IO FOR EXPORT(S)∗Monika Mrázová†J. Peter Neary‡

University of Geneva,

University of Oxford,

CEPR and CESifo

CEPR and CESifo

December 3, 2019

Abstract
We provide an overview and synthesis of recent work on models of monopolisticcompetition with heterogeneous firms in international trade, paying particular attention to competition effects, pass-through, selection effects, and linking distributions offirm characteristics and outcomes. A recurring theme is that CES preferences are extremely convenient for deriving analytic results, but also extremely restrictive in their
theoretical and empirical implications. We introduce the class of “constant-responsedemand functions” to describe some related families of demand functions that provide
a unifying principle for much recent work that explores alternatives to CES demands.
Keywords: Heterogeneous Firms; Pass-Through; Quantifying Effects of Globalization; Super- and
Sub-Convexity; Supermodularity.
JEL Classification: F12, L11, F23∗

This is a revised version of a plenary lecture presented at EARIE 2018, the 45th Annual Conference of
the European Association for Research in Industrial Economics, Athens, September 2018. We are grateful toparticipants on that occasion, at the Research Workshop on International Trade in Villars, Switzerland, the2019 IEE Research Day in Geneva, a conference on International Economic Integration in Tübingen, the 2019
CESifo Area Conference on Global Economy in Munich, ERWIT 2019 in London, SAET 2019 in Ischia, the2019 Hitotsubashi Summer Institute in Tokyo, EEA 2019 in Manchester, ETSG 2019 in Bern, and a seminar
at University College Dublin. We also thank the editor, Paul Heidhues, two referees, Paolo Bertoletti, Luis
Cabral, Toshihiro Okubo and John Vickers, for helpful comments and discussions. Monika Mrázová thanks
the Fondation de Famille Sandoz for funding under the “Sandoz Family Foundation - Monique de Meuron”
Programme for Academic Promotion. Peter Neary thanks the European Research Council for funding under
the European Union’s Seventh Framework Programme (FP7/2007-2013), ERC grant agreement no. 295669.
†
Geneva School of Economics and Management (GSEM), University of Geneva, Bd. du Pont d’Arve 40,
1211 Geneva 4, Switzerland; e-mail: monika.mrazova@unige.ch.
‡Department of Economics, University of Oxford, Manor Road, Oxford OX1 3UQ, UK; e-mail: peter.neary@economics.ox.ac.uk.

1Introduction

In this paper we aim to provide an overview of recent research in international trade, from theperspective of the theory of industrial organization (henceforth IO). There are two aspects tothis perspective, mirroring the two different interpretations of our title. On the one hand, weare interested in “IO for Exports”: what is the structure of export markets?; how does thatstructure affect which firms export?; does trade fosters competition?; and a host of othersimilar questions. On the other hand, we want to think about “IO for Export”: which IO
models are used in trade? As is well-known, the answer to the latter question is not any ofthe many varieties of oligopoly used in IO itself, but rather the monopolistically competitivemodel of Chamberlin (1933), as formalized by Dixit and Stiglitz (1977); especially with
Constant-Elasticity-of-Substitution (CES) preferences, introduced as a special case by Dixit
and Stiglitz (1977), brought to center stage by Dixit and Norman (1980) and Krugman(1980) among many others, and extended to firm heterogeneity by Melitz (2003). Ironically,
though monopolistic competition originated in IO, it is used relatively rarely there, so it
could be described as “IO for export only.”1
While monopolistic competition is by far the dominant paradigm in international trade,
it is by no means the only approach. First there is a large literature on “strategic” trade
policy, dating from the 1980s, that uses off-the-shelf IO models of oligopoly to address
standard issues of trade policy, mostly at the level of a single industry.2 Second, there
have been some attempts to explore the implications of oligopoly in general equilibrium for
trade questions, both of the traditional kind such as trade patterns, gains from trade andpricing-to-market,3 and also with applications to topics such as cross-border mergers and
1

See Neary (2003b) for further discussion. Of course, studies of monopolistic competition, in both CES
and non-CES cases, are not completely absent from IO. Examples include advanced textbook treatments byAnderson et al. (1992) and Vives (1999), as well as studies of existence by Pascoa (1993) and Pascoa (1997),
and of optimality by Kuhn and Vives (1999).
2
See Brander (1995) for an overview, and Mrázová (2011) and Mrázová et al. (2013) for applications to
trade agreements.
3
See, for example, Neary (2003a), Atkeson and Burstein (2008), Neary (2016), Gaubert and Itskhoki(2018), and Nocke and Schutz (2018).

multi-product exporters.4 Finally, there is a small literature on the theory of superstar firmsthat compete oligopolistically, while coexisting with a monopolistically competitive fringe.5
However, there are good reasons why the monopolistically competitive paradigm has
remained dominant in trade. The availability of large data sets on exporting firms, andthe desire to allow for entry and exit and to take account of general-equilibrium feedbacks
to and from factor markets, make bespoke modelling of individual sectors infeasible, and a“one-size-fits-all” approach to the choice of market structure very convenient.
Two general themes emerge from our analysis, one old, one new. First, the widespreadassumption of CES preferences, implying demand functions that are log-linear in price,
is extremely convenient for deriving analytic results, but also extremely restrictive in its
theoretical and empirical implications. Second, we introduce the class of “constant-responsedemand functions” to describe some related families of parametric demand functions that
imply a constant absolute or relative response of some firm-level variable to an exogenouschange in marginal cost. As we shall see, this class of demands, all of which nest the CES
case, provides a unifying principle for much recent work, including our own, that explores
alternatives to CES demands.6
The plan of the paper is as follows. Section 2 introduces the basic underlying modeland an approach to comparing different demand functions that we use throughout. It also
discusses recent evidence that questions a key implication of CES preferences. Each of thefour subsequent sections considers a substantive question, and asks, which demand functions
arise naturally in trying to answer it? As we show, each question can be formulated interms of a constant response of a target function to a change in costs, and so we are led
to seeking demand functions that are consistent with such a constant response. Section 3
considers the central topic of competition effects, and shows that the CES demand function
4

See, for example, Neary (2007), Eckel and Neary (2010) and Eckel et al. (2015).
See, for example, Neary (2010), Shimomura and Thisse (2012), Parenti (2018), and Cabral (2018).
6
For other approaches to going “beyond CES,” see Zhelobodko et al. (2012), Bertoletti and Epifani (2014),
Parenti et al. (2017), Dhingra and Morrow (2019), and Matsuyama and Ushchev (2017). For the most part,these papers consider non-parametric families of demand functions, whereas in this paper we focus on families
with explicit functional forms.
53

is a key benchmark in categorizing them, both at the firm level in the cross-section and ingeneral equilibrium when the economy responds to exogenous shocks. Section 4 considers
the issue of how cost changes are passed through to goods prices, and introduces the BulowPfleiderer (B-P) and Constant-Proportional-Pass-Through (CPPT) demand functions that
correspond to constant rates of absolute and proportional pass-through respectively. Section 5 turns to consider the central topic of selection effects, and introduces the ConstantElasticity-of-Marginal-Revenue (CEMR) demand functions that provide a natural reference
point for understanding them. Section 6 explores the relationship between the distributionof firm productivity, on the one hand, and the distributions of the level and growth rate ofsales, on the other; and it introduces the Constant-Revenue-Elasticity-of-Marginal-Revenue(CREMR) demand functions that arise naturally in this context. Finally, Section 7 provides
an overview of the constant-response demand functions introduced in earlier sections, andintroduces a general demand function that nests three of them. Section 8 concludes. Proofs
and further details can be found in our other papers: Mrázová and Neary (2017) for Sections 2, 3 and 4, Mrázová and Neary (2019) for Section 5, and Mrázová et al. (2015) for
Section 6.

2

Preliminaries

2.1

A Core Model

We begin by presenting a simple core model of a monopoly firm. This serves to introduce
the notation and setting that we will use throughout the remainder of the paper.
Consider a monopoly firm facing an inverse demand function p(x), giving price as astrictly decreasing function of firm output x. This is consistent with monopolistic competition where the firm takes the “perceived” demand function as given. In general equilibrium,
the demand function has extra arguments: we will see examples of that later. As for technology, we adopt the simplest specification popularized by Dixit and Stiglitz (1977): all firms
4incur a fixed cost f, often assumed to be constant across firms, and a constant marginal
cost of production c.7 Profit maximization therefore implies simple first- and second-orderconditions:p + xp0 = c and 2p0 + xp00 < 0(1)

In words, marginal revenue must equal marginal cost, and marginal revenue must be declining
in output at the optimum. It will prove convenient to write these conditions in terms of two
key demand parameters:

ε(x) ≡ −xp00 (x)
p(x)
>
0
and
ρ(x)
≡
−
xp0 (x)p0 (x)(2)

Here ε(x) is the demand elasticity and ρ(x) the convexity of demand; these are unit-freemeasures of the slope and curvature of the demand function, respectively. Reexpressing thefirst- and second-order conditions in (1) in terms of these parameters yields, first, a simple
expression linking the price-cost margin or markup, p/c, to the elasticity of demand, and,second, a restriction that convexity must be less than two:
ε
p
=
c
ε−1

2.2

and ρ < 2(3)

Demand Functions and Demand Manifolds

Throughout the paper, we will see many different parametric forms of demand function, and
it is desirable to have some way of comparing them. As is foreshadowed in the previous
section, for many questions of interest only the elasticity and convexity of demand matter.
Hence it makes sense, following Mrázová and Neary (2017), to illustrate different demand
7
Dixit and Stiglitz should not be held responsible for the widespread use of CES preferences: theypresented them only as an example, albeit with the unique implication of constrained efficiency (see their
Section I), and each of them is on record as expressing reservations about them: see Dixit (2004) and Stiglitz(2004). However, they deserve credit for popularizing, and perhaps making respectable, the very simple{f, c} technology. Ironically, this is another example of IO for export only: it is now ubiquitous in trade,
whereas more complicated cost functions are often used in the IO literature.

