K ) = 0, i0K (γ K ) = 0, iK (γ K ) = ∞, iK (γK ) > 0,i0K (γK ) > 0, and i00K (γK ) > 0 for all γK ∈ (γ K , γ K ). The marginal cost of improving the technology
is small when the innovation is small (γK,t ' γ K < 1), but it becomes increasingly expensive whenaiming for bigger innovations, especially approaching the upper bound γ K > 1.32
We assume that the firm faces similar constraints when improving AX,t and AL,t . R&D costfunctions for land and labor-augmenting technologies are defined accordingly as iX (·) and iL (·),
along with γ X , γ X , γ L and γ L , which can differ from γ K and γ K . Recall that factor augmenting
technologies are all task-specific and the R&D costs must be incurred for each of Mt and Nt tasks.
This means the total R&D costs for all M- and N-tasks are:RK,t = Mt · iK (AK,t /AK,t−1 ),RX,t = Mt · iX (AX,t /AX,t−1 ),(26)

RL,t = Nt · iL (AL,t /AL,t−1 ).
The objective of the representative firm is to maximize the single period profit, because it lives
only for one period and its knowledge will become public next period. By taking the output ineach period as numéraire, the period profit is given by
πt = Fb(Mt , Nt ) − RK,t − RX,t − RL,t − rt Kt − τt Xt − wt Lt ,(27)

where rt , τt , and wt are interest rate, land rent, and wage rate, respectively.
We keep the demand side of the economy as standard as possible. The economy is populated
by a representative households. The size of the representative household (i.e., population) evolves
according to
Lt = L0 gLt , L0 > 0, gL > 0 : given.

(28)

As in the Ramsey-Cass-Koopman model, the period utility of the household is given by the product31

With a slight abuse of notation, hereafter iK (·) represents a function, not a variable.
This can be explained by congestion in R&D activities. When many researchers are devoted to improvementsin the same task for a given time period, some of them will end up inventing the same innovation. The risk of
duplication become more prominent as the input to R&D increases, which makes the R&D cost function iK (·)convex(or, equivalently, the R&D output function ΦK (·) concave). See Horii and Iwaisako (2007) for a simple micro
foundation.
32

23

of the number of household members and the per capita period felicity function:33
ut = Lt u(Ct /Lt ) = L0 gLt u(Ct /L0 gLt ),(29)

where Ct /Lt is per capita consumption. Following the standard in the literature, we assume thefelicity function takes the CRRA form. Then, from (28) and (29), the intertemporal objectivefunction of the household can be written asU=∞X

L0 (βgL )t

t=0(Ct /L0 gLt )1−θ − 1
,
1−θ

(30)

where θ > 0 is the degree of the relative risk aversion and β ∈ (0, 1/gL ) is the discount factor.34
The representative household owns capital, Kt , and land, Xt , in addition to labor Lt . The
household also own the representative firm and receives the profit πt , although in equilibrium wewill show that it becomes zero due to perfect competition. For simplicity, we assume that the
supply of land is exogenous:t
Xt = X0 gX, X0 , gX > 0 : given.

(31)

As in the case of population, the available quantity of land can be either constant gX = 1, shrinking gX ∈ (0, 1), or growing gX > 1.35 Physical capital accumulates through the savings of thehousehold.
Kt+1 = (rt + 1 − δ)Kt + τt Xt + wt Lt + πt − Ct ,K0 > 0 : given,(32)

where (rt + 1 − δ)Kt + τt Xt + wt Lt + πt represents the household’s income. The household is subject
to the non-Ponzi game condition:

lim

T →∞

!−1
T
Y
(rt + 1 − δ)KT +1 ≥ 0

(33)

t=0

This completes the description of the model economy.
Before proceeding to the analysis of the model, let us make sure that it conforms to our definition of the multi-factor neoclassical growth model, given by (1) and (2) in Definition 1. First,
the aggregate production function (23) has exactly the same form as (19), which belongs to thedefinition of the multi-factor neoclassical production function (1). In fact, Proposition 3 guarantees
33

Strictly speaking, we are using a discrete time version of the standard setting. An equivalent formulation incontinuous time can be found, for example, in Chapter 2 of Barro and Sala-i Martin (2004).
34
We assume β < 1/gL , or equivalently βgL < 1, so that the household put smaller weights for future generations
than the current generation.
35
Recall that we just call the third production factor Xt land for convenience. If Xt literally means the acreage ofland, then gX > 1 should be ruled out unless we want to consider space migration.

24

that, if the elasticity of substitution between Kt and Xt is unity and the economy has a BGP in
equilibrium, then the aggregate production function can always be written in the form of (23) atleast along the BGP. Our microeconomic setting gives an example of such an economy. Second, by
substituting (27) into (32), we obtain the evolution of capital in the same form as (2), where thetotal R&D expenditure is defined as Rt = RK,t + RX,t + RL,t . The difference between Definition 1
and the current model is that we now have a complete description of the economy, including how
the speeds of technological changes are determined. We are now ready to explore whether thiseconomy can generate a BGP in equilibrium, paying special attention to whether there is a BGP
with strictly positive rate of capital-augmenting technological progress.

6.2

R&D by Firms and the Direction of Technological Progress

Let us examine the behavior of the representative firm in the economy described above, focusing
on the role of R&D. The representative firm maximizes profit (27) subject to the production
and R&D functions (21)-(26) with respect to {Kt , Xt , Lt , AK,t , AX,t ,AL,t }, taking as given {rt ,τt , wt , AK,t−1 , AX,t−1 , AL,t−1 }. For convenience, let us define by µt ≡ Mt /Nt as the relative task
intensity in final good production. Then, because Fb(·) in (23) is a CRS function, we can write
Fb(Mt , Nt ) = Nt Fb(µt , 1) ≡ Nt f (µt ), FbM (Mt , Nt ) = f 0 (µt ), and FbN (Mt , Nt ) = f (µt ) − µt f 0 (µt ).36
Using this notation, we can conveniently express the first order conditions for factor demands.
The firm demands capital so as to satisfyrt = (αMt /Kt ) f 0 (µt ) − iK (γK,t ) − iX (γX,t ) .

(34)

The RHS of (34) represents the (net) marginal product of Kt in producing output Yt . It is givenby the product of two parts. The first part, αM/K, is the marginal product of Kt in increasing
the number of M-tasks performed in the firm, while the second part is the net marginal product of
Mt in producing the final output. Note that, in the second part, the innovation cost for an M-task,iK (γK,t ) + iX (γX,t ), is subtracted from the marginal product of Mt , f 0 (µt ). This happens because
when the firm operates more M-tasks, it needs to pay R&D costs to increase AK,t and AX,t in thesetasks so as to keep up with other M-tasks.37 The demand for land X is determined in a similarway,τt = ((1 − α)Mt /Xt ) f 0 (µt ) − iK (γK,t ) − iX (γX,t ) ,(35)

where (1 − α)M/X is the marginal product of Xt in performing more M-tasks. Lastly, the firm
36 b
FM (·) and FbN (·) represent the partial derivative of function Fb(·) with respect its first and second arguments,

respectively.
37
Recall that, although we omit subscripts for individual tasks, AK,t and AX,t are task-specific and therefore
the R&D cost will increase with the number of tasks, given the target rate of technological improvement. Fromthe symmetry of tasks within each group (M or N), it is always optimal to spend equal amounts of R&D costs forindividual tasks.

25

employs labor according towt = AL,t f (µt ) − µt f 0 (µt ) − iL (γL,t ) ,(36)

where the first part says that an additional unit of Lt can perform AL N-tasks. By substituting(34), (35), and (36) into (27), it can be confirmed that the firm achieves zero profit, πt = 0. Thisis due to the constant-returns-to-scale property of the firm’s problem.
Now, let us turn to R&D. We first explain the R&D condition for improving the laboraugmenting technology AL,t . The representative firm chooses AL,t , or equivalently the speed of technological progress γL,t ≡ AL,t /AL,t−1 ∈ [γ L , γ L ], according to first order condition ∂πt /∂AL,t = 0.
Simplifying this condition yields:
R&D for N-tasks: γL,t i0L (γL,t ) + iL (γL,t ) = f (µt ) − µt f 0 (µt ).

