A Model of Growth Through Creative Destruction
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Aghion, Philippe, and Peter Howitt. 1992. “A Model of Growth Through Creative Destruction."
Econometrica 60, no. 2: 323-351.

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Econometrica,

Vol. 60, No. 2 (March, 1992), 323-351

A MODEL OF GROWTH THROUGH
CREATIVE DESTRUCTION

BYPHILIPPE AGHION AND PETER HowirrT
A model of endogenous growth is developed in which vertical innovations, generated
by a competitive research sector, constitute the underlying source of growth. Equilibrium

differenceequation,accordingto whichthe amountof
is determinedby a forward-looking

research in any period depends upon the expected amount of research next period. One
source of this intertemporal relationship is creative destruction. That is, the prospect ofmore future research discourages current research by threatening to destroy the rents

createdby currentresearch.The paper analyzesthe positiveand normativepropertiesof
stationaryequilibria,in which research employmentis constant and GNP follows arandomwalkwith drift, althoughunder some circumstancescyclicalequilibriaalso exist.
Both the averagegrowthrate and the varianceof the growthrate are increasingfunctionsof the size of innovations,the size of the skilled labor force, and the productivityof
research as measured by a parameter indicating the effect of research on the Poisson
arrival rate of innovations; and decreasing functions of the rate of time preference of therepresentative individual. Under laissez faire the economy's growth rate may be more orless than optimal because, in addition to the appropriability and intertemporal spillover
effects of other endogenous growth models, which tend to make growth slower than
optimal, the model also has effects that work in the opposite direction. In particular, thefact that private research firms do not internalize the destruction of rents generated bytheir innovations introduces a business-stealing effect similar to that found in thepartial-equilibrium patent race literature. When we endogenize the size of innovations wefind that business stealing also makes innovations too small.
KEYWORDS:Endogenous growth,innovations,creativedestruction.

1. INTRODUCTION
THE MAIN CONTRIBUTION of the literatureon endogenousgrowthpioneered by

Romer (1986) and Lucas (1988) has been to endogenize the underlyingsourceof sustained growth in per-capitaincome, namely the accumulationof knowledge. There are many channelsthroughwhich societies accumulateknowledge,
includingformal education,on-the-jobtraining,basic scientificresearch,learning by doing,processinnovations,and productinnovations.This paper examinesa channel that has receivedlittle attentionin the endogenousgrowthliterature,
namelythat of industrialinnovationswhich improvethe qualityof products.
This channel introducesinto endogenousgrowththeorythe factorof obsolescence; better productsrenderpreviousones obsolete. Obsolescenceexemplifies
an importantgeneral characteristicof the growthprocess, namelythat progresscreates losses as well as gains. It also embodies Schumpeter'sidea of creative'The authorswish to acknowledgethe helpful commentsand criticismsof Roland Benabou,
Olivier Blanchard,Patrick Bolton, Louis Corriveau,Mathias Dewatripont,Dick Eckaus, Zvi
Griliches,ElhananHelpman,Rebecca Henderson,Louis Phaneuf,WilliamScarth,Nancy Stokey,
Patrick Rey, and the Co-Editor and referees of this journal.
323

324

PHILIPPE AGHION AND PETER HOWITT

destruction(1942, p. 83, his emphasis):
The fundamentalimpulsethat sets and keeps the capitalistengine in motion comes
fromthe new consumers'goods,the new methodsof productionor transportation,thenew markets,.... [This process] incessantlyrevolutionizesthe economic structurefrom within,incessantlydestroyingthe old one, incessantlycreatinga new one. This
processof CreativeDestructionis the essentialfact aboutcapitalism.