5





4.0

3.0

2.0

1.0



0.0
-2.0

-1.0

0.0

1.0

2.0

3.0



Figure 1: The Admissible Region in Convexity-Elasticity Space

functions in the space of these two parameters, as in Figure 1. The shaded area represents
the admissible region implied by the restrictions in equation (3): at a profit-maximisingequilibrium, a monopoly firm must be at a point on its demand function where the elasticity
is greater than one, and the convexity is less than two.8
The space illustrated in Figure 1 may seem unfamiliar at first, so it is desirable to havesome landmarks. One such is the linear demand curve: it implies that convexity is zero,and so it is represented by the vertical line at ρ = 0 in Figure 2. It turns out that a more
important benchmark is that of CES demands:

p(x) = βx−1/σ⇒ε=σand ρ =

σ+1
σ

(4)

This is the only exception to the general rule that both ε and ρ vary with sales: in the CES
case their values are uniquely determined by the elasticity of substitution σ. Eliminating
σ from those expressions gives the relationship between elasticity and convexity implied by8
Of course, the admissible values of ε are unbounded above and those of ρ are unbounded below. The
range of values illustrated in the figures is broadly in line with the available empirical evidence: see forexample Mrázová and Neary (2017), footnote 8 and Section IV.A.

6

CES demands:
ε=

1
ρ−1

(5)

This relationship is illustrated by the curve labelled “CES” in Figure 2; it ranges from an
asymptotically infinite value of ε as ρ approaches one to a value of ε equal to one when ρequals two. Each point on this curve represents a particular CES demand function corresponding to a particular value of σ; for example, the point {ρ = 2, ε = 1}, just on theboundary of the admissible region, corresponds to the Cobb-Douglas special case.
Translog

4.0

CES

3.0
Linear
2.0
CARA
1.0

LES

0.0
-2.0

-1.0

0.0

1.0

2.0

3.0



Figure 2: Demand Manifolds for Some Common Demand Functions

The linear and CES demand functions are not the only ones that can be illustrated in(ρ, ε)-space. As shown in Mrázová and Neary (2017), any demand function can be illustrated
in this space by a smooth curve, which they call the “demand manifold” corresponding to
the demand function. (The only exception is CES demands, where as we have seen the
manifold is a single point.) Figure 2 illustrates the manifolds for three other widely-useddemand functions: the negative exponential or CARA (“constant absolute risk-aversion”)
system, the Stone-Geary LES (“linear expenditure system”), and the translog (which from afirm’s perspective is observationally equivalent to the “Almost Ideal” system of Deaton and7Muellbauer (1980)).9 (Explicit expressions are in Appendix B.)
There are a number of advantages to considering demand functions through the lensof their manifolds. First, different demand functions can be directly compared with eachother. For example, Figure 2 shows that, despite their very different origins and systemwide properties, all three of CARA, LES and translog demands have properties at the firmlevel that are midway between those of linear and CES demands. In particular, for a given
demand elasticity they are all less convex than CES demands, though they are more so thanlinear demands.
Second, demand manifolds make it possible to relate assumptions about demand directly
to comparative statics properties: we will see examples of this later. Finally, though thelocation of a demand function in (x, p)-space is affected by the values of all its parameters,
the location of its manifold in (ρ, ε)-space is often unaffected by, or invariant to, the values ofthese parameters. Mrázová and Neary (2017) call this property “manifold invariance”, andprovide necessary and sufficient conditions for it to hold. Not all demand functions exhibit
this property with respect to some of their parameters, far less to all of them. The latter istrue of the CES for example: the CES point-manifold is invariant with respect to changes in
the level parameter β but not to changes in the elasticity of substitution σ. However, many
demand functions have manifolds that are invariant to some of their parameters, and some(including all those other than the CES in Figure 2) have manifolds that are invariant toall of them. This makes it much easier to think about the implications of different demand
assumptions using the demand manifold than the demand function itself, since the latter isnever invariant to any parameter changes.
9

The non-CES demand functions whose manifolds are shown in Figure 2 have been explored in monopolistic competition with heterogeneous firms by a number of authors. Melitz and Ottaviano (2008) consider
linear demands; Behrens and Murata (2007) CARA demands; Simonovska (2015) the LES; and Feenstra
and Weinstein (2017) the translog.

8

493

PRICES, MARKUPS, AND TRADE REFORM

2.3

Evidence on Variable Markups

Of the demand functions in Figure 2, the CES is by far the most widely-used in international
trade. However, it makes implausible empirical predictions, in particular that markups
are always the same, and depend only on the preference parameter σ: p/c = σ/(σ − 1).
International trade economists have always been uneasy with these implications, even more
so in recent years as persuasive evidence against them has become available.

FIGURE 4.—Distribution of marginal
costs1989
and markups
in 1989 and 1997. Sample only in(a) India,
and 1997
cludes firm–product pairs present in 1989 and 1997. Outliers above and below the 3rd and 97th
percentiles are trimmed.

(b) Chile, 2001-2007

Figure 3: Empirical Evidence on Markup Distributions
Deoffset
Loeckeral. (2016)
Lamorgese et al. (2014)
higher markups. The results indicateSources:that firms
the etbeneficial
costandreductions from improved access to imported inputs by raising markups. The overall
effect, taking into account the average declines in input and output tariffs between 1989 and 1997, is that markups, on average, increased by 12.6 percent.
A key reference is De Loecker et al. (2016), who calculate markups for a sample of IndianThis increase offsets almost half of the average decline in marginal costs, andas a result, the overall effect of the trade reform on prices is moderated.52
without
making priorassumptions
aboutthemarketAlthough firms
tempting,
it is misleading
to draw
conclusions aboutpro- structure or the form of demand.
competitive effects of the trade reform from the markup regressions in ColFigure3(a)fromis theirpaper
thatfortheumn 3 of Table
IX. Thereasonthat oneneedsshows
to control
thedistribution
impacts of of markups is very far from being
52

concentrated at a single value. A possible explanation is that such markup heterogeneity

These results are robust to controlling India’s de-licensing policy reform; see Table A.I in the
Supplemental Material.

arises from aggregation across sectors with different elasticities of substitution. However,
Figure 3(b) from Lamorgese et al. (2014), who use data on Chilean firms, show that heterogeneity persists when the data are disaggregated by sector. This evidence clearly rejects the
hypothesis of monopolistic competition with CES preferences. One way to allow for variablemarkups is to retain CES preferences but assume an oligopolistic market structure as inAtkeson and Burstein (2008). In this paper we explore instead how non-CES preferences in9monopolistic competition have been used to throw light on a variety of substantive questionsin international trade.

3

Competition Effects

The first substantive topic we wish to discuss is that of competition effects. The term“competition effects” can be used in two different senses in this context.10 On the one hand,it can refer to the fact that markups vary across firms in the cross-section. On the other
hand, it can refer to the fact that any exogenous shock to industry equilibrium affects firms
indirectly by changing the competitive environment in which they operate, in addition to its
direct effect on individual firms. As we will see, CES demands are a key reference point forcompetition effects in both senses: with CES demands, competition effects do not arise in
the cross section, while, in response to a change in market size, the direct effect of the shockon firm profits is exactly offset by the induced general-equilibrium competition effect.

3.1

Competition Effects in the Cross Section

Consider first competition effects in the cross-section sense. To understand how they operate,consider Figure 4, which repeats the CES locus, henceforward labelled “SC”, from Figure 2.
This locus divides the admissible region in (ρ, ε)-space into two sub-regions that imply very
different comparative statics properties. Crucially, they differ in how the elasticity of demandvaries with firm output:
ε
εx =
x



ε+1
ρ−
ε


(6)

This is negative when ρ is less than its CES value of σ+1. (Recall equation (4).) Mrázová
σ
and Neary (2019) call demand functions with this property “subconvex”, and in Figure 4
10

See, for example, Krugman (1979), Melitz (2003), Melitz and Ottaviano (2008) and Zhelobodko et al.
(2012).