(37)

The firm’s private benefit from improving technology AL,t is to become able to perform a larger
number of N-tasks, which increases the final output Yt = Fb(Mt , Nt ). The RHS of (37) shows
the marginal benefit, FbN (Mt , Nt ) = f (µt ) − µt f 0 (µt ). The LHS corresponds to the marginal costof performing a larger number of N-tasks through augmenting labor efficiency AL,t (given labor
employment Lt ). This can be achieved by two steps. First, by intensifying the R&D efforts in theexisting N-tasks to raise labor efficiency, the representative firm can save a certain amount of labor,
which is just enough to perform one additional N-task. The cost associated with this activity isgiven by the first term γL,t i0L (γL,t ), which we call the intensive marginal R&D cost. The saved
labor is then used to perform a new N-task, which means the representative firm needs to invest in
R&D for one more N-task, which costs iL (γL,t ). This extensive marginal R&D cost is represented
by the second term in the LHS.
It is easy to see that condition (37) has a unique solution for γL,t as a function of µt = Mt /Nt .
From the property of the R&D cost function,38 the LHS of (37), which represents the marginal
cost of improving AL so as to operate one additional N-task, is a strictly increasing function ofγL,t = AL,t /AL,t−1 . It takes a value of 0 when γL,t = 1 and goes to infinity as γL,t → ∞.39 The RHS
is the marginal product of task-N, FbN (Mt , Nt ), which is a strictly positive and decreasing functionof µt = Mt /Nt . Therefore, there always exists a unique interior solution for γL,t , which we denoteby γ
bL (µt ). Note that γbL (µt ) > 1 and γ
bL0 (µt ) > 0 holds for any µt > 0. Intuitively, when the firm
is performing relatively fewer N-tasks (i.e., when µt ≡ Mt /Nt is higher), the benefit of increasing
AL,t to operate another N-task is larger, and therefore it is optimal to improve the labor-saving
38
Recall that the R&D cost function satisfies iL (1) = 0, i0L (1) = 0, iL (γL ) > 0, iL (∞) = ∞, i0L (γL ) > 0, and
00iL (γL ) > 0 for all γL ≥ 1. The same holds for iK (·) and iX (·).
39
The derivative of the LHS of (37) with respect to γL,t is γL,t i00L (γL,t ) + 2i0L (γL,t ), which is strictly positive forall γL,t ≥ 1 from iL (γL,t ) > 0 and i0L (γL,t ) > 0. When γL,t = 1, the LHS becomes i0L (1) + iL (1) = 0. As γL,t → ∞,the LHS also goes to infinity because iL (∞) = ∞ and γL,t i0L (γL,t ) > 0.

26

technology AL,t at a faster pace (bγL (µt ) should be higher).
Next, we examine the R&D investments for capital- and land-augmenting technologies. Similarly to the case of labor-augmenting technology, γK,t ≡ AK,t /AK,t−1 and γX,t = AX,t /AX,t−1 needto satisfy the first order conditions, ∂πt /∂AK,t = 0 and ∂πt /∂AX,t = 0. Combining these two
equations, we obtain two intuitive conditions that determine the allocation of R&D efforts among
capital- and land-augmenting technologies, as well as the condition that specifies the optimal combined amount of R&D for M-tasks.40
R&D allocation:

γK,t i0K (γK,t )α=,γX,t i0X (γX,t )1−αCombined R&D: γK,t i0K (γK,t ) + iK (γK,t ) + γX,t i0X (γX,t ) + iX (γX,t ) = f 0 (µt ).

(38)
(39)

Observe that γK,t i0K (γK,t ) and γX,t i0X (γX,t ) in condition (38) have the form similar to the intensive
marginal R&D cost in (37). Since both of them are respectively strictly increasing in γKt and γX,t ,this condition can be expressed as an upward sloping curve in the (γK,t , γX,t ) space, originatingfrom point (1, 1). This condition states that the allocation of R&D activities for improving capitalaugmenting technology AK,t and labor-augmenting technology AX,t must be reasonably balanced,
because the R&D cost function is convex. However, it does not say that AK,t and AX,t mustgrow at the same speed. As the RHS of condition (39) shows, the allocation should depend on therelative contribution of capital and land in performing M-tasks. When capital’s relative contribution
is higher (i.e., when α is higher), more resources should be allocated to R&D for the capitalaugmenting technology. In addition, the slope and convexity of the R&D cost function also affectthe optimal allocation. For example, if it is relatively difficult to improve the land productivity,
i.e., if the marginal R&D cost i0X (γX,t ) increases more rapidly with its argument than i0K (γK,t ),then it is optimal to improve AX,t slower than AK,t .
Given allocation rule (38), condition (39) specifies the optimal combined size of R&D investments. Capital and land are used in M-tasks, and therefore improving capital- and land-augmenting
technologies will enable the firm to operate more M-tasks. This marginal benefit is represented
by the RHS of (39), f 0 (µt ) = FbM (Mt , Nt ). The LHS is the marginal cost of R&D, which has
two parts, γK,t i0K (γK,t ) + iK (γK,t ) and γX,t i0X (γX,t ) + iX (γX,t ), because both capital- and landaugmenting technologies are to receive some R&D subject to the allocation condition (38). In eachof the two parts, the first term represents the intensive marginal R&D cost, whereas the second
term is the extensive marginal R&D cost, similarly to condition (37).
Note that the first part of the LHS of (39) is increasing in γK,t , whereas the second term is in
The first order condition for AK,t yields (γK,t /α)i0K (γK,t ) + iK (γK,t ) + iX (γX,t ) = f 0 (µt ), whereas that for AX,tgives (γX,t /1 − α)i0X (γX,t ) + iK (γK,t ) + iX (γX,t ) = f 0 (µt ). Condition (38) is obtained by subtracting the second
equation from the first. Condition (39) is from adding α times the first equation and (1 − α) times the second
equation.
40

27

γL

Equilibrium Innovation
Possibility Frontier (EIPF)
Eq. with µ2 (> µ 1)
Eq. with µ1µ↑shifts the
combined R&D cond.

γK

γX

µ↑
Combined R&D cond.
in (γK , γX ) space

R&D allocation cond.

Figure 2: Determination of the direction of the technological change. When µt increases, thecombined R&D curve shifts towards the origin, resulting in lower γK and γX and higher γL .
γX,t .41 Therefore, as shown in Figure 2 the locus of (γK,t , γX,t ) that satisfies the combined R&D
condition (39) is downward sloping on the (γK , γX ) plane. The intersection of the R&D allocation
condition and the combined R&D condition gives the optimal rates of innovation for capital- andland-augmenting technologies. We denote the intersection by (bγK (µt ), γbX (µt )), which uniquely
exists for any µt > 0. When µt gets higher, the RHS of (39) declines, which pushes the combined
R&D condition towards the origin. This means that both γ
bK (µt ) and γbX (µt ) are decreasing inµt . Intuitively, when Mt /Nt is higher, there is an abundance of M-tasks relative to N-tasks, which
reduces the incentives to further improve the M-task related technologies. The following lemma
summarizes the results from this subsection.
Proposition 5. (Direction of Technological Change) The representative firm chooses thegrowth factor of each factor-augmenting technology according to (37), (38) and (39). For any given
relative task intensity µt ≡ Mt /Nt > 0, the solution to this system of equations uniquely exists,and given by γbK (µt ), γbX (µt ), and γbL (µt ). In addition, γbK (µt ) < 0 and γbX (µt ) < 0 are decreasingbX (0) = γ X > 1, γbK (∞) = γ K < 1, andcontinuous functions of µt , with γ
bK (0) = γ K > 1, γbL (µt ) is an increasing continuous function with γ
bL (0) = γ L <γbX (∞) = γ X < 1. On the contrary, γ
1 and γbK (∞) = γ L > 1.
The equilibrium point in Figure 2 depicts the direction of the technological progress implied by
Proposition 5 in the 3-dimensional space. As µt changes, the optimal direction moves along the
thick curve, which represents the equilibrium innovation possibility frontier (EIPF). The lemma
41This can be confirmed in the same way as in footnote 39

28

implies that when µt is sufficiently low, the firm chooses a strictly positive rate of capital-augmenting
technological progress (bγK (µt ) > 1).
Note that Mt also represents the amount of capital composite, which combines effective capitaland effective land, whereas Nt is equal to the amount of effective labor. Therefore, µt can be thought
to represent the relative scarcity of effective production factors. In this interpretation, Proposition
5 says that the direction of technological progress is chosen so that it enhances effective factors
which are relatively in short supply. Observe that there is no market size effect or scale effects in
our model, because the rates of innovation depend only on the ratio µ = Mt /Nt . This result comesfrom the assumption that the cost of R&D is proportional to the number of tasks. It is consistent
with the observation that larger firms spend more on R&D while they do not necessarily achieve
higher productivity improvements.
Another point to note is that µt is endogenous and depends on the amount of R&D activities
in period t. Therefore, Proposition 5 should be understood as an equilibrium relationship between
the task intensity and the direction of technological change, rather than causality. To see how they
are determined in each period, we need to consider the equilibrium dynamics.

6.3

Equilibrium Dynamics

The equilibrium path of this economy is given by the sequence of output, consumption, production
factors, technologies, and R&D investments, {Yt , Ct , Kt , Xt , Lt , AK,t , AX,t , AL,t , RK,t , RX,t , RX,t }∞t=0 ,which satisfy the representative firm’s optimization, the representative consumer’s utility maximization, and the market clearance for output and production factors. The economy is endowed with
K0 , X0 and L0 at time 0, as well as the initial levels of publicly available technologies, AK,−1 ,AX,−1and AL,−1 .
While the equilibrium involves many variables, we can analytically characterize its dynamic
path in terms of only three variables: relative task intensity µt = Mt /Nt , the amount of capitalper effective labor kt ≡ Kt /AL,t Lt , and consumption per effective labor ct ≡ Ct /AL,t Lt . Below, weconstruct the equilibrium mapping from {µt , kt , ct } to {µt+1 , kt+1 , ct+1 } for t ≥ 0. The mapping andthe initial conditions µ0 and k0 , together with the transversality condition for ct , will pin down theequilibrium path of {µt , kt , ct }, from which the path of all variables in the model can be recovered.
Before so doing, it is convenient to define the net aggregate output in the economy as Vt =Fb(Mt , Nt ) − RK,t − RX,t − RL,t , which means the aggregate output minus the total R&D costs inthe economy. The net output per effective labor can be written as a function of µt :Vt /Nt = f (µt ) − µt (iK (bγK (µt )) + iX (bγX (µt ))) − iL (bγL (µt )) ≡ v(µt ).

