An Assessment of CES and Cobb-Douglas Production

Functions 1

Eric Miller

E-mail: eric.miller@cbo.gov

Congressional Budget Office

June 2008

2008-05

1Working papers in this series are preliminary and are circulated to stimulate discussion and

critical comment. These papers are not subject to CBO’s formal review and editing processes.

The analysis and conclusions expressed in them are those of the authors and should not be

interpreted as those of the Congressional Budget Office. References in publications should

be cleared with the author. Papers in this series can be obtained at www.cbo.gov (select

Publications and then Working Papers). I would like to thank Bob Arnold, Juan Contreras,

Bob Dennis, Matthew Goldberg, Angelo Mascaro, Delciana Winders, and Thomas Woodward

for helpful comments.
Abstract

This paper surveys the empirical and theoretical literature on macroeconomic production

functions and assesses whether the constant elasticity of substitution (CES) or the CobbDouglas specification is more appropriate for use in the CBO’s macroeconomic forecasts.

The Cobb-Douglas’s major strengths are its ease of use and its seemingly good empirical

fit across many data sets. Unfortunately, the Cobb-Douglas still fits the data well in

cases where some of its fundamental assumptions are violated. This suggests that many

empirical tests of the Cobb-Douglas are picking up a statistical artifact rather than an

underlying production function. The CES has less restrictive assumptions about the

interaction of capital and labor in production. However, econometric estimates of its

elasticity parameter have produced inconsistent results. For the purpose of forecasting

under current policies, there may not be a strong reason to prefer one form over the

other; but for analysis of policies affecting factor returns, such as taxes on capital and

labor income, the Cobb-Douglas specification may be too restrictive.
It takes something more than the usual “willing suspension of disbelief” to talk seriously of

the aggregate production function.

(Robert Solow, 1957, p. 1)

1. Introduction

A macroeconomic production function is a mathematical expression that describes a systematic relationship between inputs and output in an economy, and the Cobb-Douglas

and constant elasticity of substitution (CES) are two functions that have been used extensively. These functions play an important role in the economic forecasts and policy

analysis of the CBO and others. For example, the CBO uses production functions to

forecast potential output and the medium-term outlook for income shares. Additionally, several of the models that the CBO uses to analyze policy changes, such as the

multisector Aiyagari (1994) model and the overlapping generations model, assume that

aggregate output in the economy can be described by a production technology.1

In all of these cases the CBO assumes that the economy’s underlying production technology takes the Cobb-Douglas form. This specification has long been popular among

economists because it is easy to work with and can explain the stylized fact that income shares in the United States have been roughly constant during the postwar period.

Economists have also been somewhat well disposed toward Cobb-Douglas because it

gives simple closed-form solutions to many economic problems. However, empirical and

theoretical work has often questioned the validity of the Cobb-Douglas as a model of

the U.S. economy. Some economists believe that the more general CES may be a more

1Although Cobb-Douglas does restrict the elasticity of substitution between the demand for labor

and capital in production, it does not address labor-supply issues such as the substitution between labor

and leisure, or the trade off between consumption spending and saving to increase the capital stock.

These are issues which the CBO has examined extensively. For a further discussion see CBO (2008).

1
appropriate choice.

This paper will examine the major strengths and weaknesses of these two functional

forms and suggest which form is better able to forecast income shares under given policies. This is important for budget estimation because one cannot predict the future path

of the federal budget without first predicting how national income will be apportioned

between capital and labor. These two sources of income are subject to different tax

treatments and as a result their relative sizes are an important determinant of future

tax revenues. While the focus of this paper is forecasting, many of the issues it raises

are pertinent to policy analysis as well.

The paper begins with a review of some basic concepts in production theory. Readers

with a solid understanding in this area should skip ahead to section 3, which discusses

some general theoretical problems with the use of aggregate production functions. Section 4 reviews the empirical performance of the CES and Cobb-Douglas. The paper

concludes with a recommendation to continue using the Cobb-Douglas in spite of theoretical concerns because it appears that additional costs and parameter uncertainties

from the use of the CES are not outweighed by its benefits.

2. Production Functions

A production function is a heuristic device that describes the maximum output that can

be produced from different combinations of inputs using a given technology. This can

be expressed mathematically as a mapping f : R

N

+ → R+ such that Y = f(X), where X

is a vector of factor inputs (X1, X1, · · · , Xn)

0 and f(X) is the maximum output that can

be produced for a given set of inputs Xi ∈ R+.

2 This formulation is quite general and

2

It is possible to have a production function that produces a vector of outputs, but in order to

simplify the analysis this paper limits its scope to functions that map a vector of inputs to a single

output.