10



SC

4.0

SuperConvex

Sub-Convex

3.0

2.0

1.0

0.0
-2.0

-1.0

0.0

1.0

2.0

3.0



Figure 4: The Sub- and Super-Convex Regions

they are represented by the set of points in the region to the left of the SC locus.11 Whenthe demand curve exhibits this property, larger firms face a lower elasticity of demand andhence have higher markups in the cross section, and all firms enjoy higher profit margins
as they move down their demand curves over time. (The arrows in the figure indicate how
elasticity changes as firm output rises.) In the converse case, of superconvexity, all theseproperties are reversed. However, subconvexity is theoretically more plausible, both on
introspective grounds (see Marshall (1920));12 and also in that it leads to more plausible
predictions (see Dixit and Stiglitz (1977) and Krugman (1979)). Hence we can conclude
that CES demands rule out competition effects, and subconvex demands are necessary and11

Formally, subconvexity is a local property, and is defined as log p(x) being concave in log x at a point.
Mrázová and Neary (2019) show that this property is equivalent to those stated in the text. It is possible forthe same demand function, with given parameters, to be subconvex for some levels of output and superconvex
for others. (For an example, see Mrázová and Neary (2017), Online Appendix B10.) However, this property
is rare, at least as far as most widely-used demand functions is concerned. Moreover, as the argument in
the text shows, the manifold implied by such a demand function must be horizontal at the point where it
crosses the SC locus, since it is falling in output to the left and rising to the right: see the horizontal arrows
in Figure 4.
12
In Chapter 3 of Book 4 of his Principles, Marshall introduced his “law of demand” – in modern parlance,
demand curves slope downwards – while in Chapter 4 he introduced what he called “the general law ofvariation of the elasticity of demand”: “The elasticity of demand is great for high prices, and great, or atleast considerable, for medium prices; but it declines as the price falls; and gradually fades away if the fallgoes so far that satiety level is reached.” Hence the property is sometimes called “Marshall’s Second Law of
Demand.”11

sufficient for monopolistic competition to exhibit the empirically plausible case where larger
firms have higher markups.

3.2

Competition Effects of a Globalization ShockConsider next the second sense of competition effect: the general-equilibrium effect of an
exogenous shock on the competitive environment faced by all firms. An important andinteresting example of this is the effects of globalization, in the sense of an increase in thesize of the global economy.
To see how competition effects arise following a globalization shock, we need to go beyond
the “firm’s eye view” we have adopted so far, and consider the determination of industry
equilibrium.13 Also, since we want to allow for firms to sell in many markets, we distinguishfor the first time between their total output y and the amount they sell to an individual
consumer x. Following Krugman (1979), we take the simplest setting, where all consumers
are identical, there are L of them in each country, and the world consists of k identicalcountries, so y = kLx. In addition, though this is not needed for all our results, it isconvenient to assume that preferences are additively separable, so the demand function for atypical good is given implicitly by the consumer’s first-order condition u0 (x) = λp, where λis the marginal utility of income.14 With these assumptions, we can write the ex-post profits
of a typical firm as follows:
π(c , λ, k ) ≡ max p(y, λ, k) − c y
− − +

y

where p(y, λ, k) = λ−1 u0 (y/kL)(7)

In previous sections we considered the demand function from a firm’s perspective only. Now
13This section is based on Mrázová and Neary (2017), Section III.B. See also Zhelobodko et al. (2012),
Bertoletti and Epifani (2014), Mrázová and Neary (2014), and Bache and Laugesen (2015).
14
Writing the demand for each good as p(y, λ, k), a function of its own quantity consumed, a single demandside aggregate λ, and the exogenous shock k, corresponds to generalized additive separability, introduced
by Pollak (1972). Our results, up to and including equation (10) below, hold in this case, though theirinterpretation is simpler when, as in (11), we specialize to additive separability, which implies that λ equals
the marginal utility of income.

12

we want to solve explicitly for the industry equilibrium, so we need to consider demand
determinants additional to their sales. In (7) these are k, the exogenous globalization shock,equal to the number of identical countries in the world, and λ, the marginal utility of income
under additive separability, which plays the role of the level of competition in the industry.
This is exogenous to each firm, but is determined endogenously in industry equilibrium by
the free-entry condition, which requires that an entrant has zero expected value. Denotingby v̄(λ, k) the expected value of firm profits, this can be written as follows:Z cv̄(λ, k ) ≡− +v(c, λ, k)g(c) dc = fe ,v(c, λ, k) ≡ max (0, π(c, λ, k) − f )(8)cHere g(c) is the distribution of firm marginal costs. Prospective firms know this but do notknow their own marginal cost ex ante; to find that out they have to pay a sunk entry costfe in order to make a draw from the distribution. This sunk cost must therefore equal the
expected return to sampling from g(c), which with rational expectations and risk neutrality
equals v̄(λ, k).
We can solve for the effects of globalization on every firm’s profits by totally differentiating(7):∂ log π(c, λ, k) ∂ log π(c, λ, k) d log λd log π(c, λ, k)=+d log k∂ log k∂ log λd log k
|
{z
} |
{z
}
(M )

(9)

(C)

where the change in the level of competition is determined in turn by (8):d log λ∂ log v̄(λ, k)=−d log k∂ log k

∂ log v̄(λ, k)∂ log λ(10)Equation (9) shows that globalization is a double-edged sword from the perspective of firms.
On the one hand it has a market-size effect, given by (M), which tends to raise each firm’s
profits. On the other hand it has a competition effect, given by (C): because all firms’ profits
rise at the initial level of competition, the latter must increase to ensure that the expectedvalue of a firm remains equal to the fixed cost of entry; this in turn tends to reduce each firm’s
13profits. The net outcome is indeterminate in general. However, with additive separability,
equation (9) takes a particularly simple form:
ε(c, λ, k)d log π(c, λ, k)= 1 −d log kk)|{z} | ε̄(λ,
{z }
(M )

(11)

(C)

Now the market expansion effect is just one: other things equal, a 1% increase in market size
leads to a 1% increase in the profits of each firm. As for the competition effect, it depends
on the ratio of the demand elasticity the firm faces to ε̄, which is defined as follows:Z cε̄(λ, k) ≡v(c, λ, k)ε(c, λ, k)g(c) dcv̄(λ, k)c

(12)Here ε̄ , which we can interpret as the elasticity faced by the average firm. Higher competition
reduces an individual firm’s profits by ε, while it reduces the average value of a firm by ε̄.
Recalling from (6) that elasticity falls with output if and only if demand is subconvex, we
see from (11) that the ambiguity in the general case is resolved in a very simple way whenpreferences are additively separable: globalization raises profits of larger firms and reduces
those of smaller firms if and only if demands are subconvex.






T0

f

A

T1



Figure 5: The Matthew Effect of Globalization

14

Figure 5 illustrates these effects in the case of subconvexity. The curve labelled Π0shows the initial profit profile across firms of different productivity, while that labelled Π1shows the profit profile after globalization, assuming that demand is strictly subconvex.
The productivity level denoted T0 is that corresponding to threshold firms in the initial
equilibrium: they make zero profits and so are indifferent between producing or not; whilethat denoted A corresponds to the average firm that faces the demand elasticity ε̄. As canbe seen, in line with equation (12), the direct, market-expansion, effect dominates for firmsof above-average size, ε < ε̄, so they expand; the indirect, competition, effect dominates forfirms below average size, ε > ε̄, so they contract; and threshold firms cease to be profitableand exit. One way of expressing this outcome is that, with subconvex demand, globalization
has a clear “Matthew Effect” on profits, in the sense that “To those who have, more shallbe given.”15 An important consequence of these responses is that the average productivityof active exporters rises. Finally, recall that all these effects are reversed with superconvex
demands. Moreover, the CES case is very special: the direct market-size effect and theindirect competition effect of globalization exactly offset each other, so the profit profile in
Figure 5 is unaffected.

4Pass-ThroughThe next substantive question we address is that of pass-through from costs to prices. The
topic is of interest in itself but also as an illustration of how differently the same problem isaddressed in IO and in international economics.

4.1

Pass-Through in Industrial Organization

The archetypical pass-through question posed in industrial organization is both simple andimportant: if marginal cost rises by a euro, by how much will a profit-maximizing firm
15
In Mrázová and Neary (2017) we show that, subject to a technical qualification, the same effects arise
for the profiles of outputs and prices: firms with above-average productivity sell more at lower prices.

15raise its price; i.e., how large is dp/dc? The answer is easy to calculate from the first-ordercondition in (1):16
p + xp0 = c ⇒

1
dp
=
dc
2−ρ

(13)

Thus a shock to costs is passed through to prices by more the more convex is the demand
function. Moreover, the threshold for full, or “euro-for-euro”, pass-through is that the degreeof convexity is exactly one:
ρ−1
dp
−1=
=0
dc
2−ρ

(14)

Demand functions implying constant pass-through are members of the Bulow-Pfleidererfamily, due to Bulow and Pfleiderer (1983):17
 1−κ

κ
p(x) = β x
+γ ,

β

1−κ
<0
κ

(15)

This implies a constant pass-through coefficient equal to κ: dp/dc = κ. Special cases ofthis include linear demand, with convexity ρ equal to zero and a constant pass-through ofκ = 1/2; and log-linear direct demand, p(x) = β 0 (log x + γ 0 ), corresponding to a limitingcase of (15) with exactly one-for-one pass-through, κ = 1.18 The latter demand function
has convexity ρ equal to one, and so is the demand function implied by the condition in(14) holding globally. In general, values of κ > 1 and ρ > 1 correspond to log-convex direct
demand, and values of κ < 1 and ρ < 1 to log-concave demand.
16

This condition can be found in Cournot (1838), p. 79, which even allows for variable marginal cost. See
also Bontems (2019).
17This demand function is more familiar when written, in the notation of Bulow and Pfleiderer, as p =α − βxδ , when δ 6= 0, and p = α − β log x. We write it differently to facilitate comparison with other demand
functions to be introduced later.
κ18and γ = γ 0 1−κThe limit has κ → 1, β → ∞, and γ → −1. To see this, set β = β 0 1−κκ − 1 and evaluate
the limit when κ → 1, using for example a limited expansion.