The present paper constructs a simple model of growth through creative
destruction,by modellingthe innovationprocess as in the patent-raceliterature
surveyedby Tirole (1988, Ch. 10) and Reinganum(1989). The expected growthrate of the economydepends upon the economy-wideamountof research.The
paper shows that equilibriumin such an economyis determinedby a forwardlooking differenceequation, accordingto which the amountof researchin any
period depends upon the expected amount of researchnext period, similarto
the differenceequationthat defines equilibriumin the two-periodoverlappinggenerationsmodel of money (Azariadis(1981), Grandmont(1985)).
More specifically,the model assumes, followingSchumpeter,that individual
innovationsare sufficientlyimportantto affect the entire economy.A period is
the time between two successive innovations.The length of each period is
random, because of the stochastic nature of the innovationprocess, but therelationshipbetween the amount of researchin two successiveperiods can bemodelled as deterministic.The amount of researchthis period depends negativelyupon the expected amountnext period, throughtwo effects.
The first effect is that of creative destruction.The payoff from researchthis
period is the prospectof monopolyrents next period. Those rents will last only
until the next innovationoccurs, at which time the knowledgeunderlyingthe
rents will be rendered obsolete. Thus the expected present value of the rents
depends negativelyupon the Poisson arrivalrate of the next innovation.The
expectation of more research next period will increase that arrivalrate, andhence will discourageresearchthis period.
The second effect is a generalequilibriumeffect workingthroughthe wage ofskilledlabor,whichcan be used either in researchor in manufacturing.In order
to be consistentwith the conditionsfor labor-marketequilibrium,the expectation of more researchnext period must correspondto an expectationof higher
demandfor skilled labor in researchnext period,which implies the expectationof a higherreal wage of skilled labor. Higherwages next period will reduce themonopolyrents that can be gained by exclusiveknowledgeof how to produce
the best products. Thus the expectation of more research next period will
discourageresearchthis period by reducingthe flow of rents expectedto accrue
to a successfulinnovator.
This functionalrelationshipbetween researchin two successiveperiodshas aunique fixed point, which defines a stationary equilibrium.The stationary
equilibriumexhibitsbalancedgrowth,in the sense that the allocationof skilledlabor between researchand manufacturingremainsunchangedwith each inno-

MODEL OF GROWTH

325

vation; the log of GNP follows a randomwalk with drift. This is not always,
however,the only equilibriumin the model. As in the overlapping-generations
literaturethe functionalrelationshipcan also be satisfiedby cyclicaltrajectories.
One noteworthyimplicationof the negative dependencyof currentresearchupon future research is the possible existence of what we call a "no-growthtrap,"a cyclicalequilibriumin which the level of researchoscillatesdeterministically between two levels each period, and in which the lower of these twolevels is zero. An economyin such an equilibriumwould stop growingin finite
time, because with no research there would be no innovation,and hence theperiodwith no researchwould never come to an end. The (rational)expectationthat the next innovationwould be followed by a very high level of researchwould discourageanyone from undertakingthat innovation.
Another implicationis that the average growth rate of the economy is notnecessarilyincreasedby an increase in the productivityof research.In particular, a parameterchange that makes researchmore productivein some states of
the world can discourageresearch in other states, by increasingthe threat ofobsolescencefaced by the productof researchin those other states, to such an
extent that the averagegrowthrate is reduced.
From a normative point of view, the average growth rate in stationary
equilibriummay be more or less than sociallyoptimalbecause of the presenceof conflictingdistortionaryeffects. Specifically,althoughthe model includesthe
appropriabilityand intertemporalspillover effects which generate a less than
optimalgrowthrate in Romer's(1990)model, it also has effects that workin the
opposite direction.In particular,there is a "business-stealing"effect of the sortfamiliar from the patent-race literature (Tirole (1988, p. 399)). That is, researchers do not internalizethe destructionof existing rents created by theirinnovations.When the size of innovationsis taken as given,the businessstealing
effect can lead to too much growth.In addition,we find that when the size ofinnovationsis endogenized,the business stealing effect tends to make innovations too small.
Other papersin the endogenousgrowthliteraturethat modelverticalproductinnovationsinclude Segerstrom,Anant, and Dinopoulos (1990), who assume
that the time between successiveinnovationsis deterministic.They have a richer
intersectoralstructurethan the present paper, and address a different set ofquestions. Stokey (1988) models vertical productinnovationsand obsolescencein a perfectly competitive model where innovations are the unintentional
by-productof learning by doing. The cost-reducinginnovations of Shleifer(1986) can also be interpretedas vertical productinnovations.His model does
not endogenizegrowth,however,except in a limited dichotomoussense; that is,
the long-runaverage growthrate is fixed by the exogenouslyspecified rate ofinvention,except in the singularcase where no inventionsare ever implemented
and the economystops growing.
Corriveau(1988) has a discrete-timeanalysisof endogenousgrowthbased on
cost-reducinginnovationsas in Shleifer,in which the possibilityof simultaneous
discoveriescreates a differentkind of "business-stealing"effect. In his model