(40)

Then, substituting the profit (27) into the budget constraint (32), we can express the growth of29

aggregate capital supply in terms of µt , kt and ct :Vt + (1 − δ)Kt − Ctv(µt ) − ct
Kt+1==
+ 1 − δ.
Kt
Kt
kt

(41)

Dynamics for µt+1 . The growth factor of µt+1 is defined by µt+1 /µt = (Mt+1 /Mt )/(Nt+1 /Nt ).
By using (21), (22), (28), (31) and (41), its value in equilibrium can be written as(gX γbX (µt+1 ))1−α
µt+1
=
µt
gL γ
bL (µt+1 )γ
bK (µt+1 )v(µt ) − ct
+1−δ
kt

α
,

(42)

where γbK (µt ), γbX (µt ), and γbL (µt ) are the rates of technological innovations defined in Proposition
5. While equation (42) gives a relationship between the period-t variables {µt , kt , ct } and the next
period’s µt+1 , it is not easy to understand how µt+1 is determined since both the LHS and the RHS
depends on µt+1 .
To interpret it intuitively, let us decompose the dynamic relationship in (42) into two steps.
First, we define the pre-R&D relative factor intensity by
µpre
t+1 ≡

α
1−α
α

AK,t Kt+1
AX,t Xt+1
g 1−α v(µt ) − ct
= X
+ 1 − δ µt ,AL,t Lt+1
gL
kt

(43)

where the last equality is from (28), (31), (41) and the definition of µt . It is the value of µt+1 before
technologies are improved from their period-t state. Second, µpret+1 and the post-R&D value of µt+1
is related by the growth of technological levels γ
bK (µt ), γbX (µt ), and γbL (µt ) as follows:
µpre
t+1 =

γ
bL (µt+1 )µt+1 ≡ Γ(µt+1 ).
γbK (µt+1 )α γbX (µt+1 )1−α(44)Note that, Proposition 5 implies that function Γ(µt+1 ) is a strictly increasing function with limµ→0 Γ(µ) =0 and limµ→∞ Γ(µ) = ∞. Therefore, its inverse function µt+1 = Γ(−1) (µpret+1 ) is well-defined for allµpret+1 > 0, and is a strictly increasing function.
Using this inverse function and (43), the dynamic relationship (42) can be written asµt+1 = Γ(−1)

1−α
gX
gL



α !
v(µt ) − ct
+ 1 − δ µt ≡ ψ µ (µt , kt , ct ).
kt

(45)

Equation (45) explains how µt+1 is determined given the period-t variables µt , kt , ct . We write
this mapping as µt+1 = ψ µ (µt , kt , ct ). It provides a natural 2-step interpretation of the equivalentequation (42). The argument of function Γ(−1) (·) in (45) represent the pre-R&D relative task
intensity, which is determined by the relative supply of production factors, as well as the period-t
technology levels. Then, function Γ(−1) (·) describes how R&D in period t+1 transforms the relative
task intensity.

30

Dynamics for kt+1 . From (28) and (41), the growth factor of kt ≡ Kt /AL,t Lt is obtained as
1
kt+1
=
kt
gL γ
bL (µt+1 )
v(µt ) − ct
+1−δ .
kt(46)

While µt+1 is present in the RHS, we can replace it with (45) so that the RHS depend only on thevariables in period t.
kt+1 =1gL γ
bL (ψ µ (µt , kt , ct ))(v(µt ) − ct + (1 − δ)kt ) ≡ ψ k (µt , kt , ct ).

(47)

This dynamic equation simply represents the process of capital accumulation per effective labor.
The expression (v(µt ) − ct + (1 − δ)kt ) shows the sum of the net saving and the undepreciated partof existing capital, per effective labor in period t. It must be divided by gL γ
bL because of the growthof effective labor between period t and t + 1.
Dynamics for ct+1 . The representative household maximizes the intertemporal utility function(30) subject to the budget constraint (32) and the non-Ponzi Game condition (33). The Euler−θ. By substituting the market interest rateequation for this problem is Ct−θ = (rt+1 + 1 − δ)βgLθ Ct+1

(34) into the Euler equation and then applying it to the definition ct ≡ Ct /AL,t Lt , we obtain thegrowth factor of consumption per effective labor:
β 1/θ
ct+1
=
ct
γ
bL (µt+1 )
αµt+1 0f (µt+1 ) − iK (bγK (µt+1 )) − iX (bγX (µt+1 )) + 1 − δ
kt+1

1/θ
.

(48)

By replacing the period-(t + 1) variables in the RHS by (45) and (47), we can rewrite equation (48)as
ct+1 =

β 1/θ ctγbL (ψ µ (µt , kt , ct ))αψ µ (µt , kt , ct )  0 µf (ψ (µt , kt , ct )) − iK (bγK (ψ µ (µt , kt , ct )))ψ k (µt , kt , ct )!1/θµ
−iX (bγX (ψ (µt , kt , ct ))) + 1 − δ≡ ψ c (µt , kt , ct ).

(49)

Equations (45), (47) and (49) constitute the equilibrium mapping from {µt , kt , ct } to {µt+1 , kt+1 , ct+1 }for all t ≥ 0.
Boundary Conditions. To obtain the equilibrium path of {µt , kt , ct }∞t=0 , we need three boundary
conditions. First, since K0 , X0 , L0 , AK,−1 ,AX,−1 and AL,−1 are given, we can construct µpre0 , thepre-R&D relative task intensity for period 0. Using it with the inverse function of Γ from (44), we
have the initial value of µt :µ0 = Γ(−1)

(AK,−1 K0 )α (AX,−1 X0 )1−α
AL,−1 L0
31


.

(50)

Second, using µ0 , the initial value of kt is readily obtained by
k0 =

K0
.
γ
bL (µ0 )AL,−1 L0(51)

Finally, the initial value of ct must be chosen so as to satisfy the non-Ponzi game condition andthe transversality condition, which jointly requires the condition (33) to hold with equality.42 By
using the Euler equation, the terminal condition can be written as
lim

T →∞



βgL γ
bL (µT )1−θTc−θT kT = 0.

(52)

The next subsection will show that the economy has a BGP that satisfies this terminal condition.

6.4

The Balanced Growth Path

Now we are ready to derive the balanced growth path (BGP) of this economy. It will be shown thatthe direction of technological progress, including strictly positive capital-augmenting technologicalprogress, is endogenously chosen so that the BGP is achieved in equilibrium. Let us start bydefining the BGP in this economy and then showing that the three key variables are constant onit.
Lemma 3. Define a BGP as an equilibrium path where the growth factors of {Yt , Kt , Xt , Lt , Ct ,Rt , Mt , Nt } are all constant.43 Then, on any BGP, the values of µt , kt and ct must be constant.
Proof. See Appendix Section A.1.
Let us denote the BGP values of µt , kt and ct by µ∗ , k ∗ and c∗ , respectively. Note that the
RHS of (42), (46) and (48) represent the growth factors of µt , kt and ct , and therefore they takethe value of 1 on the BGP. Rearranging them, we obtain the values of µ∗ , k ∗ and c∗ as follows.
First, from (42) and (46), the BGP value of µt ≡ Mt /Nt will satisfy(gX γ
bX (µ∗ ))1−α (bγK (µ∗ ))α = (gL γbL (µ∗ ))1−α .

(53)

We can confirm that there exist a unique value of µ∗ > 0 that satisfies condition (53). Since
0 (µ∗ ) < 0, γ
0 (µ∗ ) < 0 and γγbKbX
bL0 (µ∗ ) > 0 from Proposition 5, the LHS is strictly downward slopingwith respect to µ∗ , while the RHS is strictly upward sloping. This guarantees the uniqueness ofµ∗ . An intuitive way to interpret (53) is to multiply the both of its sides by (gL γbL (µ∗ ))α .
(gX γ
bX (µ∗ ))1−α (bγK (µ∗ )gL γ
bL (µ∗ ))α = gL γ
bL (µ∗ ) = g ∗ .
42

(54)

The transversality condition from the household’s maximization problem takes a similar form to (33) but has“≤” instead of “≥”.
43
Here we slightly extend Definition 2 by requiring constancy of the growth factors of Mt and Nt , i.e., the numbersof tasks performed in the economy.