2
can be applied at both microeconomic (i.e., individual firm) and macroeconomic (i.e.,

overall economy) levels. While production functions were originally designed with the

individual firm in mind, macroeconomists came to realize that this methodology provides

a useful tool for estimating certain parameters that cannot be directly measured from

national accounts data. The most important of these is the elasticity of substitution

between capital and labor.

Elasticity of substitution in production is a measure of how easy it is to shift between

factor inputs, typically labor and capital. This measure is defined as the percentage

change in factor proportions resulting from a one-unit change in the marginal rate of

technical substitution (MRTS). MRTS is the rate at which labor can be substituted for

capital while holding output constant along an isoquant; that is, it is the slope of the

isoquant at a given point. Thus, for a two-input production function, Y = f(K, L), the

elasticity of substitution between capital and labor is given by

σ =

%∆(K/L)

%∆MRT S =

"

d(K/L)

dMRT S #

∗

"

MRT S

(K/L)

#

=

∂ln(K/L)

∂MRT S (1)

where σ can be thought of as an index that measures the rate at which diminishing

marginal returns set in as one factor is increased relative to the other (Nelson 1964).

When σ is low, changes in the MRTS lead to small changes in factor proportions. In

the extreme case of fixed proportions or Leontief (1941) technology,

Y = min(aK, bL) where a, b > 0 (2)

the resulting isoquants are L-shaped and σ = 0. This implies that changes in the MRTS

will not cause any changes in factor proportions, so output is maximized by producing

in fixed ratios. The other extreme is linear production technology,

3
Y = aK + bL (3)

where capital and labor are perfect substitutes. Here the MRTS is constant so the

isoquants are straight lines and σ = ∞. The Cobb-Douglas form,

Y = AKαL

1−α

(4)

lies between these two extremes, with σ = 1. This specification creates isoquants that

are gently convex and easy to work with.

Figure 1. Isoquant Maps for Production Functions with Different Elasticities

4
Elasticities of substitution provide a powerful tool for answering analytical questions

about the distribution of national income between capital and labor. If we assume that

markets are competitive, then factors will be paid their marginal product. Hence, the

wage rate will equal the marginal contribution from an additional worker and the return

on capital will match the contribution in output that a marginal increment of capital

provides. The elasticity of substitution can now be written as

σ =

%∆(K/L)

%∆(w/r)

=

∂ln(K/L)

∂ln(w/r)

(5)

where w is the wage rate and r is the rental rate of capital. Differing values of σ have

different implications for the distribution of income.

If σ = 1, any change in K/L will be matched by a proportional change in w/r

and the relative income shares of capital and labor will stay constant. Any increase in

the capital-labor ratio over time will be exactly matched by a percentage increase in

the MRTS and an identical percentage increase in w/r. As a result, constant shares of

output are allocated to capital and labor even though the capital-labor ratio may change

over time. During the post-World War II period, the long-term trend in factor shares in

the United States appears to have been roughly constant while the capital-labor ratio

has been steadily increasing.3 This stylized fact initially led many economists to believe

that σ = 1 in the United States.

If σ > 1, then a given percentage change in K/L will exceed the associated percentage

change in w/r. For example, an increase in the capital stock would raise the ratio K/L

but lower w/r by a smaller percentage, hence the share of capital in total income would

rise as the capital-labor ratio increased. The opposite result occurs when σ < 1: an

increase in the ratio K/L would tend to lower capital’s share because the relative price

3While the overall labor share has been relatively constant during this period, there have been

significant movements at the industry level (Solow (1958), Jones (2003), and Young (2006)).

5
of labor would rise in response to the increase in the amount of capital per worker.

The production functions that we have examined so far have been quite generic. In

order to generate testable predictions, it is necessary to impose some further restrictions.

Following Barro and Sala-i-Martin (2004), we define a production function of the form

Y = f(K, L, A), where K is capital, L is labor, and A is a measure of technology, as a

neoclassical production function if the following three conditions are met:

1) Constant returns to scale. The function f exhibits constant returns to scale. If capital

and labor are multiplied by a positive constant, λ, then the amount of output is also

multiplied by λ:

f(λK, λL, A) = λf(K, L, A) for all λ > 0 (6)

This definition includes only the two rival inputs, capital and labor; we did not define

constant returns to scale as f(λK, λL, λA) = λf(K, L, A).

2) Positive and diminishing returns to private inputs. For all K > 0 and L > 0, f

exhibits positive and diminishing marginal products with respect to each input:

∂f

∂K > 0,

∂

2

f

∂K2

< 0

∂f

∂L > 0,

∂

2

f

∂L2

< 0 (7)

If we hold constant the levels of technology and labor, then each additional unit of

capital delivers additional output, but these additions decrease as the stock of capital

rises. The same is true for labor.