16

4.2

Pass-Through in International Economics

Pass-through is also widely studied in international economics, both pass-through from tariffs
to domestic prices in international trade, and pass-through from exchange rates to domestic
prices in international monetary economics. However, the approach is totally different fromthat in industrial organization: compare for example Gopinath and Itskhoki (2010) with
Weyl and Fabinger (2013), both important papers that refer to large literatures but have
only a single reference in common.19 It is not merely the answers that differ between the two
fields, but the questions too. In IO the focus is on absolute or euro-for-euro pass-through as
we have seen, whereas in international economics the question posed relates to proportional
pass-through: how large is d log p/d log c? Both approaches make perfect sense in theirrespective contexts: IO scholars are typically analysing the effects of a cost or tax change ona single industry, whereas in international economics the focus is on a change in policy or
exchange rates at the economy-wide level. The first question is a partial-equilibrium one, so
it is natural to work with absolute changes, whereas the second is a general-equilibrium one,mandating a focus on relative prices and proportional changes. However, it is important tounderstand the differences between the two.
Once again, the expression for proportional pass-through can be derived from the firstorder condition (1):ε−1 1d log p=d log c
ε 2−ρ

(16)

Unlike the corresponding term for absolute pass-through, (13), this depends on the elasticity
of demand as well as its convexity. So, the threshold for full (100%) proportional passthrough is:
d log pερ − ε − 1−1==0d log c2−ρ(17)This is equivalent to the demand function taking the CES form, as we have already seenthat only in that case is the ratio p/c constant. (To show this explicitly, substitute for the19

The papers cite 41 and 86 references respectively, of which only Atkeson and Burstein (2008) is common
to both.

17

convexity of CES demand from (4) into (17).) This suggests that pass-through will be morethan 100% if and only if the demand function at the initial equilibrium is more convex thana CES demand function. We will return to this below.
We saw in the previous sub-section that an important benchmark is the Bulow-Pfleiderer
demand family, which implies constant absolute pass-through. So it is natural to ask if there
are demand functions other than the CES that imply constant proportional pass-through(CPPT): d log p/d log c = κ, κ 6= 1. The answer to this question is the CPPT family,
introduced in Mrázová and Neary (2017):20

p(x) =κ κ−1β  κ−1,x κ +γx

β > 0, κ 6= 1

(18)This family also has the property that it implies a constant proportional response of markups
to marginal costs: d log(p/c)/d log c = κ − 1.

4.3

Comparing Absolute and Proportional Pass-ThroughCasual inspection suggests that the CPPT demand functions in (18) are very different fromthe Bulow-Pfleiderer demands in equation (15). However, it is difficult to compare the two
directly: they have different functional forms with three parameters each. Instead, we use
the approach introduced in Section 2.2 of comparing the demand manifolds implied by thedemand functions. This also allows us to compare visually the conditions that imply constantabsolute and proportional pass-through.
By calculating the convexity of the Bulow-Pfleiderer demand function in equation (15),
we can show that the implied demand manifold is given by:

ρ(ε) = 2 −

1
κ

(19)

which is independent of ε. Similarly, by calculating both the elasticity and convexity of the20

The parameter γ must be strictly positive to ensure that the elasticity of demand exceeds one.

18

CPPT demand function in equation (18), the implied demand manifold is given by:

ρ(ε) = 2 −

1 ε−1
κ ε

(20)

These manifolds are illustrated for different values of κ in the two panels of Figure 6.




SC

4.0

SC

4.0

 = 1.0
3.0

3.0

 = 0.33

 = 0.5

 = 1.0

 = 2.0

 = 0.2

2.0

2.0

1.0

1.0

 = 0.3

 = 0.5

 = 2.5

 = 0.1

0.0
-2.0

-1.0

0.0

1.0

2.0

3.0

0.0



-2.0

(a) Bulow-Pfleiderer Demands

-1.0

0.0

1.0

2.0

3.0



(b) CPPT Demands

Figure 6: Constant Pass-Through Demand Manifolds

It is clear that the manifolds are much easier to compare than the corresponding demand
functions. Whereas each demand function has three distinct parameters, each manifold isinvariant with respect to two of these parameters, and depends only on the pass-throughcoefficient κ. The manifolds show clearly how demand adjusts as output rises (which in thesubconvex region is equivalent to the elasticity falling). Bulow-Pfleiderer demands have constant convexity (given the one-to-one relationship between demand convexity and absolutepass-through from (13) and (19)), while CPPT demands become more convex as elasticity
falls.
The demand manifolds in Figure 6 are not only the manifolds implied by the BulowPfleiderer and CPPT demand functions, they are also contour lines corresponding to different
values of κ, the degree of either absolute or proportional pass-through. As a result, they
can be combined with the demand manifolds for any other demand functions to read off the19rates of pass-through implied by those demands. For example, recalling the CARA, LES
and translog demand manifolds in Figure 2, we can see that all three imply a higher level ofabsolute pass-through for large firms; LES demands imply a constant rate of proportional
pass-through for all firms, equal to 0.5, whereas CARA demands imply that the rate ofproportional pass-through decreases with firm size, and translog demands imply that it
increases.

5

Selection Effects

We turn next to one of the most distinctive and novel features of models with heterogeneousfirms: their ability to predict which firms will select into which activities. Consider first
the question of which firms will choose to export. Here the key prediction of the Melitz(2003) model is that more productive firms select into exporting. It turns out that this isan extremely robust result. It requires only that ex post profits π are decreasing in c, whichmust be true in all models of this kind. To see this, write the maximized value of operating
profits as follows:π(τ, c) ≡ max π̃(x, τ, c),π̃(x, τ, c) = (p(x) − τ c)x

x

(21)

where τ is an iceberg trade cost: τ units must be produced and shipped to the export market
in order for one unit to arrive. For simplicity, assume that π(τ, c) is continuous in {τ, c}.
Differentiating (21) and invoking the envelope theorem, we can see that operating profits
are unambiguously decreasing in c:

πc = π̃c = −τ x < 0(22)This result is independent of the form of the demand function; in particular, it is not sensitive
to whether demands are CES or not. Hence, ranking firms by their productivity, or inverse

20

marginal cost, unambiguously predicts their ranking by total profits, Π = π − f . So there
must be a unique threshold level of productivity, with all firms more productive than thethreshold choosing to export and all those less productive choosing not to.



 fE

F
First-Order
Selection Effects

c1E
Exit

Second-Order
Selection Effects

c1S

E

c1F

Exports

c1
FDI

 fF
Figure 7: Selection into Exports versus FDI

Consider next the case where each firm has the option of choosing to serve a market in
two alternative ways, either by exporting (E) or by engaging in foreign direct investment(FDI). Firms therefore face a “proximity-concentration trade-off”: FDI entails operating aforeign plant and selling to foreign consumers without incurring the trade cost, so τ = 1;
however, it foregoes the benefits of concentrating production at home, since it incurs a fixed
cost fF that is higher than the fixed cost of exporting fE :Π(τ, c) = max ΠF (c) = π(1, c) − fF

(23) ΠE (τ, c) = π(τ, c) − fE
The outcome under CES demand has been shown by Helpman et al. (2004) and is illustrated in Figure 7. Profits under both regimes are linear in an increasing power function of
productivity; profits under FDI are lower than under exporting for the lowest-productivity
firms, since FDI incurs a higher fixed cost; however, profits under FDI increase faster with21

productivity because FDI incurs lower access costs; and the two profit schedules intersect
only once as shown.21 As the lower set of labels indicates, there is now three-way selection,with the least efficient firms not serving the market as before, firms of intermediate efficiency
exporting, and the most efficient firms engaging in FDI.
However, Mrázová and Neary (2019) show that this predicted selection pattern is lessrobust than that for the choice between exporting or not. To see why, consider the upper
set of labels in Figure 7. These categorise productivity levels not in terms of their predicted
outcomes but in terms of the choices they offer to firms. Low-productivity firms, with costs
higher than cS (represented by points to the left of c1−σin the figure), face the same choiceS
as before: FDI is not profitable, so the only decision they face is whether to export or not.
Mrázová and Neary (2019) call the outcomes of such a decision a “first-order selection effect”:
firms face a choice between one activity, exporting, whose profits vary with productivity, andanother, not exporting, whose profits are the same for all firms. This is in contrast to the“second-order selection effect” that arise from the choices of high-productivity firms withcosts lower than cS . For such firms both exports and FDI are profitable, and their profitability
in both modes varies with productivity. Hence, the conventional sorting, where more efficientfirms select into FDI, is only guaranteed if the two curves have a single intersection as in
Figure 7. With a slight abuse of terminology we will refer to this configuration as the
single-crossing property.22
When might we expect the single-crossing property to hold? One consideration whichsuggests that second-order selection effects may be less robust than first-order ones comesfrom Appendix A. We show there that CES demands are the only case in which variable
profits are a power function of marginal cost, and so total profits are affine in a simple
21

To ensure that both exports and FDI are observed we require that the two curves intersect in the positive
quadrant, which is equivalent to the “freeness” of trade (the variable trade cost adjusted by the degree ofsubstitutability) being greater than the ratio of fixed costs: τ 1−σ > fE /fF .
22
In the language of monotone comparative statics, the condition for ΠE and ΠF to have only a single
intersection is that the function ΠF (c) − ΠE (τ, c) is single crossing from above. For details see Mrázová and
Neary (2019). For further background on monotonic comparative statics, see Milgrom and Roberts (1990)
and Vives (1990).