32

The LHS represents the growth factor of Mt on the BGP, while the RHS is that for Nt . Therefore,
this condition means that the relative factor intensity µ∗ = Mt /Nt is determined so that Mt and
Nt grow at the same speed on the Equilibrium Innovation Possibility Frontier (EIPF), as depicted
in Figure 2. Because of the CRS property of production function Yt = Fb(Mt , Nt ), the value ofequation (53) also represents the the economic growth factor g ∗ ≡ Yt+1 /Yt .
Second, from (48), the BGP value of kt = Kt /(At Lt ) isk∗ =βαµ∗ (f 0 (µ∗ ) − iK (bγK (µ∗ )) − iX (bγX (µ∗ ))),γbL (µ∗ )θ − β(1 − δ)(55)

where µ∗ is obtained from (53). Intuitively, (55) means that the capital-effective labor ratio on the
BGP is determined so that the interest rate r∗ implies constant consumption per effective labor,c∗ .44 Third, from the resource constraint (46), the BGP value of c∗ = Ct /AL,t Lt must satisfyc∗ = v(µ∗ ) + (1 − δ)k ∗ − gL γ
bL (µ∗ )k ∗ ,(56)

where µ∗ and k ∗ are given by (53) and (55). These three equations uniquely give the BGP in thiseconomy.
Proposition 6. There exists a unique BGP that satisfy µ∗ > 0, k ∗ > 0, c∗ > 0, and the terminal
condition (52).
Note that (53) is equivalent to the technology condition (20) in Section 5.3, except that now the
speeds of technological progress are endogenously determined. This difference has an important
implication for the plausibility of capital-augmenting technological progress on the BGP. In theexample model in Section 5.3, the rates of innovation for three factor-augmenting technologies are
exogenously given, and they must happen to satisfy a knife-edge condition (20). In contrast, in thefull model of this section, the representative firm chooses the rates of technological progress so as
to maximize its profit.
This balanced growth path accommodates the two previously-conflicting stylized facts: (i) a lessthan-unitary elasticity of substitution between labor and capital, and (ii) strictly positive capitalaugmenting technological change in the long run. To demonstrate this result, section 7 explores themodel numerically. The primary benefit of the exercise is to show that the endogenous growth model
has a globally stable BGP under a range of parameter values. Regardless of the initial combinationof technologies and endowments of production factors, the economy converges to the BGP with apositive rate of capital-augmenting technical change, where condition (20) is endogenously satisfied.
It will also be shown that the model can be readily calibrated to standard macroeconomic facts
describing the BGP of the United States.
Using (34), condition (55) is shown to be equivalent to the Euler equation on the BGP, γbL (µ∗ ) = β(r∗ + 1 − δ).
The LHS means the growth of consumption per capita (when consumption per effective labor is constant).
44

33

7

Numerical Analysis and Stability

The multi-factor Uzawa steady state growth theorem presented in Proposition 1 and 3 requires thatthe growth rates of factor-augmenting technologies conform to a particular log-linear relationshipas in equation (20). Section 6 demonstrates that this log-linear relationship can be an endogenous
outcome of a model with directed technical change, suggesting that this requirement is not actually
restrictive. The analytic results, however, did not demonstrate that the economy will necessarily
converge to the BGP with capital-augmenting technical change.
In this section, therefore, we investigate the local and global stability of the three factor endogenous growth model. To do so, we present a series of numerical examples for which we can check
both local and global stability computationally. Whenever possible, we ensure that our numericalexamples are consistent with macroeconomic data characterizing the balanced growth path of theUnited States. We stress, however, that this is not a complete calibration, and the results would beinsufficient for a complete quantitative analysis. Our contribution is theoretical, and the numericalresults are meant to be informative about stability. As discussed in section 8, future empiricaland quantitative work to fully pin down the parameters of such a growth model is a particularly
promising area for future work.

7.1
7.1.1

Calibration
Preliminaries

In this section, we describe the partial calibration procedure that we use to discipline our numericalexamples. We start by making some important assumptions. In line with Section 6, we assume
that there is some aggregate variable X that incorporates all factors of production other than laborand capital.45 For ease of exposition, we call this factor land.
Next, we need to make two important functional form assumptions. We start by assuming thatthe aggregate production function (23) is CES,

Yt =

α

η (AK K) (AX X)

1−α

 −1


+ (1 − η)(AL L)

−1


 

1−

,

 > 0,

0 < η < 1.

(57)

Next, we assume that the cost function for R&D (25) takes the form46λiZ (γZ ) = ζZ γZ − 1 ,45ζZ > 0,λ > 1,(58)

This is done for transparency and tractability. The theorem only requires that any one factor non-reproducible
factor is combined with capital with a unit-elastic elasticity of substitution.
46
As in (24), R&D cost function (58) can also be represented in terms of R&D production function: γ Z = 1 andΦ(iZ ) = (iZ /ζZ )1/λ . This Φ(iZ ) function satisfies the required conditions: Φ0Z (0) = ∞, Φ0Z (iZ ) > 0 and Φ00Z (iZ ) < 0
for iZ > 0 and ΦK (∞) = ∞.

34

for Z = K, X, and L. In words, we assume that the cost of R&D investment differs across typesof technology due to the ζZ parameters, but they have the same degree of the convexity λ.
Finally, we need to decide on a period length. In our endogenous growth model, one period
captures the length of time in which a firm can monopolize its R&D outcomes, and, equivalently, thehorizon for firm decision-making. We take a period length of 10 years as a reasonable approximation.
7.1.2

External Calibration

We set several parameters exogenously based on existing literature. To start, we assume log preferences as in Golosov et al. (2014), who use an identical period length. We also set the elasticity of
substitution between labor and the capital composite exogenously. Existing studies generally estimate this parameter assuming that there are only two factors of production, reproducible capitaland labor. Thus, any existing match from the literature to our model is imperfect. We stress againthat this is meant to be an illustrative example demonstrating that the stability of the three-factor
model for a wide range of parameters. For our baseline analysis, we take  = 0.8 from Oberfield andRaval (2014), which is similar to estimates by Antras et al. (2004), Klump et al. (2007), Herrendorf
et al. (2015) and Alvarez-Cuadrado et al. (2018). Karabarbounis and Neiman (2014) and Piketty(2014) estimate elasticities that are greater than one.
In the next section, we describe how the remaining parameters are calibrated. We are notable to pin down λ, the degree of diminishing marginal returns in R&D. Intuitively, λ governs thediminishing returns to reallocating research inputs between different types of technology. However,research allocations are stable on the balanced growth path, making it difficult to pin down thisparameter. In the baseline, we take λ = 2, so that the cost function is quadratic. We also showthat our stability results do not depend on this choice.
7.1.3

BGP Moments

To calibrate the remaining parameters, we match the model to balanced growth from the UnitedStates.47 Average annual growth in GDP per capita in the United States from 1950-2014 was
2.0% year (γL∗ = 1.22), population growth was 1.1%/year (gL = 1.12), and the relative price of∗ = 1.06). The most obvious interpretation of X is that is ainvestment goods fell by 0.6%/year (γKa land-energy composite. Interpreting X as land would suggest that it is fixed over time (gX = 1),
which we will take as the baseline.
We use the notation κZ to denote the income share of factor Z. In our model, output net
of R&D spending is fully divided between labor, reproducible capital, and land. The remainderof gross output would show up as profits in the national accounts. In equilibrium, this portionof gross output is exactly equal to the R&D share of expenditure of gross output. The average47

See Appendix Section A.2 for details regarding the data used in the calibration.

35

Table 2: Calibration
Parameter
λ
gX

θ
gL
β
δ
α
η
ζL
ζK
ζX

Baseline
2.0
1.0
0.8
1.00
1.21
0.88
0.38
0.69
0.19
0.53
9.35
9.28

Description
R&D Convexity
Growth of X
Elast. Sub.
IES
Population growth
Discount factor
Depreciation
Cap. Distribution
CES Distribution
Research efficiency
Research efficiency
Research efficiency

SourceFree Parameter
Definition of X
Oberfield and Raval (2014)
Golosov et al. (2014)
BEA
Calibrated
Calibrated
Calibrated
Calibrated
Calibrated
Calibrated
Calibrated

labor share of income over this period was approximately κ∗L = 65% (Jones, 2016), and based on
Table 1 we take κ∗X = 10%, as it is a stand in for all forms of natural capital. Moreover, datafrom the BEA suggests that the R&D share of expenditure is 2.7% of GDP on average. This
immediately implies that κ∗K = 22.3%. Now, the cobb-douglas structure of the capital composite
yields α = κ∗K /(κ∗K + κ∗X ) = 0.69.
The annual capital-output ratio in the United States is 2.90, when measuring capital in terms of
final output as in our model. With our ten year time scale, we target a value of 0.29. Consumptionof fixed capital in the United States is approximately 11% of GDP (Rognlie, 2016). Thus, δ =(100/29) · .11 = 0.38. Similarly, with κ∗K = 22.3%, r∗ = (100/29) · 22.3 = 0.77. Putting thesetogether yields a real rate of return equal to R∗ = 1 + r∗ − δ = 1.39, which corresponds to anannual rate of return of 3.3%. With log preferences, β = γL /R∗ = 0.88.
Noting that we set λ exogenously – and test for robustness – the remaining unknown parameters
are given by the set {ζK , ζX , ζL , η}. To find these parameters, we perform an ad hoc search over the
relevant parameter space. In addition to matching the moments discussed above, we also target the
non-R&D investment rate of 21%. In the future, this will be updated to a more formal procedure.
The results are presented in Table 2.
As demonstrated in Table 3, the ad hoc calibration is able to match several key moments fromthe data.48 . As noted above, our goal is not to provide a complete quantitative model of the U.S.
economy. Still, the fact that the model is able to recreate patterns observed in data suggests that
it will be useful as a quantitative tool in future work.49
48

A more formal calibration will be added soon
In terms of non-targeted moments, it is worth noting that we predict an R&D share of output of 6.7% which ishigher than the standard measure of R&D expenditure, which makes up approximately 2.7% of GDP. This result is
not surprising. Given that our model is attributing all changes in productivity to R&D, it should incorporate moretypes of investment than business investment (e.g., education expenditures).
49

36

Table 3: Moments
Moment
γL∗
∗
γK
κ∗L
κ∗X
K/Y
R/Y
s∗

7.2
7.2.1

Description
Y /L growth
AK growthLabor share
Land Share
Capital-Output
R&D-Output
Savings Rate