3) Inada conditions. The marginal product of capital (labor) approaches infinity as

6
capital (labor) goes to zero and approaches zero as capital (labor) goes to infinity:

lim

K→0

∂f

∂K = lim

L→0

∂f

∂L = ∞

lim

K→∞

∂f

∂K = lim

L→∞

∂f

∂L = 0 (8)

The two most popular neoclassical production functions are the Cobb-Douglas and

the CES. The Cobb-Douglas is a simple production function that is thought to provide

a reasonable description of actual economies. It was created by labor economist Paul

H. Douglas and mathematician Charles W. Cobb in an effort to fit Douglas’s empirical

results for production, employment, and capital stock in U.S. manufacturing into a

simple function (Cobb and Douglas 1928). This functional form has been extremely

popular among economists because of its ease of use and its extreme flexibility. It can

be written as

Y = AKαL

1−α

(9)

where Y is output, K is capital, L is labor, α is a constant that takes values between

zero and 1, and A is the level of technology (A > 0). It can be easily demonstrated that

this function exhibits constant returns to scale and σ = 1. If we assume that markets are

competitive and that factors are paid their marginal product, then α and 1−α are equal

to capital and labor’s share of output respectively. As mentioned earlier, an elasticity of

substitution equal to unity implies that these factor shares will remain constant for any

capital-labor ratio because any changes in factor proportions will be exactly offset by

changes in the marginal productivities of the factor inputs. Thus, the observed income

shares will be constant through time.

The Cobb-Douglas is a special case in a more general class of production functions

7
with constant elasticity of substitution.4 Following Klump and Preissler (2000), we can

rewrite equation (1) as

σ =

"

d(K/L)

dMRT S #

∗

"

MRT S

(K/L)

#

=

 ̃f

0

(k)[  ̃f(k) − k

 ̃f

0

(k)]

−k

 ̃f

00(k)

 ̃f(k)

(10)

This definition can then be transformed into a second-order partial differential equation

in k with the solution

y =

Y

L

=  ̃f(k) = γ1[k

ρ + γ2]1/ρ (11)

or

Y = f(K, L) = γ1[Kρ + γ2L

ρ

]

1/ρ (12)

where γ1 and γ2 are constants of integration and ρ = (σ −1)/σ. If we set α = 1/(1 +γ2)

and C = γ1(1 + γ2)

1/ρ then we arrive at the standard CES form introduced by Arrow,

Chenery, Minhas, and Solow (1961) (hereinafter referred to as ACMS):

Y = C[αKρ + (1 − α)L

ρ

]

1/ρ (13)

where C is a measure of technical progress and the coefficients α and 1 − α are distribution parameters between zero and 1 and can be used to determine factor shares. The

substitution parameter ρ can be used to derive the elasticity of substitution σ.

This formulation of the CES function has been criticized as being unduly restrictive

because it assumes that technological progress has no effect on the marginal productivities of input factors.5

If we set AK = γ1 and AL = γ2γ

ρ

1 we obtain the functional form

4The CES is a special case of an even more general class of production functions. For a further

discussion see Christensen, Jorgenson and Lau (1973).

5Barro and Sala-i-Martin (2004) have proposed a CES variant C[α(bK)ρ + (1 − α)((1 − b)L)ρ

]

1/ρ

that generalizes the ACMS function by allowing for biased technological change. Klump and Preissler

(2000) show that this specification does not follow directly from the definition of σ; instead, the authors

suggest starting with equation (14) and then applying a suitable normalization procedure.

8
proposed by David and van de Klundert (1965):

Y = [(AKK)

ρ + (ALL)ρ

]

1/ρ (14)

where AK represents capital-augmenting technological change and AL represents laboraugmenting technological change. From equation (14) it follows that

M arginal P roduct of Capital =

∂f

∂K = A

(σ−1/σ)

k

 

Y

K

!1/σ

(15)

M arginal P roduct of Labor =

∂f

∂L = A

(σ−1/σ)

L

 

Y

L

!1/σ

(16)

MRT S = σ > 0 (17)

In the extreme case where (σ − 1)/σ → −∞, the elasticity of substitution tends toward

zero (no substitution) and when (σ−1)/σ → 1 the elasticity of substitution tends toward

infinity (perfect substitutes).

For any variant of the CES production function, the value of σ depends on three

given baseline values: a given capital intensity (k0 = K0/L0), a given marginal rate of

substitution [fL/fK]0 = u0, and a given level of per capita production y0 = Y0/L0. A

set of CES functions are considered to be in the same “family” if they share the same

baseline values but differ only in their values of σ and one point of tangency that is

characterized by the given baseline values. Using Klump and de La Grandville’s (2000)

normalization procedure, one can benchmark current data to some initial period and

then assess how σ varies relative to different benchmark choices.6

6Klump and de La Grandville (2000) advocate the use of normalized production functions in empirical work because it makes for more consistent cross-study estimates of σ. For a discussion of how

this normalization procedure can be carried out in empirical work, see Klump, McAdam, and Willman

(2008).