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PHILIPPE AGHION AND PETER HOWITT

the payoff to currentresearchis independentof future researchbecause rents
to innovationsare assumedto accrue only in the same period as the researchfrom which they resulted. Grossmanand Helpman(1991) constructa model ofverticalproductinnovationthat explicitlyintegratesthe analysisof Segerstrom,
Anant, and Dinopoulos (1990) with that of the present paper.
Judd (1985) and Romer (1990) model growth based on horizontalproductinnovations, using the Dixit-Stiglitz(1977) model of product variety. Thesemodels involveno obsolescence;new productsare no better than existingones.
They also involveno uncertainty.King and Rebelo (1988) introduceuncertainty
into an endogenousgrowthmodel by assuminga randomrate of returnto theaccumulationof humancapital under conditionsof perfect competition.
Within the patent-raceliteraturethe paper closest to the present is that ofReinganum(1985), which also emphasizes the affinityto creative destruction.
The present paper adds to Reinganum'smodel the general equilibriumeffects
of future researchon the rents created by currentresearch,and of the level ofmanufacturingemploymenton the cost of research.The paper also generalizes
the Reinganum model by allowing the stream of innovations to continue
forever, and by explicitlyanalyzingthe effect of the future level of researchon
the prospectiverewardto currentresearch.2
Section 2 below presents the basic model of the paper. This basic model
assumesfor simplicitythat each innovationcreates an economy-widemonopoly
in the production of intermediate goods. Section 3 derives the functional
relationshipbetween research in two successive periods that defines equilibrium. It then analyzes the determinantsof the average growth rate and thevariabilityof the growthrate in stationaryequilibrium.One of those determinants is the degree of market power possessed by an intermediate-goodmonopolist,which is parameterizedin the model. Section 4 characterizesthe
welfarepropertiesof stationaryequilibriain the basic model, underthe assumption of a fixed size of innovations. Section 5 introduces the possibility of
nondrasticinnovations.Section 6 generalizes the model by allowing researchfirms to choose the size of innovationsas well as their arrivalrate. Section 7
deals with a strategicmonopsonyeffect that has been ignoreduntil this point in
the argument,by which an intermediatefirmcan extend the expectedlifetimeof
its monopoly by hiring more than the short-runprofit-maximizingamount ofskilled labor, at the cost of a higherreal wage. Section 8 relaxesthe assumptionof a single economy-widemonopoly in the productionof intermediategoods.
Section 9 containsbrief concludingremarks.

2

That is, Reinganum's comparative-statics analysis follows the common practice of the patent-race
literature in taking the reward to a successful innovation as given, whereas the following analysis
shows that the effect of a parameter change on the time path of research involves feedback fromfuture research to current research working through the two above-mentioned channels: creative
destruction and the general equilibrium wage effect on profits, both of which flow through thereward to a successful innovation.

MODEL OF GROWTH

327

2. THE BASIC MODEL

2.A. Assumptions

There are three classes of tradeableobjects:labor, a consumptiongood, andan intermediategood. There is a continuumof infinitely-livedindividuals,withidentical intertemporallyadditive preferences defined over lifetime consumption, and the constant rate of time preference r > 0. The marginalutility of
consumptionis assumedconstant;thus r is also the rate of interest.
There is no disutility from supplyinglabor. There are three categories oflabor: unskilled labor, which can be used only in producingthe consumption
good; skilled labor,which can be used either in researchor in the intermediate
sector;and specializedlabor,which can be used only in research.Each individual is endowed with a one-unit flow of labor. Let M, N, and R denoterespectivelythe mass of unskilled,skilled, and specializedindividuals.
The consumptiongood is producedusing the fixed quantityM of unskilledlabor, and the intermediategood, subjectto constantreturns.Since M is fixed,
the productionfunctioncan be written as(2.1)