Model
1.21
1.12
0.61
0.10
0.29
0.07
0.21

Data
1.21
1.06
0.65
0.10
0.29
0.03
0.21

Stability
Local Stability

Next, we turn to turn to examining the stability of the numerical model. As a preliminary analysis,
we hold fixed all the value from Table 2, except for λ. We then solve for the steady state fordifferent values of λ and numerically find the eigenvalues of the system. In the future, this process
will be updated to test a much wider range of parameters. Based on other initial analyses (notreported), we find evidence that the system is stable for a wide range of parameter values.
Table 4: Stability
λ
1.25
1.5
2
3
4

Eigenvalues
0.92, 0.57, 2.26
0.91, 0.48, 2.46
0.92, 0.39, 2.96
0.94, 0.27, 4.22
0.95, 0.19, 6.03

Stability
SaddleSaddleSaddleSaddleSaddleTable 4 shows that the numerical models is locally saddle-path stable for all the different
parameter values of λ. In the three-variable dynamics of (45), (47) and (49), variable µt and
kt are pre-determined state variables, and ct is a control variable. Because the model is in discretetime, the saddle stability requires two of the eigenvalues to be greater than one, and one of them tobe less than one, both in absolute values. Table 4 confirms this for all the configurations presented.
Thus, once the economy come sufficiently close to the balanced growth path, it converges to the
BGP, where capital-augmenting technical change is present in the long-run.
7.2.2

Global Stability

the log-linear condition on technology is not restrictive, we now turn to examining global stability.
Given time and space constraints, this preliminary analysis examines stability for a wide range ofstarting values for the state variables, µt and kt , but only a single set of parameters. Specifically,37

we examine local stability for the baseline calibration parameters reported in Table 2.

c
0.8

k

0.3

0.2

0.1

0.0
c
0.8

0.6

0.6
0.4

0.2

0.0

0.4

0.3
0.2
0.1

k

0.2
1.0
0.5

1.0

1.5

0.0

0.0

Figure 3: Global Stability. This figure shows the dynamic paths of the model starting from a wide range of valuesof the two state variables µt and kt . In particular, the we test starting values for the state variables that are up to
two times the steady state values. The fact the model converges to the steady state from such a wide range of ofstarting values strongly suggests that the steady state with positive capital-augmenting technical change is globally
saddle-path stable. The figure shows the three-dimensional space from two angles in order to give a clear picture of
the saddle paths.

To solve for the global dynamics, we use a forward shooting algorithm. There is only one control
variable, ct in the system. Since we already demonstrated that the model is locally saddle-path
stable, we know that there is only one value of ct that leads to convergence to the BGP, for any
given starting values of kt and µt . Once this value is known, it is straightforward to solve for all of
the subsequent values of all three variables in the normalized system. Thus, we can solve for the
global dynamics by iterating over guesses for ct and updating our guesses via a bisection algorithm.
We test for global stability from a wide range of parameters. We set up a two-dimensional gridfor the starting values of kt and µt , testing values up to twice the steady state value. Then, we findthe global dynamics starting from each point on the outer edge of this grid. Figure 3 presents theresults. We show the three-dimensional figure from two angles to better illuminate the transition
paths. The figures show that the economy converges to the BGP, even when they have started
from initial states that are far away from the BGP. This means that the economy endogenously
converges to the state where BGP condition (53) is satisfied. This result demonstrates that the
log-linear relationship between technological growth rates (20), which is equivalent t o (53) in our
endogenous growth model, can be viewed as an endogenous outcome and does not pose an extra
38restriction on the conditions needed for balanced growth in neoclassical models with improvementsin the productivity of capital.
The simulation results also illustrate interesting properties of the 3-factor endogenous directed
technological change model. For example, the figure demonstrates that µt converges much slower
than kt , a result that is consistent with the eigenvalues reported in Table 4. This result suggeststhat ignoring land and endogenous technical change can lead neoclassical models to overestimate
the speed of convergence to the steady state.

8

Discussion

In this section, we discuss the implications of the model, as well as some important goals for future
research.

8.1

Implications of the analysis

The multi-factor Uzawa steady state theorem and the three-factor endogenous growth model suggestthat non-accumulable factors, such as land and natural resources, and directed technical change arefundamental to understanding the evolution of the economy. There is, of course, a long literature
on both of these topics, but they are generally only included in growth models to achieve specificaims. For example, energy is generally only included in growth models when studying the depletionof finite resources (e.g., Hotelling, 1931; Heal, 1976) or climate change (e.g., Golosov et al., 2014;
Barrage, 2019). Directed technical change is often used to understand the evolution of relative
wages between different groups of workers (e.g., Acemoglu, 1998, 2002; Acemoglu and Restrepo,2016).
Our results suggests a much broader importance of directed technical change and non-accumulable
factors — these factors must be incorporated into models of economic growth in order to recreate the balanced growth facts that originally motivated aggregate growth modelling (Solow, 1956,
1994). This suggests that essentially all analyses in economic growth would benefit from including
land (or natural resources) and directed technical change if they want to be consistent with balanced growth. We think that our three-factor endogenous growth model presents a natural starting
point for such analyses.50 In this way, our results are related to those of Unified Growth Theory,
which suggests that including land in the production function is essential of understanding long-run
economic growth and the transition from Malthusian stagnation to modern growth (e.g., Galor andWeil, 2000; Hansen and Prescott, 2002; Galor, 2011). Also, Caselli and Feyrer (2007) suggest that
accounting for natural resource well is necessary for understanding cross-country dispersion in themarginal product of capital.
50
Existing work on directed technical change and the environment may also be helpful in this regard (e.g, Smulders
and De Nooij, 2003; Acemoglu et al., 2012; Hassler et al., 2016).

39

8.2

Next Steps

Estimating σKXj – The obvious next step for future research would be to determine whether
there is some factor, Xj , that has a unit-elastic elasticity of substitution with reproducible capital.
In our illustrative growth model, we assumed that this third factor was an amalgamation of all
factors not included in reproducible capital or labor. Proposition 3, however, only requires that asingle factor of production has this property. Thus, it is necessary to test many different factors to
see if they satisfy this property.51 The obvious candidates would be land, energy, and materials,
but it could also be any subcategory of these types of inputs. Given the vast array of choices andthe limited restrictions created by the theory, it would be safe to say that there is some possibility
that such a factor exists.
What if there is no Xj with σKXj = 1? — Despite the wide range of factors used in production,
it is possible that no factor has unit-elastic substitution with reproducible capital. In this case, ouranalysis suggests that it is not possible to write down a multi-factor growth neoclassical growthmodel with capital-augmenting technical change, which is clearly observed from data.
If the result of the Uzawa theorem does not fit the data, we need to question whether theassumptions of the theorem hold in reality. A remarkable property of the Uzawa theorem is thatit depends on very few assumption: (1) the economy can be expressed by a neoclassical growthmodel, and (2) there is a balanced growth path. The NIPA data strongly suggests the existence ofthe balanced growth path. Therefore, we can narrow down the concern to the assumption in the
neoclassical growth model, as given in Definition 1.
The definition consists of two parts, aggregate production function Yt = F (Kt , X1,t , ..., XJ,t ; t)and resource constraint Kt+1 = Yt −Ct −Rt +(1−δ)Kt . In Uzawa theorem, the latter is only utilized
in Lemma 1, which showed that the K/Y ratio must be constant in the BGP. The result of thislemma is clearly visible in the NIPA data depicted by Figure 1, where Y and K grows at the samerate, confirming the statement that “capital inherits the trend of output” (Jones and Scrimgeour,2008). Therefore, as long as an economic analysis uses aggregate output Y and aggregated capitalK, as defined in NIPA, this resource constraint seems to do no additional harm.52
The remaining suspect, then, is the aggregate production function Yt = F (Kt , X1,t , ..., XJ,t ; t).
It assumes that the there is a mapping from aggregated factor inputs to the aggregate output.
While vast majority of all macroeconomic models use some form of aggregate production function,
it is neither a weak assumption nor a fully proven property. For example, Figure 1 shows that the
movement of the relative price of capital depends the type of capital (e.g., equipment and structure).
51

While, of course, avoiding the pitfalls of multiple hypothesis testing.
Another concern with the resource constraint might be that it assumes a one-to-one conversion between Ct andKt+1 . However, as discussed in Section 3, if the conversion rate changes over time, we can redefine the units of capitalso Definition 1 still applies (see equation 3). In this sense, it does not pose a real restriction.
52

40

Even within equipment category, the relative price of different capital goods changes dramaticallyover time. The same can be said for different kinds of output. The aggregate production function
implicitly assumes that capital and output can be aggregated and there are stable relationshipbetween these aggregates. If there is no Xj with σKXj = 1, it might suggest that the aggregation
created the problem. Exploring disaggregated models seems important in this respect.