22

transformation of productivity as in Figure 7. Thus, with demands other than CES, we needto establish whether the single-crossing property holds and so whether the second-orderselection effects will be as illustrated in Figure 7.
Checking directly whether the single-crossing property holds in any particular application
is not easy. However, it is easy to check for an important sufficient condition. Recall that π
is continuous in c, and assume also that the two profit functions ΠF and ΠE have different
slopes at any point where they intersect. Given this, the single-crossing property is equivalent
to ΠF being steeper than ΠE at any productivity level where they intersect, the configuration
shown in Figure 7. This suggests a sufficient condition for second-order selection effects which
is much easier to check: if ΠF is steeper than ΠE at every level of productivity, then single
crossing must hold. This condition is equivalent to πc (1, c) < πc (τ, c) at every value of c,which, provided π is twice differentiable in (τ, c), is equivalent to a restriction on its second
derivative: πcτ is positive, or π(τ, c) is supermodular in (τ, c). A necessary and sufficientcondition for πcτ > 0 can be found in turn by differentiating (22) with respect to τ :d log x∂x
= −x 1 +πcτ = −x − τ∂τd log c(24)Hence π(τ, c) is supermodular in {τ, c} if and only if −d log x/d log c > 1; i.e., if and only if
the elasticity of output with respect to marginal cost is greater than one in absolute value.
(In what follows we denote this by “EMR” since it is also the elasticity of output withrespect to marginal revenue.) To understand this condition, note that supermodularity is a
restriction on a second derivative, that is, on a difference in differences: not whether higher
trade costs reduce profits, but whether they do so by less for lower-productivity firms. The
final term on the right-hand side of (24) shows that they are more likely to do so the moreelastic is the volume sold to an increase in costs. Offsetting this is the first term, equal toone: this reflects the fact that higher trade costs reduce profits by less for a firm that isselling less to begin with. Whether or not supermodularity obtains depends on the balance23between these two forces.
An alternative way of writing the condition for supermodularity in (24) allows us toillustrate it in demand-manifold space. It can be expressed in terms of demand parameters
by noting that it equals the elasticity of demand times the elasticity of price with respect tomarginal cost, already given in (16) above:−d log pε−1d log x=ε=d log cd log c2−ρ(25)Supermodularity therefore requires that the following expression be positive:d log x− 1+
d log c


=

ε+ρ−3
2−ρ

(26)

Thus, from (24), selection into FDI by large firms is only guaranteed if the EMR is greaterthan one in absolute value, which from (26) is equivalent to the demand function being
sufficiently elastic and/or sufficiently convex that ε and ρ sum to more than three.

p tc  0

 tc  0p( x)  x 1/ 

 tc  0p

ᇱ

p( x)    x

MR(x)

MR(x)



x

(a) CES(b) Inverse Translog



 x

(c) Linear

Figure 8: Second-Order Selection Effects and Demand
To understand these results, it is helpful first to consider Figure 8, showing the demand
and marginal revenue functions for three canonical special cases. Panel (a) shows the CES
case, which is an example where the EMR is greater than one. It is easy to check from (25),using the CES values for ε and ρ from (4), that the EMR in this case equals σ, which isgreater than one. Hence, if we compare two firms facing the same CES demand function,
one with marginal costs 10% lower than the other, the more productive firm will have an
24

output more than 10% higher than the less productive one. This helps explain the result ofHelpman et al. (2004), which exhibits another example of the Matthew Effect: in this case,more efficient firms enjoy higher profits when they engage in FDI.
Panel (c), by contrast, shows the case of linear demands, which is an example where the
EMR is less than one for larger firms. Intuitively, for such large firms the additional trade
cost they must pay if they export is small, since their production cost is low to begin with,and trade costs are proportional to production costs; moreover it reduces their sales very
little, since the marginal revenue curve is not steep. Hence, supermodularity fails for larger
firms under linear demands, and it can be shown that the largest ones will indeed exhibit
reverse selection effects, choosing to export rather than to engage in FDI.
Finally, Panel (b) shows a knife-edge case with the property that the EMR equals one.
As a result, the proximity-concentration trade-off yields the same outcome for all firms
irrespective of their productivity: the ΠX and ΠF curves in Figure 7 are parallel. Hence,no firms will find it profitable to engage in FDI since it incurs a higher fixed cost, and sono selection will be observed. Mrázová and Neary (2019) show that this demand function
0is the inverse translog, p(x) = βx (log x + γ 0 ). It is a limiting case of a family of demand
functions that exhibit a constant EMR: d log x/d log c = −κ. Mrázová et al. (2015) call thisthe “CEMR” family:23

p(x) =β  κ−1x κ +γxβ(κ − 1) > 0(27)

The corresponding CEMR manifold is ρ(ε) = 2 − κ1 (ε − 1), which implies a family of affine
curves in (ρ, ε)-space, some of which are illustrated for different values of κ in Figure 9. Of
these, the inverse translog, where the EMR is unity, corresponds to the boundary between
the sub- and supermodular regions in (ρ, ε)-space, the curve labelled “SM” in Figure 9.
23
Mrázová and Neary (2019), who first considered these demands in this context, called them the Inverse
PIGL (“price-independent generalized linear”) demand family, drawing on Muellbauer (1975). The inverseκtranslog is a limiting case of the CEMR family as κ → 1, β → ∞ and γ → −1. To see this, set β = β 0 κ−1and γ = γ 0 κ−1
κ − 1 and evaluate the limit when κ → 1.

25



SM

4.0

=1

=2

=4

3.0
 = 0.5

2.0

 = 0.25

1.0
SC
0.0
-2.0

-1.0

0.0

1.0

2.0

3.0



Figure 9: CEMR Demand Manifolds

6

Linking Productivity and Sales Distributions

The final substantive topic we want to discuss goes to the heart of firm heterogeneity inmodels of monopolistic competition. It concerns the relationship between the shape of theunderlying distribution assumed for firm productivities, and the shape of the predicted distribution of firm outcomes, whether the level or the rate of growth of output or sales.24
There is a relatively small number of results of this kind. First is the canonical result in
the field, due to Helpman et al. (2004) and Chaney (2008): in the Melitz (2003) model ofheterogeneous firms, if firm productivities follow a Pareto distribution and demands are CES,
then firm sales must also follow a Pareto distribution. Second, going beyond the canonicalresult, Head et al. (2014) retain the assumption that demands are CES, and show that if
firm productivities follow a Lognormal distribution instead of a Pareto then firm sales mustalso follow a Lognormal distribution. This result is important as there is abundant evidence
that the distribution of firm sales is approximately Pareto for the “long tail” of large firms,
but is more closely approximated by a Lognormal distribution for the whole population offirms, especially smaller ones: see, for example, Bee and Schiavo (2018).25
24

This section draws on Mrázová et al. (2015).
Some authors have explored models of heterogeneous firms with other productivity distributions, including generalizations or combinations of the Lognormal and Pareto such as mixtures of thin- and fat-tailed
25

26

Both these results share two features. First, they assume CES demands, and so imply
that all firms have the same markups which do not vary over time. Second, they exhibit aproperty that Mrázová et al. (2015) call “self-reflection”: the distribution predicted for theendogenous random variable takes the same form as, or “reflects,” the distribution assumed
for the exogenous random variable. Self-reflection is an attractive property: for example, it isdesirable to understand in what circumstances the analytically tractable Pareto distribution
can be used for both productivity and sales.26 Hence, it is desirable to know whether there
are demand functions other than the CES that are consistent with this property.
Mrázová et al. (2015) show that the answer to this question is “yes” by deriving conditions
for self-reflection of two distributions, where the firm-level variables are linked by a functional
relationship. They introduce a class of distributions, the “Generalized Power Function”(GPF) class, that nests many well-known families of distributions, including the Pareto,
Lognormal and Fréchet. They then show that, if the distributions of both variables are
members of the same family in this class, it then follows that the relationship between
the two variables must take a simple power-law form. Moreover, these three conditions
are necessary and sufficient for each other in the sense that any two imply the third. In
the present context, the variables of interest are productivity and sales revenue, so therequirement for self-reflection is that sales revenue is a power-law function of productivity.
Since productivity is the inverse of marginal cost, this is equivalent to a constant elasticity
of sales revenue with respect to marginal cost: d log r/d log c = −κ. Of course, marginal
cost equals marginal revenue in equilibrium, so this condition can also be expressed as a“Constant Revenue Elasticity of Marginal Revenue,” whence the acronym “CREMR”.
The next step is to derive the demand functions that exhibit the CREMR property. It is
straightforward to express the elasticity in terms of ε and ρ, using the fact that r(x) = xp(x)Pareto or piecewise Lognormal-Pareto distributions, although these are typically not analytically tractable.
See, for example, Luttmer (2007), Eaton et al. (2011), Edmonds et al. (2012), and Nigai (2017).
26
Mrázová et al. (2015) show that self-reflection is also necessary and sufficient for models of monopolisticcompetition to exhibit Gibrat’s Law, the classic result due to Gibrat (1931) that a firm’s rate of growth
should be independent of its size.