9
Finally, we examine the role that production functions play in estimating labor’s

share of income. If we assume that markets are competitive and returns to scale are

constant, then labor’s share SL can be modeled solely as a function of the capitaloutput ratio k

∗

. The relationship between SL and k

∗

is unaltered by changes in factor

prices, factor quantities, or the presence of labor-augmenting technical progress. When

our production function is CES (σ 6= 1), then the relationship between SL and k

∗

is

monotonic. If labor and capital are substitutes (σ > 1), a lower capital intensity will

increase the labor share; if they are complements (σ < 1), then the converse holds. In

models that use more general production functions, the relationship between SL and

k

∗ need not be monotonic: the labor share can go up and then down as some variable

driving changes in k

∗

(such as real wages or interest rates) varies.

3. Production Functions: Theoretical Problems

Reducing the U.S. economy to a simple mathematical function is bound to mask many

of the complex interactions that characterize a real economy. However, if this strippeddown model can produce reasonably accurate predictions, then it can be a useful forecasting tool regardless of how well it describes reality.

However, once we move beyond simple forecasting and attempt to draw inferences

from model parameters, internal consistency becomes much more important. All of the

estimated parameters in the CES and Cobb-Douglas models have a behavioral interpretation. This means that if the model is not internally consistent, then these parameters

are not describing a meaningful economic relationship. However, even if our model is

misspecified and the parameters are in fact statistical artifacts, they may still be useful for forecasting purposes. The theoretical literature on aggregation shows that a

macroeconomic production function has economic content only if a very stringent set of

10
conditions is met. Since these conditions aren’t satisfied in real economies, it is likely

that the good fit observed in empirical studies of aggregate production functions is the

result of a statistical artifact.

Aggregate production functions rely heavily on the use of marginal products and

factor elasticities, both of which are microeconomic concepts that macroeconomists have

found very useful for simplifying their models. While it is common practice to estimate

these parameters for capital and labor in the whole economy, it is not entirely clear

that these measurements capture an economically meaningful relationship. It is worth

remembering that a production function does not express a relation between inputs

and actual output, but between combinations of inputs and maximum potential output.

Thus, macroeconomic parameter estimates have an economic interpretation when the

production possibility frontiers (PPF) for the individual firms can be combined to form

an aggregate PPF for the entire economy. As we shall see, a very stringent set of

conditions is required for meaningful aggregation across firms.

The first problem that arises is how to develop a sensible measure of aggregate capital.

Following Jorgenson et al. (2000), capital enters our production function exogenously as

a flow of services K, which is assumed to be proportional to the physical stock of capital.7

Many parameterized production function models describe the capital stock in dollar

amounts rather than in physical quantities. This is problematic, because according to

neoclassical theory, the rate of interest is determined by the overall amount of physical

capital in the economy. Since the monetary value of a capital asset is influenced by

the rate of return it generates over its productive lifetime, the model will suffer from

endogeneity problems.8 One can get around this difficulty (to some extent) by using the

BEA’s quantity index, which measures changes in the quality-adjusted stock of physical

7The constant of proportionality accounts for changes in quality and treats new capital as an equivalent number of efficiency units of older capital.

8Joan Robinson popularized this line of argumentation during the Cambridge Capital Controversies.

For a further discussion of this issue see Cohen and Harcourt (2003).

11
capital. However, it is important to keep in mind just how diverse U.S. capital stock

really is. Everything from steel mills to palm pilots comes under this heading, and the

Bureau of Economic Analysis is faced with the monumental task of boiling down this

heterogeneity to a single index number.9 Additionally, there is some arbitrariness in the

way that income is partitioned between capital and labor in the national accounts. If

an individual is employed by a firm, her compensation comes in the form of a wage and

counts as labor income. On the other hand, if this individual owns her own business

where she performs the exact same work, her compensation is considered proprietors’

income, a separate category in the national income and product accounts, and researchers

are left to determine what part of proprietors’ income to treat as labor income and what

part capital income.10

Assuming that it is possible to create a meaningful measure of capital services in the

economy, it remains to be seen whether aggregation of firms’ PPF leads to a coherent

PPF for the economy as a whole. The following argument draws heavily on Felipe and

Fisher’s (2003) survey of the theoretical literature on aggregation. The first major result

presented is Leontief’s theorem, which provides the necessary and sufficient conditions

for aggregation of any twice differentiable production function. The theorem states that

aggregation is possible if and only if the MRTS of the variables in the aggregate production function are independent of the variables that are not included. In an economy

where each firm has a production function f(K, L, A) and aggregation occurs across K

and L, we can construct an aggregate production function F if and only if

∂(fK/fL)

∂fA

= 0 (18)

9Similar problems (though not as severe) arise in the measurement of services from labor. While

labor hours provide a convenient numeraire, it is clear that not all labor hours are created equal. For a

discussion of how to adjust labor hours for differences in human capital see Ho and Jorgenson (1999).