y =AF(x),where F' > 0, F" < 0, y is the flow output of consumptiongood, x the flow ofintermediate input, and A a parameter indicating the productivityof theintermediateinput.
The intermediategood is producedusing skilledlabor alone, accordingto thelinear technology
(2.2)

x=L,

where L is the flow of skilled labor used in the intermediatesector.
Research produces a random sequence of innovations.The Poisson arrival
rate of innovationsin the economy at any instant is A4(n, R), where n is theflow of skilled labor used in research, A a constant parameter, and b a
constant-returns,concave productionfunction. Both A and b are given by thetechnologyof research.There is no memoryin this technology,since the arrival
rate depends only upon the current flow of input to research, not upon past
research.Assume that skilledlaboris an essentialfactorin research:4(0, R) = 0.
Then an economy that allocates no skilled labor to research will not grow,because it will experienceno innovations.(The "linear"case where 0(n, R) = n,that is, where R = 0, will be used frequently.)
Time is continuous,and indexed by r> 0. The subscriptt = 0,1... denotes
the intervalstartingwith the tth innovation(or with r= Oin the case of t = 0)and ending just before the t + 1st. The length of each intervalis random.All
prices and quantitiesare assumedto remainconstantwithin each interval.If nt
is applied to researchin interval t, the length of the intervalwill be exponentially distributedwith parameterA4(nt, R).
Each innovationconsists of the inventionof a new intermediategood, whose
use as input allows more efficient methods to be used in producing the328

PHILIPPE AGHION AND PETER HOWITT

consumptiongood. Real-worldexamplesinclude such "input"innovations3as
the steam engine, the airplane,and the computer,whose use made possiblenew
methods of productionin mining,transportation,and banking,with economywide effects. An innovationneed not, however, be as revolutionaryas theseexamples,but might consist instead of a new generationof intermediategood,
similarto the old one.
Specifically,use of the new intermediate good increases the productivity
parameterA in (2.1) by the factor y > 1. There are no lags in the diffusionof
technology.4The most modern intermediategood is alwaysproduced,so that:(2.3)

At =AOyt(t = 0,where Ao is the initialvalue givenby history.(Of course,it is alwayspossible toproducethe consumptiongood using an old technology,with a correspondingly
old intermediategood.)A successfulinnovatorobtains a patent which it can use to monopolize theintermediatesector. (Section 8 relaxes this assumptionby allowingfor a finite
number of monopolisticcompetitors.)The patent is assumed to last forever.
However,the monopolylasts only until the next innovation,at which time theintermediategood is replaced by the next vintage. All markets are perfectlycompetitiveexcept that for intermediategoods.

2.B. The IntermediateMonopolist's Decision Problem

For ease of presentationthe analysisstartsby assumingthat innovationsare
alwaysdrastic;that the intermediatemonopolistis unconstrainedby potential
competition from the previous patent. This assumption will be relaxed in
Section 5 below. The intermediatemonopolist'sobjective is to maximizethe
expected present value of profits over the currentinterval.When the intervalends so do the profits.The only uncertaintyconcernsthe length of the interval.
Except in Section 7 below, the monopolist is assumed to take as given theamountof researchat each time, and hence also takes as giventhe length of theinterval.

3 Scherer(1984)combinesprocess-and input-orientedR and D into a measureof "used"R andD, which he distinguishesfrom pure product R and D. He estimates that during the period
1973-1978in U.S. industrythe socialrate of returnto "used"R and D lay between71%and 104%,
whereasthe returnto pure productR and D was insignificant.
4 Gradual diffusion could be introducedby allowing the productivityparameterafter eachinnovationto follow a predeterminedbut gradualpath asymptoticallyapproachingthe limit At, andthen to jump to At upon the next innovationand follow a gradualpath approachingAt+1. This
would produce a cycle in researchwithin each interval,as the gradualrise in productivitywould
inducemanufacturingfirmsto hire more and moreworkersout of researchuntilthe next innovation
occurs.

329

MODEL OF GROWTH

Let xt be the flow of the intermediategood produced by the monopolist
during interval t. By (2.2), xt also equals employment of skilled labor in
manufacturing.The inverse demand curve facing a monopolist chargingthe
price pt (relativeto the numeraireconsumptiongood) is the marginalproduct(2.4)

Pt =AtF'(xt).