27and the expression for the elasticity of output with respect to marginal cost from (25):d log rε − 1 d log x(ε − 1)2==−
d log cε d log cε(2 − ρ)(28)

Mrázová et al. (2015) show that the demand functions which ensure that this is constant,which they call CREMR demands, are given by the following:p(x) =

κβ(x + γ) κ+1 ,x

κ > 0, x + γ(κ + 1) > 0, β > 0(29)Once again, this is a generalization of CES demands, and reduces to them when γ is zero. The
results of Mrázová et al. (2015) imply that it is the only demand function that is consistent
with self-reflection of the productivity and sales distributions in monopolistic competition,
when both distributions are members of the same family from the GPF class. Mrázová et al.
(2015) show that it also has other desirable properties: in particular, it implies distributions
for both sales and markups that are both theoretically plausible and empirically realistic,in that they provide a better fit for these distributions than other widely-used demand
functions.

=1



=2

=5

SC

4.0
 = 0.5

3.0

 = 0.2

2.0

1.0

0.0
-2.0

-1.0

0.0

1.0

2.0

3.0

Figure 10: CREMR Demand Manifolds

28

Finally, we can calculate the CREMR demand manifold, which is given by the following:

ρ(ε) = 2 −1 (ε − 1)2
κ
ε

(30)

Figure 10 illustrates some examples of this for different values of κ. Once again, theseare very different from the demand manifolds for standard demand functions, as shown in
Figure 2.

7Constant Response Demand Functions
Response

Demand Functionp(x; β, γ, κ)

Demand Manifold
ρ(ε; κ)Bulow-Pfleiderer

dp
=κ
dc


 1−κ
β x κ +γ

2 − κ1CPPT

d log p=κd log c

CEMR/Inverse PIGL

d log x= −κd log c

β
x

CREMR

d log r= −κd log c

κ
β
κ+1
(x
+
γ)
x

β
x

κ
 κ−1
 κ−1
x κ +γ κ−1x κ +γ2 − κ1 ε−1ε
2 − κ1 (ε − 1)2

2 − κ1 (ε−1)ε

Table 1: Constant-Response Demand Functions
Table 1 summarizes the demand functions that have been discussed in the paper. Eachof them is a “constant-response demand function,” which is necessary and sufficient for an
exogenous change in marginal cost to induce an equal absolute or relative response in somefirm-level variable, whether price, output or sales. Each of them nests CES demands, andreduces to the CES case when the parameter γ is zero.27 Putting this differently, only CES
27

This is immediate in three of the four cases. In the CPPT case we have not shown it explicitly, as it ispclear from (17) that CES demands imply that dd log
log c = 1. For all four demand functions, the CES special
case imposes restrictions on κ. Setting ε and ρ in the final column of Table 1 equal to their CES values(given in (4)), the implied values of κ with CES demands are: σ/(σ − 1) for Bulow-Pfleiderer, one for CPPT,
σ for CEMR, and σ − 1 for CREMR, where σ is the CES elasticity of substitution and demand.

29

demands imply constant responses to costs of more than one of these variables, and each of
the four demand functions in Table 1 allows for variable markups if and only if γ is non-zero.
The explicit expressions for the different demand functions given in the third column of
the table are suggestive. Yet, even though they are written in such a way as to make themas comparable as possible, their behavior relative to each other is not immediately obvious.
It is easier to compare their properties using their manifolds, as shown in the fourth column,
and as illustrated in Figures 6, 9, and 10. While each demand function depends on threeparameters, each of the manifolds depends only on κ, the parameter that measures the
response of the variable in question to the change in marginal cost. Thus, as we have seen inprevious sections, for a given value of κ, each manifold is a locus of values of ε and ρ which
ensure that the relevant response to an increase in marginal cost is constant and equal to κ.
Except for the Bulow-Pfleiderer case, the other three demand functions in Table 1 havemanifolds that are mostly quite different from those of the better-known and more widelyused demand functions shown in Figure 2. If we concentrate on the subconvex case, we
can see that these demand functions allow for much more concavity than most widely-useddemands. This is especially true for firms that are small and relatively unresponsive toshocks (i.e., for high values of ε and low values of κ). Mrázová et al. (2015) point out that
this ability to allow small firms to face very concave demand may be why CREMR demands
give a better fit to both sales and markups than other demand systems. By contrast, forlarger firms (i.e., for lower values of ε), Bulow-Pfleiderer demands have constant convexity by
construction, whereas the others become steadily more convex: CPPT and CEMR asymptote
to Cobb-Douglas, and CREMR to a general CES with elasticity of demand σ equal to κ + 1.
While the manifolds in Table 1 are invariant with respect to the other two demandparameters β and γ, this does not imply that these have no role. Consider first the nonCES parameter γ: its sign is a key determinant of whether a given demand function issub- or superconvex. More specifically, if, for a given value of κ, a demand function can beeither sub- or superconvex, then it is superconvex if and only if γ is positive. Table 3 in30

Appendix C illustrates the possible configurations in detail. As for the parameter β, its rolein these demand functions is similar to the one it plays in the CES itself: it affects only thelevel of demand, and is taken as given by firms, but is determined endogenously in generalequilibrium. We saw in Section 3 how this gives rise to competition effects. In general, any
exogenous shock to an industry equilibrium will have both a direct effect on individual firms
and an indirect competition effect working through the free entry condition and the demand
function. As we saw in Section 3, the competition effect operates in a particularly simple
way when demand is generated by additively separable preferences, since it affects only β,
which is inversely proportional to the marginal utility of income. Finally, a further role
played by the β parameter arises if we want to move from demands to preferences. All of thedemand functions in Table 1 can be integrated to get the corresponding preference system,
which allows us to solve for the endogenous value of β. This is relatively straightforward
when cross-price effects take a simple form, as in the case of additive separability or Kimballpreferences. (Mrázová et al. (2015) discuss this further for CREMR demands.) Hence, thesedemand functions can be used in explicit calculations of the general-equilibrium welfare
effects of policy and other shocks.
Finally, in both theoretical and empirical applications it may be convenient to work withdemand functions that nest some or all of the families in Table 1. By introducing just one
extra parameter we can write a generalized constant-response demand function that nests
the CPPT, CEMR and CREMR cases.28 This takes the following form:

p(x) =β α(x + γ)θ
x(31)

It is obvious by inspection that this nests the CPPT, CEMR and CREMR demand functions.
Moreover, we can check that it implies a constant proportional response to a change in
28

We can introduce yet one further parameter to write a generalized demand function that nests all
four cases in Table 1. However, whereas the constant response implied by Bulow-Pfleiderer demands is anabsolute one, we focus here on nesting the three demand functions (CPPT, CEMR and CREMR) that imply
a constant proportional response to costs of a variable that we can also nest by a general constant response
function implied by (31).

31marginal cost of a Cobb-Douglas function of quantity and price, f (x) = xµ p(x)1−µ . Write
the constant response as −κ0 . It can be written as a function of µ and of the elasticity andconvexity of demand by making use of the relationship between output and marginal costfrom (25):
d log f (x) d log x1−µ ε−1d log f (x)
=
=− µ−
= −κ0
d log cd log x d log c
ε
2−ρ

(32)

This can be rearranged to get the demand manifold:ρ(ε) = 2 −µε − 1 + µ ε − 1
κ0
ε

(33)

It can be checked that the parameters α and θ in (31) are uniquely related to the parameters
κ0 and µ in (33):α≡κ0 + 1 − 2µ
κ0

and

θ≡

κ0
κ0 + 1 − µ(34)

The three special cases correspond to values of µ equal to zero in the CPPT case (whenκ = −κ0 ), equal to one in the CEMR case (when κ = κ0 ), and equal to 21 in the CREMR case(when f (x) is the square root of revenue rather than revenue itself, so κ = 2κ0 ). Figure 11

Figure 11: Nesting Constant-Response Demand Functions

illustrates schematically how the different constant-response demand functions relate to each32

other: all except Bulow-Pfleiderer demands are nested by (31), and the CES is the only
demand function that is nested by all.29

8Conclusion

In this paper we have reviewed and extended recent work by ourselves and others on monopolistic competition in benchmark models of international trade.30 We have concentrated
on the implications of such models for a number of important results, including competition effects, pass-through, selection effects, and the size distribution of firms. One unifying
principle we have emphasized is that the assumption of CES preferences and demands yields
enormous analytical tractability, but also has very restrictive and often counter-factual implications. In a CES world, competition effects take a very simple form: all firms have the same
markup, and globalization shocks leave the distribution of profits across firms unchanged;
pass-through from costs to prices is always exactly 100%; larger firms always select into
activities with lower access costs; and the distributions of productivity and sales typicallyexhibit the same form. A second unifying principle that we have highlighted is the classof constant-response demand functions that provide a parsimonious suite of alternatives to
the CES case. Each of these functions adds a single parameter to the CES, so avoiding thecounter-factual prediction of constant markups, while preserving one or other of its key properties. Given the dominance of monopolistic competition in trade, these demand functions
provide a route to more realistic modelling and empirical work. Moreover, the generalized
constant-response demand function that we introduced in Section 7 will hopefully prove
helpful in giving a unified perspective on the different individual constant-response functions
and in understanding how they relate to each other.
29

Matsuyama and Ushchev (2017) illustrate in a similar way links between non-parametric homothetic
departures from CES.
30
As already mentioned, we have not discussed trade under oligopoly, which provides an alternative explanation for the evidence of variable markups cited in Section 2.3. (See, for example, Neary (2003, 2016) andAtkeson and Burstein (2008).) Nor have we discussed the extensive recent literature in trade that uses calibration methods to quantify the effects of exogenous shocks, often in CES-based frameworks. (See Costinot
and Rodrı́guez-Clare (2014) for an overview.)