10The CBO convention (CBO (2006)) has been to use the same average fraction as is found in the

corporate sector. However, tax considerations may induce people to report their income in ways that

shift the balance between capital and labor.

12
That is, aggregation is possible if and only if the marginal rate of substitution between

K and L is independent of A. In the current example both capital and labor enter as

arguments, so technology must be neutral with respect to both. If we construct the

production function in per capita terms, then we need to aggregate only across capital.

In this case, we can have a coherent production function when technological change is

labor augmenting. Let us return for a moment to the problem of heterogeneous capital.

If we have a production function f = f(k1, · · · , kn, L), where each ki represents a unique

type of capital, then f = F(K, L), where K is the aggregate stock of capital if and only

∂

∂L 

∂Q/∂ki

∂Q/∂kj

!

= 0 for all i 6= j (19)

Thus, Leontief’s theorem requires that labor has no effect on the substitution possibilities between the capital inputs. This makes intuitive sense because if labor has an

effect on substitution possibilities, then it becomes impossible to collapse the capital

stock to a single dimension. Clearly this condition is not satisfied in the real world,

where the choice of capital is influenced by the quantity and quality of labor available.

Franklin Fisher expanded on this work and found that if one assumes that labor

is efficiently allocated to firms, then an aggregate production function can be found to

exist if and only if all of the micro units are identical except for their capital efficiency

coefficient. When firms choose the amount of labor to hire, a labor aggregate exists only

if there is no specialization in employment and aggregate output exists only if there is

no specialization in production, i.e., firms all produce the same market basket of goods

at different scales. What does all this mean for our aggregate production function? We

give Felipe and Fisher (2008) the last word on this matter:

The conditions under which aggregate production functions exist are so stringent that real economies surely do not satisfy them. The aggregation results

13
pose insurmountable problems for theoretical and applied work in fields such

as growth, labour or trade. They imply that intuitions based on micro variables and micro production functions will often be false when applied to

aggregates.

Proponents of the neoclassical production function argue that despite these theoretical shortcomings, aggregate production functions can be defended on instrumentalist

grounds if they provide a reasonably good description of the data. To test this proposition, Fisher (1971) simulated numerous fictitious simple economies where he knew the

conditions for successful aggregation would be violated. He fit the Cobb-Douglas to the

generated data and found that without exception it yielded an almost perfect fit. Fisher

et al. (1977) ran a similar experiment and found that as the constancy of the labor

share decreased, the Cobb-Douglas’s fit decreased substantially. When the experiment

was repeated using the CES, the authors were unable to predict the model’s fit based

on changes in the labor share.

Fisher’s findings have been confirmed by subsequent experiments in which the requisite conditions for the existence of a production function were clearly violated. Shaikh

(1974) showed that any production series Y , K would fit the Cobb-Douglas well, provided that factor shares are constant and capital and labor are uncorrelated with the

rate of technological growth. To illustrate this point, he created a data set that spelled

out the word HUMBUG when values of Y/L were plotted against values of K/L. Additionally, Shaikh chose his data so that it would produce the same capital share as Solow’s

(1957) famous paper on economic growth. Using Solow’s (1957) procedure, Shaikh fit

the Cobb-Douglas to his data and found an extremely high R2 value.11

Nelson and Winter (1982) used simulation analysis to generate a data set from their

evolutionary model of the economy. The model makes the following assumptions: firms

do not maximize profits, technology is available only in fixed coefficients, and firms

11In a short note, Solow (1974) claims to have refuted Shaikh’s study. However, Solow’s reply completely ignores the problem of identification.

14
operate in conditions of disequilibrium. Surprisingly, the fitted Cobb-Douglas provided

an excellent description of this data and generated elasticity of substitution parameters

that were very close to the factors’ share of revenue.

In each of these experiments, the researchers created a model economy in which

neoclassical theory would provide a poor causal explanation for the observed data. Yet

in all cases the Cobb-Douglas still produced a remarkably good fit. These findings show

that the Cobb-Douglas functional form is flexible enough that it can fit the data well

even when it does not have a meaningful economic interpretation.

By the principle of Occam’s razor, if many theories can explain a set of empirical

results, we should accept the explanation that requires the fewest assumptions. Indeed,

the simplest explanation for the Cobb-Douglas’ excellent empirical fit in many real world

studies is the presence of an accounting identity that exists in any real economy, regardless of the underlying production technology. Observe the following equation:

Y (t) = W(t) + P(t) = wL(t) + rK(t) (20)

where Y is real value added at time t, W is the wage bill, P is total payments to

owners of capital, w is the real average wage rate, r is the real rate of return, L is

the total labor employed, and K is the total capital stock. This accounting identity

shows how output is broken down between wages and profit and does not rely on the

standard neoclassical assumptions. From this definition we can use elementary calculus

to construct a mathematical function that looks very much like the Cobb-Douglas. We

follow Felipe and McCombie (2005a) and begin by differentiating equation (20) with

respect to time and dividing through by total output to get

Y ̇

Y

=

wL

Y

 

L ̇

L

!