Thus the monopolist chooses xt to maximize [AtF'(xt) - wt]xt, taking as given

At and the wage wt of skilled labor.
wage" as wt- wt/At, and the "marginalDefine the "productivity-adjusted
revenue function"as G(X) F'(x) + xF"(x). Assume that marginalrevenue isdownward-sloping
and satisfiesInada-typeconditions.
1: G)(x) < 0 for all x > 0, limxOG

ASSUMPTION(X) = oo,limx., 0(x)= 0.

Then for any positive wt the monopolist'schoice of output xt is given by the
first-ordercondition(2.5)>9t=o( xt ) 1

or
(2.6)

xt =(wt),

where x is the function 6 -'. The flow of monopoly profits is(2.7)-rt=A t(v),where ir(w)-(x(co))2F"(I(wG)). Note that x and vF are each strictly positive-

valued and strictlydecreasingfor all positive wt.
An examplesatisfyingAssumption1 is the Cobb-Douglasexample,in which
the consumption-good technology is F(x) = xa, 0 < a < 1, which yields(2.8)

Pt = wt/a,

Tt= (

a

,xt

= (wt/a2)

2.C. Research

There are no contemporaneousspilloversin research;that is, a firm employing the amounts z, s of the two factors in researchwill experienceinnovations
with a Poisson arrivalrate of A4(z, s), independentlyof the inputs of otherfirms.The objectiveof a firm in choosing z and s at each date is to maximize
the flow of expected profitsfrom research:(2.9)

A4(z, s)Vt+1-wtz-w's,

where Vt+ is the value of the t + 1st innovation,and w' is the wage rate ofspecializedlabor.

330

PHILIPPE AGHION AND PETER HOWITT

Because p has constant returns, and because the total flow of specializedlabor must equal R in equilibrium,it follows from the Kuhn-Tuckerconditions
for maximizing(2.9) that
(2.10)

wt > 'pf(nt)AVt+j, nt > 0, with at least one equality,

where p(nt) -- (nt, R), and nt is the economy-wideflow of skilled labor usedin researchduringintervalt. Note that
(2.11)

p(O)= 0, and (p'(n) > 0,(p"(n) < 0

for all n > 0.

As we shall see, all research is conducted by outside research firms rather
than by the incumbentmonopolist. Because of constant returns to scale thenumber of research firms is indeterminate. The value Vtl to an outside
researchfirmis the expectedpresentvalue of the flow of monopolyprofits ?t+1
generatedby the t + 1st innovationover an intervalwhose length is exponentially distributedwith parameterAp(nt+1):(2.12)

t+1= r+A(nt+1)

The reasonwhy the monopolistchooses to do no researchis that the value tothe monopolist of making the next innovationwould be Vt+I- Vt, which isstrictlyless than the value Vt+ 1 to an outside firm. This is an example of theArrow effect, or replacementeffect (see Tirole (1988, p. 392)). The efficiency
effect, or rent-dissipationeffect, according to which an outside firm might
receive a smallerpayofffrom an innovationthan would the present incumbent,
because of havingto compete with the present incumbent,is absent in the caseof drasticinnovationsbecause the flow of profit rt?+ in (2.12) is independentof
whether the firm earningthose profitshas access to the previouspatent.S
There is an importantintertemporalspilloverin this model. An innovation
raises productivityforever.It allows each subsequentinnovationto raise At by
the same multiple y, and with the same probabilityAp(nt), but from a starting
value that is higher by the multiple y than it would otherwisehave been. The
producerof an innovationcaptures(some of) the rents from that productivity
gain, but only duringone interval.After that the rents are capturedby other
innovators,building upon the basis of the present innovation, but withoutcompensatingthe present innovator.6This intertemporalspilloverplays a rolein the welfare analysisof Section 4 below.
5If, instead of a constant-returns research technology, each firm had an identical researchtechnology with rising marginal cost, then the monopolist might do some research, but the Arroweffect would imply that the monopolist would do less research than each outside research firm, as
shown by Reinganum (1985, Proposition 2) in a similar context.
6
This is the spillover identified by Romer (1990, pp. S83-S85).