33

The fact that the monopolistically competitive paradigm has become the dominant one
in international trade may seem surprising from an IO perspective. However, there are goodreasons for this. The availability of large data sets on exporting firms, and the desire to takeaccount of general-equilibrium feedbacks between goods and factor markets, make a “onesize-fits-all” approach to the choice of market structure very convenient. This is despite thefact that it is sure to miss much of the supply-side specificity on which IO economists tend
to focus, including industry-specific features such as limit pricing in some industries but notin others, differences in fixed costs, the role of learning-by-doing, etc. Moreover, as we haveshown, relaxing the assumption of CES preferences in monopolistic competition makes its
view of firms more realistic and its predictions more flexible.
Of course, while leaving the comfort zone of CES goes part of the way to a more realistic
view of imperfectly competitive markets, it raises new problems. To paraphrase Tolstoy(1878), CES demand functions are all alike, every non-CES demand function is non-CES in
its own way.31 The CES benchmark is sure to retain a central place in this field, both becauseof its tractability and also because it alone exhibits all the constant-response properties that
we have discussed.
In conclusion, we have already noted some ironies in the selective way in which international trade has borrowed from industrial organization in how it models firms in open
economies. In particular, Dixit and Stiglitz proposed using additively separable preferences
as a way of modelling product differentiation, and viewed CES preferences as just a special
illustrative case; whereas it is the latter that has become the workhorse model for exploringa huge range of issues in trade. There is a further irony. The substantive issue Dixit andStiglitz addressed was the efficiency of the market equilibrium, and in particular whether
an unregulated monopolistically competitive market would lead to too many or too few varieties, and to firms that are too large or too small relative to the social optimum. Thisissue has not been addressed much in trade, but in recent years there has been a revival of31

From the opening sentence of Anna Karenina: “Happy families are all alike; every unhappy family is
unhappy in its own way.”34

interest in it: see for example Feenstra and Kee (2008) and Dhingra and Morrow (2019). In
ongoing work with Mathieu Parenti (see Mrázová et al. (2015)) we show how the tools from
Section 6 above can be used to throw light on this issue.

AppendicesA

Constant Response of Profits Demand Functions

We would like to characterize the demand functions with the property that operating profitsπ= κ. We know that this holds
have a constant proportional response to marginal cost: dd log
log cin the CES case, where π = Bc1−σ .32 To find the necessary condition for this property, note
that, since π = (p − c) x, we have in general:d log π = d log (p − c) + d log x(35)

Equation (25) gives the proportional response of output to changes in marginal cost, d log x/d log c.
The next step is to consider d log (p − c). Consider first d (p − c). From the firm’s first-ordercondition this equals d (−xp0 ) = −p0 dx − xp00 dx = − (p0 + xp00 ) dx. Hence:d log (p − c) =(p0 + xp00 ) dx
p0 + xp00
d (−xp0 )==d log x = (1 − ρ) d log x
−xp0
xp0
p0

(36)

Collecting terms gives, from (25) and (35):d log πd log (p − c) d log x
d log x=+= (2 − ρ)= − (ε − 1)d log cd log cd log cd log c32(37)Here B is exogenous from the firm’s perspective, but it depends on the price index and on aggregate
income, and so is endogenous in general equilibrium. See, for example, Melitz (2003), equation (4).

35

Clearly the CES case is the only one that yields a constant value of this. Hence CES demands are necessary and sufficient for a constant proportional response of operating profits
to marginal cost. This in turn implies that there is no other demand function which guarantees that profits are affine in a power-law transformation of costs, so we must seek otherconditions, such as supermodularity of the profit function in production and trade costs, tobe sure that second-order selection effects follow the same pattern as they do in the CES
case.

B

Manifolds for Some Common Demand Functions
Demand Function

Demand Manifold

CARAx(p) = γ + δ log pρ(ε) = 1εStone-Geary/LESβp(x) = γx+1

ρ(ε) = 2εTranslog/AI

x(p) = p1 (γ + δ log p)ρ(ε) = 3ε−1
ε2Table 2: Some Common Demand Functions and Their Manifolds

Table 2 gives some common demand functions and their manifolds, as illustrated in Figure 2.
Further details are given in Mrázová and Neary (2017). As elsewhere in the paper, thedemand functions are written from the perspective of an individual firm in monopolisticcompetition. They also depend on aggregate expenditure and on indices of the prices of
all goods, including the good in question; these are taken as given by the firm, since it isof measure zero, and are subsumed into the demand parameters. (See Dixit and Stiglitz(1993) for further discussion.) All three manifolds are invariant with respect to these othervariables. To take a specific example, Stone-Geary demands are more familiar when writtenR
as: x(i) = α(i) + (p(i))−1 I − i0 ∈Ω p(i0 )α(i0 )di0 , which, when γ = (−α(i))−1 and β =

36

γ I−

R


p(i0 )α(i0 )di0 , is equivalent to the expression in Table 2. The latter expression is the

special case of the CPPT demands from (18) when κ = 1/2.

C

The Role of γ in Determining Superconvexity
γ<0

γ>0

BP:
n/a
BP:
CPPT:
n/a
CPPT:
κ<1
CEMR:
n/a
CEMR:
CREMR: Subconvex CREMR:

Subconvex
Subconvex
Subconvex
Superconvex

BP:
Subconvex BP:
CPPT:
n/a
CPPT:
κ>1
CEMR:
Subconvex CEMR:CREMR: Subconvex CREMR:

Superconvex
Superconvex
Superconvex
Superconvex

Table 3: Possible Configurations of Sub- and Superconvexity
Table 3 illustrates schematically the conditions under which each of the non-CES constantresponse demand functions is either sub- or superconvex, depending on the values of the γand κ parameters. In four of the eight cases, the demand functions can only be either sub- or
superconvex, irrespective of the sign of γ. The remaining cases are represented in Figures 6,9, and 10 by manifolds with two branches, one in the subconvex region and the other inthe superconvex one. It is in these cases that the sign of γ is crucial, with a positive value
corresponding to the superconvex branch.

37

References
Anderson, S. P., A. De Palma, and J.-F. Thisse (1992): Discrete Choice Theory of
Product Differentiation, Cambridge, MA: MIT Press.
Arkolakis, C. (2010): “Market Penetration Costs and the New Consumers Margin inInternational Trade: Appendix,” web appendix, Yale University.
——— (2016): “A Unified Theory of Firm Selection and Growth,” Quarterly Journal of
Economics, 131, 89–155.
Atkeson, A. and A. Burstein (2008): “Pricing-to-Market, Trade Costs, and International Relative Prices,” American Economic Review, 98, 1998–2031.
Bache, P. A. and A. Laugesen (2015): “Monotone Comparative Statics for the Industry
Composition,” Working Paper, Aarhus University.
Bee, M. and S. Schiavo (2018): “Powerless: Gains from Trade when Firm Productivity
is not Pareto Distributed,” Review of World Economics, 154, 15–45.
Behrens, K. and Y. Murata (2007): “General Equilibrium Models of Monopolistic
Competition: A New Approach,” Journal of Economic Theory, 136, 776–787.
Bertoletti, P. and P. Epifani (2014): “Monopolistic Competition: CES Redux?” Journal of International Economics, 93, 227–238.
Bontems, P. (2019): “Not Too Costly: How Cost Flexibility Influences Firm Behavior,”
presented to ETSG 2019, Bern, Switzerland.
Brander, J. A. (1995): “Strategic Trade Policy,” in G.M. Grossman and K. Rogoff (eds.):
Handbook of International Economics, Volume 3, Amsterdam: Elsevier.
Bulow, J. I. and P. Pfleiderer (1983): “A Note on the Effect of Cost Changes on
Prices,” Journal of Political Economy, 91, 182–185.
38