8.3

Applications for a multi-factor model

The Uzawa steady state theorem is not a mere theoretical curiosity. It has important implications for understanding the economy. If it is possible to identify a factors that has a unit-elastic
substitution with reproducible capital, then aggregate productions can include both labor- andcapital-augmenting technical change on the balanced growth path, allowing for more informative
analysis of data and policy interventions.
Most obviously, the counter-factual notion that there is a single type of technology places
severe restrictions on our ability to describe the economy, even in an aggregate sense. In growthaccounting, there has been a great deal of work focusing on the role of investment specific technicalchange. To conform with the restrictions of the Uzawa theorem, the production functions in suchanalyses need must focus on the Cobb-Douglas case (e.g., Greenwood et al., 1997; Krusell, 1998;
Cummins and Violante, 2002) or be inconsistent with balanced growth (e.g., Krusell et al., 2000).53
Incorporating additional factors of production would allow such exercises to be conducted in a
way that is consistent both with evidence on balanced growth and evidence on the elasticity of
substitution between capital and labor. The same would be true business-cycle analyses, which arealso restricted to the Cobb-Douglas case when trying to understand the importance of investmentspecific technical change (e.g., Greenwood et al., 2000; Fisher, 2006; DiCecio, 2009). Development
accounting could also benefit from models that allow for separate contributions from labor- andcapital-augmenting technical change (Hsieh and Klenow, 2010; Schoellman, 2011). Work by Caselli
and Feyrer (2007) also demonstrates the importance of including natural resources in development
accounting exercises.
Separate from the accounting literate, models of economic growth with capital-augmenting
technical change would be useful in the study of automation, artificial intelligence, and laborsaving technical change, topics that have recently received a great deal of attention in the growth
literature (e.g., Benzell et al., 2015; Hémous and Olsen, 2016; Brynjolfsson and McAfee, 2012, 2014).
An outstanding question in this literature is how to best model the impact of technology on labormarket outcomes (Acemoglu and Autor, 2011; Acemoglu and Restrepo, 2016, 2018). Models with
only one type of technology in the long run – e.g., those constrained the Uzawa (1961) theorem –53

See, He et al. (2008) and Maliar and Maliar (2011) for discussions of how to make these models consistent withbalanced growth.

41

are naturally of limited use when trying to determine when technological change is worker-friendly
and when it is not. In standard neoclassical production functions, capital- and labor-augmenting
technical change have different impacts on the labor share of income. With low elasticities ofsubstitution, positive shocks to labor- and capital-augmenting technologies also have opposite effects
on the levels of wages and rental rates (Acemoglu, 2010; Acemoglu and Restrepo, 2018). Thus,
including both capital- and labor-augmenting technical greatly increases the usefulness of thesemodels in studying impacts of new technologies.54

9Conclusion

The Uzawa (1961) steady state theorem has long posed significant problems for neoclassical models
of balanced growth (Jones and Scrimgeour, 2008). As conventionally understood, the theorem
states that all technological change must be labor-augmenting on a balanced growth path, unlessthe production function is Cobb-Douglas. This constraint makes it impossible for the neoclassicalmodel to be consistent with data indicating that (i) the two-factor aggregate production function is
not Cobb-Douglas and (ii) the relative price of capital goods has been declining. This is a significant
limitation. Substitution between capital and labor and technological progress are the key forcesthat generate balanced growth in the standard model (see, e.g., Solow, 1956, 1994).
To provide insight into this apparent contradiction, we prove a multi-factor version of the
Uzawa (1961) steady state theorem. The extended version of the theorem says that, on a balanced
growth path, production can be represented by a neoclassical production function without capitalaugmenting technical change. As in the standard Uzawa theorem, this does not imply that thetrue production function does not have capital-augmenting technical change. More precisely, thetheorem does not rule out the existence of multiple different representations of aggregate production
along the balanced growth path. Thus, we also examine the circumstances under which there isa representation of the aggregate production function with capital-augmenting technical change.
We prove that, with more than two factors of production, neoclassical production functions canaccommodate capital-augmenting technical change, as long as there is a single factor than has aunit-elastic elasticity of substitution with reproducible capital.
This suggests a natural way to resolve the contradiction affecting standard models: add morefactors of production. In reality, there are many factors of production besides labor and reproducible
capital. If any of these has a Cobb-Douglas relationship with capital, then multi-factor neoclassical54

On a related note, much of the literature on automation has argued that factor-augmenting technologies are an
ineffective way of modelling labor-saving technical change. Instead, such papers argue for using a task-based model
along the lines of Zeira (1998) (e.g., Acemoglu and Autor, 2011; Acemoglu and Restrepo, 2016, 2018). However, as
discussed in Acemoglu and Restrepo (2016), task-based growth models are still subject to the Uzawa theorem, aslong as they can be represented by an aggregate production function. Indeed, Propositions 1 and 3 imply that thesemodels have Uzawa and Factor-Augmenting representations on the BGP. To the best of our knowledge, no task-basedmodel of endogenous growth has capital-augmenting technical change on the BGP.

42

models can be consistent with the data. We also build a three-factor endogenous growth modeland show that is converges to a globally-stable BGP with a positive rate of capital-augmenting
technical change.
We strongly suggest that future empirical work investigate whether there is a factor of production that is Cobb-Douglas with reproducible capital. If such a factor can be identified, thentractable aggregate models can be made more consistent with data and will be better able to study
a wide range of economic phenomena.

References
Acemoglu, D. (1998). Why Do New Technologies Complement Skills? Directed Technical Change
and Wage Inequality. The Quarterly Journal of Economics, 113(4):1055–1089.
Acemoglu, D. (2002). Directed technical change. Review of Economic Studies, 69(4):781–809.
Acemoglu, D. (2003). Labor-and capital-augmenting technical change. Journal of the European
Economic Association, 1(1):1–37.
Acemoglu, D. (2008). Introduction to Modern Economic Growth. Princeton University Press.
Acemoglu, D. (2010). When does labor scarcity encourage innovation? Journal of Political Economy, 118(6):1037–1078.
Acemoglu, D., Aghion, P., Bursztyn, L., and Hemous, D. (2012). The environment and directed
technical change. American Economic Review, 102(1):131–166.
Acemoglu, D. and Autor, D. (2011). Skills, tasks and technologies: Implications for employment
and earnings. Handbook of Labor Economics, 4:1043–1171.
Acemoglu, D. and Restrepo, P. (2016). The race between machine and man: Implications oftechnology for growth, factor shares and employment. NBER Working Paper, 22252.
Acemoglu, D. and Restrepo, P. (2018). Artificial intelligence, automation and work. NBER Working
Paper, 24196.
Ahlfeldt, G. and McMillen, D. (2014). New Estimates of the Elasticity of Substitution of Land for
Capital. ERSA conference papers ersa14p108, European Regional Science Association.
Alvarez-Cuadrado, F., Van Long, N., and Poschke, M. (2018). Capital-labor substitution, structuralchange and the labor income share. Journal of Economic Dynamics and Control, 87:206–231.
Antras, P. et al. (2004). Is the us aggregate production function cobb-douglas? new estimates ofthe elasticity of substitution. Contributions to Macroeconomics, 4(1):1–34.
43

Ashraf, Q. and Galor, O. (2011). Dynamics and stagnation in the malthusian epoch. American
Economic Review, 101(5):2003–2041.
Barrage, L. (2019). Optimal dynamic carbon taxes in a climate-economy model with distortionary
fiscal policy. Forthcoming in Review of Economic Studies.
Barro, R. and Sala-i Martin, X. (2004). Economic Growth. MIT Press, 2nd edition.
Benzell, S. G., Kotlikoff, L. J., LaGarda, G., and Sachs, J. D. (2015). Robots are us: Someeconomics of human replacement. NBER Working Paper, 20941.
Brynjolfsson, E. and McAfee, A. (2012). Race against the Machine: How the Digital Revolution
is Accelerating innovation, Driving Productivity, and Irreversibly Transforming Employment andthe Economy. Digital Frontier Press.
Brynjolfsson, E. and McAfee, A. (2014). The second machine age: Work, Progress, and Prosperity
in a Time of Brilliant Technologies. WW Norton & Company.
Caselli, F. and Feyrer, J. (2007). The marginal product of capital. The Quarterly Journal of
Economics, 122(2):535–568.
Chirinko, R. S. (2008). σ: The long and short of it. Journal of Macroeconomics, 30(2):671–686.
Chirinko, R. S., Fazzari, S. M., and Meyer, A. P. (2011). A new approach to estimating production
function parameters: The elusive capital–labor substitution elasticity. Journal of Business &Economic Statistics, 29(4):587–594.
Cummins, J. G. and Violante, G. L. (2002). Investment-specific technical change in the united states(1947–2000): Measurement and macroeconomic consequences. Review of Economic Dynamics,
5(2):243–284.
DiCecio, R. (2009). Sticky wages and sectoral labor comovement. Journal of Economic Dynamics
and Control, 33(3):538–553.
Epple, D., Gordon, B., and Sieg, H. (2010). A new approach to estimating the production function
for housing. American Economic Review, 100(3):905–924.
Fisher, J. D. (2006). The dynamic effects of neutral and investment-specific technology shocks.
Journal of Political Economy, 114(3):413–451.
Galor, O. (2011). Unified Growth Theory. Princeton University Press.
Galor, O. and Weil, D. N. (2000). Population, technology, and growth: From malthusian stagnation
to the demographic transition and beyond. American Economic Review, 90(4):806–828.
44

Golosov, M., Hassler, J., Krusell, P., and Tsyvinski, A. (2014). Optimal taxes on fossil fuel ingeneral equilibrium. Econometrica, 82(1):41–88.
Greenwood, J., Hercowitz, Z., and Krusell, P. (1997). Long-run implications of investment-specifictechnological change. American Economic Review, 87(3):342–362.
Greenwood, J., Hercowitz, Z., and Krusell, P. (2000). The role of investment-specific technologicalchange in the business cycle. European Economic Review, 44(1):91–115.
Grossman, G. M., Helpman, E., Oberfield, E., and Sampson, T. (2017). Balanced growth despiteuzawa. American Economic Review, 107(4):1293–1312.
Hansen, G. D. and Prescott, E. C. (2002).