331

MODEL OF GROWTH

The model also embodies Schumpeter'sidea of "creativedestruction."Eachinnovationis an act of creation aimed at capturingmonopolyrents. But it also
destroys the monopoly rents that motivated the previous creation. Creative
destruction accounts for the term Ap(nt+d) in the denominatorof Vt+1 in(2.12). More research reduces the expected tenure of the currentmonopolist,
and hence reduces the expected present value of its flow of rents.

2.D. Capital Markets

The structureof capitalmarketscan be specifiedin manydifferentways.One
is to suppose that there is a frictionlessWalrasiancredit marketin which future
expected consumptioncan be discountedat the constant rate r. Another is tosuppose that there is no credit market.Accordingto the latter specificationall
nonresearchworkersconsumetheir wage income at each instant,the ownersof
the monopoly intermediatefirm consume their flow of profits at each instant,and researchworkersreceive no pay unless their firmsinnovate,at which timethey are paid in shares of the next intermediate firm. According to either
specification,all researchfirmscould be assumedto be owned by their workers,and (2.9) would represent the expected flow of surplus to be divided among
them. The crucial assumptionthat utility is linear in consumptionmakes thesedifferent specificationsall equivalent,by removing any motive to use capital
marketsfor risk-sharing.
3. PERFECT FORESIGHT DYNAMICS AND BALANCED GROWTH

3.A. Equilibrium

At any point in time there is only one decision for society to make; namely,
how to allocate the fixed flow N of skilled labor between manufacturingand
research. Combining(2.5), (2.7), (2.10), (2.12) and the equilibriumcondition
N = nt + xt yields(3X1)?1p(n)

r + A.(n), nt >0,with at least one equality.

Condition(3.1) determinesresearchemploymentat t as a functionof research
employmentat t + 1:
(3.2)

nt = qi(nt+ )I

where ,l: [0, N)-1 R+ is a strictlydecreasingfunction wherever it is positivevalued.
The functionalrelationship frbetween research employmentin two successive periods is illustrated in Figure 1, where c(nt)is the "marginal cost of

332

PHILIPPE AGHION AND PETER HOWITT
marginal cost, benefiti (N))
(&

_

_

_

_

_

_

_

_

_

_

_

_

_

_

_

_

r
c(nt)

X,.