Cabral, L. M. (2018): “Standing on the Shoulders of Dwarfs: Dominant Firms and Innovation Incentives,” CEPR Discussion Paper No. 13115.
Chamberlin, E. H. (1933): The Theory of Monopolistic Competition: A Re-Orientationof the Theory of Value, Cambridge, Mass: Harvard University Press.
Chaney, T. (2008): “Distorted Gravity: The Intensive and Extensive Margins of International Trade,” American Economic Review, 98, 1707–1721.
Costinot, A. and A. Rodrı́guez-Clare (2014): “Trade Theory with Numbers: Quantifying the Consequences of Globalization,” in G. Gopinath, E. Helpman and K. Rogoff(eds.): Handbook of International Economics, Elsevier, 197–261.
Cournot, A. (1838): Recherches Sur les Principes Mathématiques de la Théorie desRichesses, Paris: Gratiot.
De Loecker, J., P. K. Goldberg, A. K. Khandelwal, and N. Pavcnik (2016):“Prices, Markups and Trade Reform,” Econometrica, 84, 445–510.
Deaton, A. and J. Muellbauer (1980): “An Almost Ideal Demand System,” American
Economic Review, 70, 312–326.
Dhingra, S. and J. Morrow (2019): “Monopolistic Competition and Optimum Product
Diversity under Firm Heterogeneity,” Journal of Political Economy, 127, 196–232.
Dixit, A. and V. Norman (1980): Theory of International Trade, Cambridge University
Press.
Dixit, A. K. (2004): “Some Reflections on Theories and Applications of Monopolistic Competition,” in S. Brakman and B.J. Heijdra (eds): The Monopolistic Competition Revolution
in Retrospect, Cambridge: Cambridge University Press, 123-133.
Dixit, A. K. and J. E. Stiglitz (1977): “Monopolistic Competition and Optimum
Product Diversity,” American Economic Review, 67, 297–308.
39

——— (1993): “Monopolistic Competition and Optimum Product Diversity: Reply [to Yang
and Heijdra],” American Economic Review, 83, 302–304.
Eaton, J., S. Kortum, and F. Kramarz (2011): “An Anatomy of International Trade:
Evidence from French Firms,” Econometrica, 79, 1453–1498.
Eckel, C., L. Iacovone, B. Javorcik, and J. P. Neary (2015): “Multi-Product Firms
at Home and Away: Cost- versus Quality-Based Competence,” Journal of International
Economics, 95, 216–232.
Eckel, C. and J. P. Neary (2010): “Multi-Product Firms and Flexible Manufacturingin the Global Economy,” Review of Economic Studies, 77, 188–217.
Feenstra, R. and H. L. Kee (2008): “Export Variety and Country Productivity: Estimating the Monopolistic Competition Model with Endogenous Productivity,” Journal ofInternational Economics, 74, 500–518.
Feenstra, R. C. and D. E. Weinstein (2017): “Globalization, Markups, and U.S.
Welfare,” Journal of Political Economy, 125, 1040–1074.
Gaubert, C. and O. Itskhoki (2018): “Granular Comparative Advantage,” mimeo.,
University of California at Berkeley.
Gibrat, R. (1931): Les inégalités économiques: applications d’une loi nouvelle, la loi del’effet proportionnel [Economic Inequalities: Applications of a New Law, the Law of Proportionate Effect], Librairie du Recueil Sirey.
Gopinath, G. and O. Itskhoki (2010): “Frequency of price adjustment and passthrough,” Quarterly Journal of Economics, 125, 675–727.
Head, K., T. Mayer, and M. Thoenig (2014): “Welfare and Trade Without Pareto,”
American Economic Review, Papers and Proceedings, 104, 310–316.

40

Helpman, E., M. J. Melitz, and S. R. Yeaple (2004): “Export Versus FDI with
Heterogeneous Firms,” American Economic Review, 94, 300–316.
Krugman, P. (1980): “Scale Economies, Product Differentiation, and the Pattern ofTrade,” American Economic Review, 70, 950–959.
Krugman, P. R. (1979): “Increasing Returns, Monopolistic Competition, and International Trade,” Journal of International Economics, 9, 469–479.
Kuhn, K.-U. and X. Vives (1999): “Excess Entry, Vertical Integration, and Welfare,”RAND Journal of Economics, 30, 575–603.
Lamorgese, A., A. Linarello, and F. Warzynski (2014): “Free Trade Agreements
and Firm-Product Markups in Chilean Manufacturing,” Tech. rep., Aarhus University.
Luttmer, E. G. (2007): “Selection, Growth, and the Size Distribution of Firms,” Quarterly
Journal of Economics, 122, 1103–1144.
Marshall, A. (1920): Principles of Economics, An Introductory Volume, Eighth Edition,
London: Macmillan.
Matsuyama, K. and P. Ushchev (2017): “Beyond CES: Three Alternative Classes of
Flexible Homothetic Demand Systems,” HSE Working Paper BRP 172/EC/2017, National
Research University Higher School of Economics.
Melitz, M. J. (2003): “The Impact of Trade on Intra-Industry Reallocations and Aggregate
Industry Productivity,” Econometrica, 71, 1695–1725.
Melitz, M. J. and G. I. Ottaviano (2008): “Market Size, Trade, and Productivity,”
Review of Economic Studies, 75, 295–316.
Milgrom, P. and J. Roberts (1990): “The Economics of Modern Manufacturing: Technology, Strategy, and Organization,” American Economic Review, 80, 511–528.
41

Mrázová, M. (2011): “Trade Agreements when Profits Matter,” Discussion Paper, Department of Economics, University of Surrey.
Mrázová, M. and J. P. Neary (2014): “Together at Last: Trade Costs, Demand Structure, and Welfare,” American Economic Review, Papers and Proceedings, 104, 298–303.
Mrázová, M. and J. P. Neary (2017): “Not So Demanding: Demand Structure andFirm Behavior,” American Economic Review, 107, 3835–3874.
——— (2019): “Selection Effects with Heterogeneous Firms,” Journal of the European Economic Association, 17, 1294–1334.
Mrázová, M., J. P. Neary, and M. Parenti (2015): “Sales and Markup Dispersion:
Theory and Empirics,” revise and resubmit, Econometrica.
Mrázová, M., D. Vines, and B. Zissimos (2013): “Is the GATT/WTO’s Article XXIV
bad?” Journal of International Economics, 89, 216–232.
Muellbauer, J. (1975): “Aggregation, Income Distribution and Consumer Demand,” Review of Economic Studies, 42, 525–543.
Neary, J. P. (2003a): “Globalization and Market Structure,” Journal of the European
Economic Association, 1, 245–271.
——— (2003b): “Monopolistic Competition and International Trade Theory,” in S. Brakman and B.J. Heijdra (eds.): The Monopolistic Competition Revolution in Retrospect,
Cambridge University Press, 159-184.
——— (2007): “Cross-Border Mergers as Instruments of Comparative Advantage,” Review
of Economic Studies, 74, 1229–1257.
——— (2010): “Two and a Half Theories of Trade,” The World Economy, 33, 1–19.

42

——— (2016): “International Trade in General Oligopolistic Equilibrium,” Review of International Economics, 24, 669–698.
Nigai, S. (2017): “A Tale of Two Tails: Productivity Distribution and the Gains from
Trade,” Journal of International Economics, 104, 44–62.
Nocke, V. and N. Schutz (2018): “Oligopoly in International Trade: An Aggregative
Games Approach,” presented to EARIE 2018, Athens.
Parenti, M. (2018): “Large and Small Firms in a Global Market: David vs. Goliath,”
Journal of International Economics, 110, 103–118.
Parenti, M., P. Ushchev, and J.-F. Thisse (2017): “Toward a Theory of Monopolistic
Ccompetition,” Journal of Economic Theory, 167, 86–115.
Pascoa, M. R. (1993): “Noncooperative Equilibrium and Chamberlinian Monopolistic
Competition,” Journal of Economic Theory, 60, 335–353.
——— (1997): “Monopolistic Competition and Non-Neighboring Goods,” Economic Theory,9, 129–142.
Pollak, R. A. (1972): “Generalized Separability,” Econometrica, 40, 431–453.
Shimomura, K.-I. and J.-F. Thisse (2012): “Competition Among the Big and the
Small,” RAND Journal of Economics, 43, 329–347.
Simonovska, I. (2015): “Income Differences and Prices of Tradables: Insights from an
Online Retailer,” Review of Economic Studies, 82, 1612–1656.
Stiglitz, J. E. (2004): “Reflections on the State of the Theory of Monopolistic Competition,” in S. Brakman and B.J. Heijdra (eds): The Monopolistic Competition Revolution
in Retrospect, Cambridge: Cambridge University Press, 134-148.

43

Tolstoy, L. (1878): Anna Karenina, translated by Rosemary Edmonds, Harmondsworth,Middlesex: Penguin Classics, 1954.
Vives, X. (1990): “Nash Equilibrium with Strategic Complementarities,” Journal of Mathematical Economics, 19, 305–321.
——— (1999): Oligopoly Pricing: Old Ideas and New Tools, Cambridge, Mass.: MIT Press.
Weyl, E. G. and M. Fabinger (2013): “Pass-Through as an Economic Tool: Principles
of Incidence under Imperfect Competition,” Journal of Political Economy, 121, 528–583.
Zhelobodko, E., S. Kokovin, M. Parenti, and J.-F. Thisse (2012): “Monopolistic
Competition: Beyond the Constant Elasticity of Substitution,” Econometrica, 80, 2765–
2784.

44