Malthus to solow.

American Economic Review,

92(4):1205–1217.
Hassler, J., Krusell, P., and Olovsson, C. (2016). Directed technical change as a response tonatural-resource scarcity. Working Paper.
He, H., Liu, Z., et al. (2008). Investment-specific technological change, skill accumulation, andwage inequality. Review of Economic Dynamics, 11(2):314–334.
Heal, G. (1976). The relationship between price and extraction cost for a resource with a backstop
technology. The Bell Journal of Economics, 7(2):371–378.
Hémous, D. and Olsen, M. (2016). The rise of the machines: Automation, horizontal innovationand income inequality. Working Paper.
Herrendorf, B., Herrington, C., and Valentinyi, A. (2015). Sectoral technology and structuraltransformation. American Economic Journal: Macroeconomics, 7(4):104–133.
Horii, R. and Iwaisako, T. (2007). Economic growth with imperfect protection of intellectual
property rights. Journal of Economics, 90(1):45–85.
Hotelling, H. (1931). The economics of exhaustible resources. Journal of Political Economy,
39(2):137–175.
Hsieh, C.-T. and Klenow, P. J. (2010). Development accounting. American Economic Journal:
Macroeconomics, 2(1):207–223.
Irmen, A. (2016). A generalized steady-state growth theorem. Macroeconomic Dynamics, 22(4):779–
804.
Irmen, A. (2017). Capital-and labor-saving technical change in an aging economy. International
Economic Review, 58(1):261–285.
45

Irmen, A. and Tabaković, A. (2017). Endogenous capital-and labor-augmenting technical change
in the neoclassical growth model. Journal of Economic Theory, 170:346–384.
Jones, C. I. (2005). The shape of production functions and the direction of technical change.
Quarterly Journal of Economics, 120(2):517–549.
Jones, C. I. (2016). The facts of economic growth. Handbook of Macroeconomics, 2:3–69.
Jones, C. I. and Romer, P. M. (2010). The new kaldor facts: Ideas, institutions, population, andhuman capital. American Economic Journal: Macroeconomics, 2(1):224–245.
Jones, C. I. and Scrimgeour, D. (2008). A new proof of uzawa’s steady-state growth theorem.
Review of Economics and Statistics, 90(1):180–182.
Kaldor, N. (1961). Capital accumulation and economic growth. In The Theory of Capital, pages
177–222. Springer.
Karabarbounis, L. and Neiman, B. (2014). The global decline of the labor share. The Quarterly
Journal of Economics, 129(1):61–103.
Klump, R., McAdam, P., and Willman, A. (2007). Factor substitution and factor-augmenting
technical progress in the united states: a normalized supply-side system approach. The Review
of Economics and Statistics, 89(1):183–192.
Krusell, P. (1998). Investment-specific r and d and the decline in the relative price of capital.
Journal of Economic Growth, 3(2):131–141.
Krusell, P., Ohanian, L. E., Rı́os-Rull, J.-V., and Violante, G. L. (2000). Capital-skill complementarity and inequality: A macroeconomic analysis. Econometrica, 68(5):1029–1053.
Leon-Ledesma, M. A. and Satchi, M. (2019). Appropriate technology and balanced growth. The
Review of Economic Studies, 86(2):807–835.
Li, D., Huang, J., et al. (2016). A variant of uzawassteady-state theorem in a malthusian model.
Economics Bulletin, 36(3):1371–1375.
Maliar, L. and Maliar, S. (2011). Capital–skill complementarity and balanced growth. Economica,
78(310):240–259.
Mas-Colell, A., Whinston, M. D., Green, J. R., et al. (1995). Microeconomic Theory. Oxford
University Press.
Oberfield, E. and Raval, D. (2014). Micro data and macro technology. NBER Working Paper,
20452.
46

Papell, D. H. and Prodan, R. (2014). Long run time series tests of constant steady-state growth.
Economic Modelling, 42:464–474.
Piketty, T. (2014). Capital in the 21st Century. Harvard University Press.
Rognlie, M. (2016). Deciphering the fall and rise in the net capital share: Accumulation or scarcity?
Brookings papers on economic activity, 2015(1):1–69.
Schlicht, E. (2006). A variant of uzawa’s theorem. Economics Bulletin, 5(6):1–5.
Schoellman, T. (2011). Education quality and development accounting. The Review of Economic
Studies, 79(1):388–417.
Smulders, S. and De Nooij, M. (2003). The impact of energy conservation on technology andeconomic growth. Resource and Energy Economics, 25(1):59–79.
Solow, R. M. (1956). A contribution to the theory of economic growth. The Quarterly Journal of
Economics, 70(1):65–94.
Solow, R. M. (1994). Perspectives on growth theory. Journal of Economic Perspectives, 8(1):45–54.
Swan, T. W. (1956). Economic growth and capital accumulation. Economic Record, 32(2):334–361.
Uzawa, H. (1961). Neutral inventions and the stability of growth equilibrium. Review of Economic
Studies, 28(2):117–124.
Valentinyi, A. and Herrendorf, B. (2008). Measuring factor income shares at the sectoral level.
Review of Economic Dynamics, 11(4):820–835.
Zeira, J. (1998). Workers, machines, and economic growth. The Quarterly Journal of Economics,
113(4):1091–1117.

47

A

Appendix

A.1

ProofsA.1.1

Proposition 2

Proof. Note that the share of factor Zt ∈ {Kt , X1,t , ..., XJ,t } is the same in period t and 0. Because
the production function in period 0 is F (·; 0) ≡ Fe(·),
sZ = FZ (Kt , X1,t , ..., XJ,t ; t)Zt
Z0
= FeZ (K0 , X1,0 , ..., XJ,0 ) ,
Yt
Y0

(A.1)

where FeZ (·) represents the derivative of function Fe(·) with respect to its argument (see footnote15). Note that, since function Fe(·) has constant returns to scale, its partial derivative function
FeZ (·) must be homogeneous of degree 0 (See Theorem M.B.1 in Mas-Colell et al., 1995). Therefore,
the value of FeZ (·) will be unchanged when all of its arguments are multiplied to the same proportion
eXj,t Xj,t /Xj,0 (Here we used the stationarity of Kt /Yt from Lemma 1).
g t = Yt /Y0 = Kt /K0 = A
Also, because the effective amount of production factors and the output grow at the same speed,eZ,t Zt /Yt holds on the BGP (In the case of Zt = Kt , we define A
eK,t ≡ 1). Therefore,
Z0 /Y0 = A
eX1,t X1,t , ..., A
eXJ,t XJ,t )AeZ,tsZ = FeZ (Kt , A

eX1,t X1,t , ..., A
eXJ,t XJ,t ) Zt
∂ Fe(Kt , A
Zt
=
,
Yt
∂Zt
Yt

where the validity of the second equality is guaranteed by the chain rule (See footnote 16). By
comparing the latter with (A.1), we obtain (6).
A.1.2

Lemma 2

Proof. If j ∗ = 0, α = 0 follows. Then, the lemma becomes trivial, because the RHS of (10) is
simply the original F (·; 0) function. Therefore, we focus on the case of j ∗ ≥ 1.
Let us consider a K-X1 space keeping other inputs X−1 ≡ {X2 , ..., XJ } constant. Note that,since σKX1,0 = 1, Definition 5 means that d ln(FK /FX1 )/d ln(K/X1 ) = −1 holds on this K-X1
space. Integrating this differential equation gives ln(FK /FX ) = − ln(K/X1 ) + ξe1 , where ξe1 is a1

constant of integration. Taking the exponential of the both sides gives FK /FX1 = (X1 /K)ξb1 , whereξb1 ≡ exp ξe1 > 0 is a positive constant.
Now, consider the isoquant curve of production function FK (K, X1 , ..., XJ ; 0) on this K-X1
space. The slope of the isoquant curve is dX1 /dK = −FK /FX = (X1 /K)ξb1 . Integrating this1

differential equation gives ln K = −ξb1 ln X1 + κ
b1 , where κ
b1 is another constant of integration. By

taking exponential, the isoquant curve is written as K = κ1 X1−ξ1 , where κ1 ≡ exp κb1 > 0 is abpositive constant.
Let us focus on the isoquant curve that runs through the allocation at time t = 0; i.e.,48

{K0 , X0 }. The constant κ1 representing this curve is obtained by substituting K = K0 and
X1 = X1,0 into above expression, yielding K = K0 (X1,0 /X1 )ξ1 . By substituting X1 = 1 intobξthis curve, we know that the allocation with K = K0 X0 1,0 and X1 = 1 gives the same amountof output with that with K = K0 and X1 = X1,0 , keeping other inputs constant. This means
Y0 = F (K0 X0ξ1 , 1, X2,0 , ..., XJ,0 ; 0).
b