+

wL

Y

 

w ̇

w

!

+

rK

Y

 

K ̇

K

!

+

rK

Y

 

r ̇

r

!

(21)

15
For convenience, let α = wL/Y , labor’s share of output and let (1 − α) = rK/Y ,

capital’s share of output. Now, with the help of several stylized facts we can derive the

Cobb-Douglas from equation (20). Assume that factor shares are constant and integrate

equation (21). This gives us the function

Y = Bwαr

1−αLαK1−α

(22)

If we further assume that the wage rate grows at a nearly constant rate and profit rate

does not exhibit a secular trend (  ̇r ≈ 0), then aw ̇ + (1 − α)  ̇r ≈ αw ̇ = λ, which is a

constant. Thus, we can rewrite equation (22) as the Cobb-Douglas production function

with exogenous technical change:

Y (t) = AeλtL

αK1−α

(23)

This equivalence between the Cobb-Douglas and the accounting identity holds whenever

factor shares are roughly constant and the correlation between the capital labor ratio is

weakly correlated with changes in the technology parameter.12 The above derivation was

carried out without assuming that factors are paid their marginal product, that returns

to scale are constant, or that production can be described by a particular technology.

Felipe and Holz (2001) used Monte Carlo simulation to empirically test the conditions

under which this link will hold. The pair found that the Cobb-Douglas is well described

by the accounting identity even when there are large variations in the factor shares.

However, the link between the Cobb-Douglas and the accounting identity begins to

break down when there are variations in the growth rates of wages and the return to

capital.

Felipe and McCombie (2001) examined ACMS’s CES production function and found

12This derivation applies to time-series data. For a similar transformation of cross-sectional data see

Felipe and McCombie (2005a).

16
that regression results that are purportedly well explained by a CES production function can be equally well explained by an underlying accounting identity. The original

motivation behind the CES was the observation that in a given industry, value added

per unit of labor varied across countries with the wage rate (and hence the elasticity of

substitution is not equal to unity). To explain this fact, ACMS worked backward and

hypothesized the following relationship:

ln 

Y

L



= ln[(C

1−σ

)(α

−σ

)] + σlnw (24)

From equation (24) they were able to derive the CES production function. When equation (24) was fitted to the data it gave a very high R2 and a value of σ below unity.

However, it is highly plausible that the good fit of the model has nothing to do with an

underlying production function and instead reflects the following identity, which comes

directly from the definition of the labor share SL, where

Y

L

=

w

SL

(25)

If labor’s share remains constant across units of observation then the model fit will

be very good and estimates of σ will be close to unity. If labor’s share and wages

are negatively correlated then estimates of σ will be greater than unity, while positive

correlation between wages and labor’s share will yield an estimate of σ that is less than

unity. While all of these outcomes can be explained by an underlying CES production

function, they can be more easily explained by a simple accounting identity.13

13The above argument applied to cross-sectional data, but Felipe and McCombie (2001) show that a

similar argument applies to time-series data.

17
4. CES and Cobb-Douglas: An Empirical Assessment

If income shares are forecast using a neoclassical production function, the elasticity of

substitution parameter σ is of prime importance. When σ > 1, an increasing share

of national income goes to capital as the capital-labor ratio increases. If σ < 1, then

capital’s share declines as this ratio increases. When σ = 1, income shares are unaffected

by changes in the capital-labor ratio. Cobb-Douglas models assume that σ equals unity,

but there is no theoretical reason why this must be the case. The correct choice of σ is a

purely empirical question. Ever since the pathbreaking work of Charles Cobb and Paul

Douglas in 1928, economists have been well disposed toward the Cobb-Douglas function

because it can explain the stylized fact that income shares in the United States have

been roughly constant over time while the capital-labor ratio has been rising steadily.

In their original 1928 study, Cobb and Douglas used time-series data to examine

the relationship between capital, labor, and manufacturing output in the United States.

The authors found a tight correspondence between their theoretical production function

and the observed results. Over the next few decades, a number of studies confirmed

Cobb and Douglas’s initial findings and lent strong empirical support to their model.14

However, modern economists have questioned the methodological soundness of these

early studies.

Fraser (2002) reexamined five of these original time series studies that claimed the

Cobb-Douglas provided an excellent fit to the data. Collinearity diagnostics revealed

that once a time trend was accounted for, the data in all five studies was highly collinear,

which can lead to imprecise point estimates. Fraser also examined the time-series properties of the five data sets and found a mix of stationary and nonstationary variables.