Repeating this procedure for j = 1, ..., j ∗ , we obtain
Y0 = F K0


Yj ∗

b

(Xj,0 )ξj , 1, ..., 1, Xj ∗ +1,0 , ..., XJ,0 ; 0 .
| {z }
j=1

(A.2)

j∗


−1
P∗
Define α ≡ 1 + jj=1 ξbj
and ξj = αξbj for j = {1, ..., j ∗ }. Then, using the definition of Fb(·)
in (11), it can be confirmed that (A.2) is the same as (10).
Lastly, we confirm that Fb(·) has constant returns to scale. Multiplying all the arguments inFb(z0 , zj ∗ +1 , ..., zJ ) by λ > 0 gives Fb(λz0 , λzj ∗ +1 , ..., λzJ ) = F ((λz0 )1/α , 1, ..., 1, λzj ∗ +1 , ..., λzJ ; 0).
Pj ∗ b1/α1/α Qj ∗ξbj
Note that, from the definition of α, (λz0 )1/α = λ1+ j=1 ξj z0= λz0
j=1 λ . Also, recall−ξbjthat the isoquant curve of F (·; 0) on a K-Xj space takes the form of K = κj Xj
1, ..., j ∗ , which means that if we multiply Xjbby λ and K by λ−ξj

for j =

then the output will be un-

1/αchanged. Using these, F ((λz0 )1/α , 1, ..., 1, λzj ∗ +1 , ..., λzJ ; 0) = F (λz0 , λ, ..., λ, λzj ∗ +1 , ..., λzJ ; 0) =| {z }j∗1/αλF (z0 , 1, ..., 1, zj ∗ +1 , ..., zJ ; 0), where the last equality follows because F (·; 0) has constant returns| {z }
j∗to scale. The last expression coincides with λFb(z0 , zj ∗ +1 , ..., zJ ). This confirms the constant-returnsto-scale property of Fb(·).
A.1.3

Proposition 3

Proof. From Lemma 1 and condition (12), it can be confirmed that the first argument in function
Fb(·) in (13) is multiplied by g each period. The same holds for the other arguments: AXj,t Xj,t forj = j ∗ + 1, ..., J. Since Fb(·) has constant returns to scale, the RHS of (13) is multiplied by g eachperiod. In period 0, (7) holds because it is identical with (10). Since Yt is multiplied by g every
period by the definition of the BGP, (13) holds for all t ≥ 0.
A.1.4

Proposition 4

Proof. Let us first consider the case of Zt = Kt . Note that the share of capital is the same in periodt and 0. Because the production function in period 0 can be represented in the form of (10),


Yj ∗
Kt
∂ b
K0
α
ξj
sK = FK (Kt , X1,t , ..., XJ,t ; t)=F K0
(Xj,0 ) , Xj ∗ +1,0 , ..., XJ,0
.
j=1
Yt
∂K0
Y0
49

(A.3)

Define FbM (·) as the derivative of function Fb(·) with respect to its first argument. In (A.3), the firstQ∗argument is the capital composite, M0 = K0α jj=1 (Xj,0 )ξj . Then, using the chain rule, the RHS of(A.3) becomessK = FbM (M0 , Xj ∗ +1,0 , ..., XJ,0 )αM0
dM0 K0. = FbM (M0 , Xj ∗ +1,0 , ..., XJ,0 ).
dK0 Y0Y0(A.4)

Since Fb(·) has constant returns to scale from Lemma 2, its derivative FbM (·) is a homogeneousfunction of degree 0. Recall that, from Lemma 1 and condition (12), Mt is multiplied by g every period. The same applies to AX j,t Xj,t for j = j ∗ + 1, ..., J. Therefore, every argument in
FbM (Mt , AX j ∗ +1,t Xj ∗ +1,t , ..., AX J,t XJ,t ) is multiplied by g every period, which does not change thevalue of FM (·) over time. Also αM0 /Y0 = αMt /Yt = (αMt /Kt )(Kt /Yt ) holds because Mt and Yt
grow at the same speed. From these and the chain rule,αMt αKt
sK = FbM (Mt , AX j ∗ +1,t Xj ∗ +1,t , ..., AX J,t XJ,t )kt Ytb∂ F (Mt , AX j ∗ +1,t Xj ∗ +1,t , ..., AXJ,t XJ,t ) αKt
=
,
∂Zt
Yt
where Mt = (AK,t Kt )αQj ∗

j=1 (AXj,t Xj,t ).

(A.5)

By comparing (A.3) with (A.5), we obtain (14) for thecase of Zt = Kt .
The proof of the proposition for the case of Zt = Xj,t , j ∈ {1, ..., j ∗ }, follows the same pattern
as in (A.3)-(A.5), with only the modification thatKt is replaced by Xj,t and α by ξj .
Finally, the the case of Zt = Xj,t , j ∈ {j ∗ , ..., J}, can be confirmed in the same way as in
Proposition 2, by swapping Fe(·) with Fb(·). In this case, we define FbXj (·), j ∈ {j ∗ , ..., J}, as thederivative of function Fb(·) with respect to its (j − j ∗ + 1)th argument.55
A.1.5

Lemma 3

Proof. Consider a BGP. We will show that µt , kt and ct must be constant in turn. First, from thedefinition of a BGP, Nt+1 /Nt = (AL,t+1 Lt+1 )/(AL,t Lt ) = γbL (µt+1 )gL is constant. To keep the RHSof the latter equation constant, µt must also be constant, since γbL (·) is a strictly increasing function
from Proposition 5. Second, since growth factors of Ct and Nt are constant, the growth factor of
ct = Ct /At Lt = Ct /Nt is also constant. This, in turn, means that the LHS of the Euler equation(48) is constant. Then, for the RHS of (48) to be constant, kt must be constant, since we alreadyknow that µt is constant as shown above. Similarly, the growth factor of kt = Kt /At Lt = Kt /Nt isconstant on the BGP, which means the LHS of (46) is constant. For its RHS to be constant, giventhat µt and kt are already shown to be constant, ct also needs to be constant.
55

Recall that j ∗ arguments are eliminated from function Fb(·) in definition (11).

50

A.2

Data

A.2.1

Figure 1

Real GDP. U.S. Bureau of Economic Analysis, Real Gross Domestic Product [GDPCA], retrieved
from FRED, Federal Reserve Bank of St. Louis; https://fred.stlouisfed.org/series/GDPCA, February 15, 2019. Shown in Panel (a).
Investment. U.S. Bureau of Economic Analysis, Real Gross Private Domestic Investment [GPDIC1],
retrieved from FRED, Federal Reserve Bank of St. Louis; https://fred.stlouisfed.org/series/GPDIC1,
February 15, 2019. Shown in Panel (a).
Consumption. U.S. Bureau of Economic Analysis, Real Personal Consumption Expenditures[PCECC96], retrieved from FRED, Federal Reserve Bank of St. Louis;
https://fred.stlouisfed.org/series/PCECC96, February 15, 2019. Shown in Panel (a).
Nominal Capital Stock. U.S. Bureau of Economic Analysis, Current-Cost Net Stock of Fixed
Assets, Fixed Asset Table 1.1. Not shown in Table.
GDP Deflator. Bureau of Economic Analysis, Implicit Price Deflator for Gross Domestic Product,
NIPA Table 1.1.9. Not shown in Table.
Real Capital Stock (K). Nominal Capital Stock/GDP Deflator. Shown in Panel (a).
Investment Deflator. Bureau of Economic Analysis, Implicit Price Deflator for Gross Private
Domestic Investment, NIPA Table 1.1.9. Not shown in Table.
Non-Residential Investment Deflator. Bureau of Economic Analysis, Implicit Price Deflator
for Fixed, Non-Residental Investment, NIPA Table 1.1.9. Not shown in Table.
Equipment Investment Deflator. Bureau of Economic Analysis, Implicit Price Deflator for
Equipment, NIPA Table 1.1.9. Not shown in Table.
Personal Consumption Expenditures Deflator. Bureau of Economic Analysis, Implicit Price
Deflator for Personal Consumption Expenditures, NIPA Table 1.1.9. Not shown in Table.
Relative Price of Investment (various). Investment Deflator/Consumption Deflator. Applied
to all investment, non-residential, and equipment deflators. Shown in panel (b).
51

A.2.2

Table 1

Energy Expenditure Share. Energy expenditure as a share of GDP (%). Data available from
1970–2014. Source: ‘Table 1.5: Energy consumption, expenditures, and emissions indicators estimates, 1949–2011’ at https://www.eia.gov/totalenergy/data/annual/.
A.2.3

Calibration

See information for Relative Price of Investment Goods, Nominal Capital Stock, and RealGDP from above.
Nominal GDP. U.S. Bureau of Economic Analysis, Gross Domestic Product [GDP], retrieved
from FRED, Federal Reserve Bank of St. Louis; https://fred.stlouisfed.org/series/GDP, February
15, 2019.
R&D Expenditure. U.S. Bureau of Economic Analysis, Gross Domestic Product: Research andDevelopment [Y694RC1Q027SBEA], retrieved from FRED, Federal Reserve Bank of St. Louis;
https://fred.stlouisfed.org/series/Y694RC1Q027SBEA, February 15, 2019.
Capital-Output Ratio. Nominal Capital Stock/Nominal GDP.
Labor Compensation. Bureau of Economic Analysis, (Compensation of Employees) + ( Proprietors’ income with inventory valuation and capital consumption adjustments), NIPA Table 2.1.
Labor Share of Income. Labor Compensation / Nominal GDP.
R&D Share of GDP. R&D Expenditure / Nominal GDP.
Savings Rate. (Investment Share of GDP) - (R&D Share of GDP).

52