14For an overview of these early studies see Douglas (1948).

18
In the presence of nonstationary variables standard Ordinary Least Squares estimates

are dubious because they may lead to spurious regressions.15

After the seminal work of ACMS (1961), estimating elasticities using a fitted CobbDouglas largely fell out of favor.16 Early CES studies that used cross-sectional data

tended to find elasticity values close to 1, while time series analyses tended to produce

much lower estimates (Berndt 1976). More recent studies tend to eschew flat crosssectional data which contain inherent biases (Lucas 1969) and instead focus on either

panel or time-series data. Most of these studies find values of σ that are consistently

below unity, but a great deal of variation in the results persists. Pereira (2003) surveyed

major papers in the field from the past 40 years and found that, in general, elasticity

values were below unity. Hamermesh (1993) surveyed a number of studies that used

the CES specification and labor-hours data to estimate factor elasticities in the United

States and found results ranging from 0.32 to 1.16. Again, in most cases the results

were significantly below unity. A recent survey by Chirinko (2008) looked at modern

studies of the elasticity parameter and found considerable variation in cross-study results.

However, the weight of the evidence suggested a range of σ that is between 0.4 and 0.6,

with the assumption of Cobb-Douglas being strongly rejected.

One problem with interpreting these cross-study results is that the various analyses

are not all measuring the same thing. The CES production function has a number

of different variants that can be tested using either cross-sectional or time-series data.

As mentioned earlier, the value of σ is dependent on a given capital intensity, a given

marginal rate of substitution, and a given level of per capita production. Klump and de

La Grandville (2000) argue that cross-study results become far more meaningful when

15Felipe and Holz (2001) find that spuriousness makes only a minor contribution to the high R2

in

regressions that use a fitted Cobb-Douglas.

16The notable exception is Mankiw, Romer, and Weil (1992). The authors find that when international GDP growth is estimated using a Cobb-Douglas model with human capital, the fit is quite

good. Felipe and McCombie (2005b) have criticized this result and argue that the good fit is due to an

accounting identity rather than an underlying production function.

19
they are within a “family” of CES functions that differ only in their σ values.

Another reason why the CES results are not robust across data sets is because timeseries estimates of σ are not well measured by least squares regression techniques. Kumar

and Gapinski (1974) used Monte Carlo simulation to study the small-sample properties

of CES estimators and found that the simulated parameters generally performed well

except for estimates of σ, where estimates of the true parameters exhibited enormous

variance and, in one case, bias on the order of 10109. The authors concluded that these

wild results are due to the flatness of the sum of squared error function near the true

parameter values. These results were confirmed by Thursby (1980), who showed that

parameter estimates of σ may not have an expected value and even when the first and

second moments do exist, the variance of this estimator tends to be large.

Building on this work, Pereira (2003) used Monte Carlo simulation to test the reliability of econometric estimates of the CES substitution parameter ρ. The data were

generated from Cobb-Douglas and CES production functions with various specifications

(true parameters). When the underlying data was generated by a Cobb-Douglas production function, the simulated CES estimates consistently did a poor job of identifying

the true parameter. When the true parameter was generated by a CES production function, the reliability of the simulated estimators varied greatly depending on the chosen

model specification. The experiment was rerun but this time a Box-Cox transformation

of the CES estimator was used to identify the true parameters.17 In nearly all cases, the

transformed estimator produced results with less bias and lower variance.

Additionally, there is the issue of econometric identification in production function

estimates. The fundamental idea is nicely captured in an older paper by Marschak and

Andrews (1944, p. 144):

17A Box-Cox transformation can be used to normalize error terms and reduce heteroskedasticity. For

a further discussion see Zarembka (1974).

20
Can the economist measure the effect of changing amounts of labor and

capital on the firm’s output-the “production function”- in the same way

in which the agricultural research worker measures the effect of changing

amounts of fertilizers on the plot’s yield? He cannot because the man power

and capital used by each firm is determined by the firm, not by the economist.

This determination is expressed by a system of functional relationships; the

production function, in which the economist happens to be interested, is but

one of them.

This criticism can be restated in econometrics jargon as a violation of strict exogeneity assumptions. For unbiased coefficient estimates, OLS requires that the regressors be

strictly exogenous: (E[i|x1, · · · , xn]) = 0, where iis an error term and the xi terms are

regressors. However, many of the variables that influence firms’ choice of capital and

labor are unknown to the econometrician, but nevertheless exhibit a systematic pattern.

Studies that simply regress output on the factors of production are subject to Marschak

and Andrews’ critique and are likely to violate this assumption and produce biased estimates. This problem may be overcome using two-stage least squares or a fixed-effects

estimator, but such techniques were uncommon in early studies when these methods

were still in their infancy.

Another source of inconsistency between studies may come from misspecifications of

technical progress. The vast majority of production function studies make the simplifying assumption that technological progress is Hicks-neutral, i.e., it does not change the

marginal products of capital or labor for a given ratio of inputs. Given this assumption,

one can show that Cobb-Douglas is the only functional form that is capable of explaining the U.S. experience of constant factor shares and a rising capital-labor ratio (Antras

2004). This is because Cobb-Douglas is the only functional form in which Hicks-neutral

technical change can be equivalently expressed as labor-augmenting technical change

and only the latter can deliver the theoretical result of a constant labor share that has

approximately been seen in the data. In contrast, empirical studies such as Greenwood,

Hercowitz, and Krusell (1997), Krusell et al. (2000), and Dupuy (2006)) strongly sup21
port the notion that capital-augmenting technology has a significant effect on the U.S.

economy. However, as Acemoglu (2003) has shown, such technology can change labor

income’s long-run share from one level to another, provided that it does not dominate

labor-augmenting technical progress in the long run. Thus, functional forms other than

Cobb-Douglas may be able to explain the presence of constant factor shares alongside a

rising capital-labor ratio.

Choosing the correct functional form in the presence of biased technological change is

challenging because it is impossible to determine whether the evolution of factor shares

and factor ratios over time is due to elasticity effects or technological bias unless a priori

assumptions about the structure of technological change are imposed (Diamond, McFadden, and Rodriguez (1978)). Thus, in order to generate tractable results, economists

are forced to either make very strong assumptions about how technological change affects the overall economy or assume that technological change is Hicks-neutral. If these

assumptions turn out to be unwarranted, this will result in model misspecification and

the resulting bias may be large. Antras (2004) found that in econometric models where

technological change is assumed to be Hicks-neutral, estimates of the elasticity of substitution between capital and labor are biased toward unity. If the economy’s underlying

technology turns out to be labor or capital augmenting, then the values of the technological parameters AK and AL cannot be estimated and the resulting omitted variable

bias pushes the estimated elasticity toward unity. If one assumes that AK and AL both

grow at constant rates, then estimates of σ are significantly below unity.18

Finally, there is the issue of short-run versus long-run analysis. A recent paper by

Charles Jones (2003) presents readers with four stylized facts:

1. Growth rates in U.S. per capita GDP have not shown a major trend for the last

18Given the difficulty of directly measuring the rate of technological progress in the economy, this

assumption may be equally unwarranted. Additionally, this specification should produce an upward or

downward trend in labor’s share.

22
125 years.

2. The capital share shows a significant trend in many countries and in many U.S.

industries over time.

3. Elasticity of substitution estimates are often below unity.

4. The price of capital goods in the “equipment”category e.g. computers, machine

tools, has been falling relative to the price of nondurable consumption goods, where

the falling price is taken to indicate that technological progress is being embodied

in capital goods at a faster rate than in consumption goods.

Taken together, these results are somewhat puzzling because a well-known theorem

in growth theory states that if a neoclassical growth model possesses a steady state with

positive growth and a growing capital share, either technological change must be laboraugmenting or the production function must be Cobb-Douglas. Jones (2003) believes this

result can be reconciled with the above facts if production functions are Cobb-Douglas

in the long run, but take some other form in the short run. While this hypothesis seems

plausible, it is undermined by a recent survey by Chirinko (2008), who finds that in

most studies where σ is estimated by techniques that explicitly focus on the long-run

position of the economy, the value of σ is still significantly below unity.

5. Conclusion

After surveying the literature on macroeconomic production functions, there does not

appear to be overwhelming evidence that would lead one to choose the CES over the

Cobb-Douglas for forecasting GDP and income shares. When empirical estimates are

restricted to the Cobb-Douglas form, the fit tends to be quite good. However, there

are strong theoretical reasons why this fit is likely due to an accounting identity rather

23
than an underlying production function. To some extent, the CES specification gets

around this problem because it allows for more degrees of freedom. However, it still

suffers from serious identification problems. The generality of the CES is also a liability

for forecasting purposes because it is difficult to get an estimate of σ that is entirely

consistent across studies.

Aggregation issues pose a serious theoretical challenge to both functional forms and

one should be aware of these problems in order to avoid making strong inferences that

cannot be justified. In spite of this, the Cobb-Douglas has in the past been able to

produce reasonably accurate long-term economic forecasts. Thus, despite the strong

theoretical case against the continued use of Cobb-Douglas, it does not appear that

using a CES framework would significantly improve the CBO’s ability to forecast GDP

and income shares. For analysis of policies affecting factor returns, such as taxes on

capital and labor income, however, the Cobb-Douglas specification is too restrictive.

24
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