b(nt ,

!~~

~

Xlim

~~~~~~~~~~~i(m)f(N)________ __ _Xr+x

O

,I

,

.

.

n noA

n

,
I

n

I

I
,

,

I

jn
nfn

I.

,

n

I

research

N employment

FIGURE 1.-The effect of future researchon currentresearch:no = q,(n1)and n1 = 4(n2). The
pair (0, ng) constitutesa no-growthtrap.

research" and b(nt+l) the "marginal benefit of research," defined by
6(N

-nt)

A(p'(nt)yir(6(N

- nt+

1))

b(nt+,) r+Ap(n)+1)

By (2.11) and Assumption 1, c is strictly increasing, b is strictly decreasing, andc(nt) -* oo as n t - N. It follows that in the case illustratedin Figure 1, wherec(O) <b(O), tJ(nt+1) is well defined on [0, N), and is positive and decreasing if
and only if 7 nt+1 < i. In the case where c(0) < b(0), fr(nt+1) is identically zero.
In economic terms, there are two reasons for the negative dependency of
current research on future research, corresponding to the two places in whichnt+1 enters the expression for the marginal benefit of research, b(nt+,). That is,
a foreseen increase in research next period discourages research this period (a)by raising future wages and hence reducing the flow of profits ir(CO(N- nt+))
to be captured from the next innovation, and (b) by raising the rate of creative
destruction A p(nt+) next period and hence shortening the expected lifetime of
the monopoly to be enjoyed by the next innovator.
A perfect foresight equilibrium (PFE) is defined as a sequence {nt}o satisfying(3.2) for all t > 0. In Figure 1 the sequence {no, n1, n2,... }constructed from the
7The criticalvalue ni is definedby c(O)= b(nl),unless limn , Nb(n)> c(O),in whichcase n = N.

MODEL OF GROWTH

333

counterclockwisespiral startingat no constitutes a PFE. A stationaryequilibrium correspondsto a PFE with nt constant. It is defined as the solution ton =nThere exists a uniquestationaryequilibrium.As Figure 1 shows,if c(O)< b(O)then n is positive,and is definedby,j( Ny 7( (1( Nn)n)r + Ai(n)Ap()(nIn this case growth is positive because innovationsarrive at the Poisson rate
0. If c(0) > b(O)then n = 0 and there is no growth,because the Poisson
arrivalrate of innovationsis A p(O)= 0. Henceforth,assumethat c(O)< b(O)andn > 0.
Other equilibria may also exist. A two-cycle is a pair (n0,n1) such thatno = qi(nl) and nl = qi(n0). It defines a PFE of period two. If both no and nl are
positive, the PFE is a "real" two-cycle. If either no or nl is zero, it is a
"no-growthtrap." In a real two-cycle, the prospect of high research in odd
intervals discouragesresearch in even intervals, and the prospect of low research in even intervalsstimulatesresearchin odd intervals.8A no-growthtrap
is the extremecase in whichthe prospectof high researchin odd intervalsshuts
down research completely in even intervals. Although the no-growth trap
defines an infinitesequence {nt}0,the oscillationwill cease after one innovation.
From then on no growthwill occurbecause no innovationswill occur. It is clear
from Figure 1 that a no-growthtrap exists if lim, -f i(w) = 0 and r is smallenough. A real two-cycle will exist if a no-growth trap exists and9 c'(n)+A(n)>b'() > O.
Consider the Cobb-Douglas example: F(x) = x', with a linear research technology p(n) n. From (2.8), the equation(3.3) defininga positive n is

1 -a
(3.4)

1=

r+

A

8
Shleifer(1986)also findsdeterministiccyclesin a modelof multipleequilibriawith innovations.
The source of multiplicityin Shleifer'smodel is a contemporaneousstrategiccomplementarity,
whereby the incentive to innovate this period is stronger the more innovationsare occurring
elsewhere in the economy this period. No such strategiccomplementarityexists in the present
model, in whichmore researchthis periodraisesthe marginalcost of researchwithoutaffectingthe
marginalbenefit.BecauseShleiferassumesthat imitationdestroysthe returnfrominnovationsafter
one period, his model does not exhibitthe dependencyof currentresearchupon future researchwhich underliesthe cycles, as well as the other equilibria,in the present model. Deneckere andJudd(1986)also generatecyclesin a modelof innovations.Theircyclesarisefromlocal instabilityof
a uniqueequilibriumratherthan from multipleequilibria.Like Shleifer,they also do not allow forany effect of futureresearchupon currentresearch.
9Considerthe second-iteratefunction+2(n). Geometrically,12 is definedby reversingthe spiralillustratedin Figure 1; thus no = /i2(n2). Supposea no-growthtrap exists. Then 0 = +f2(n) < n forsmallbut positiven (see Figure1). Because c'(ni) + b'(n) > 0, thereforethe counterclockwise
spiralin Figure1 spiralsout in the neighbourhoodof ni,so that +/,2(n) > n for n close to but less than nu.
By the continuityof qf2 there must exist an no strictlybetween 0 and n, such that /i2(n0) = no.
Evidentlyno,O(n0) constitutea real two-cycle.

334

PHILIPPE AGHION AND PETER HOWITT

and the conditionfor n to be positive is
1 <Ay

(3.5)

(

N/'r.

Since fr(oo)= 0, a no-growth trap exists when r is small. Since10 b'(nf)/c'(n) -O 0
uniformlyin r as a -O 0, a real two-cycleexistswhen a and r are small enough.

3.B. Research in Stationary Equilibrium

The rest of the paper focuses on stationaryequilibria.Comparative-statics
analysisof (3.3) shows the followingproposition.
PROPOSITION
1: The amount of research employment n1in a stationary equilibrium increases with: (a) a decrease in the rate of interest r; (b) an increase in thesize y of each innovation; (c) an increase in the total endowment N of skilledlabor; or (d) an increase in the arrival parameter A.

This propositionis intuitive:(a) A decrease in the rate of interest increases
the marginal benefit to research, by raising the present value of monopoly
profits. (b) An increase in the size of each innovation also increases themarginalbenefit to research, by raising the size of next interval'smonopoly
profitsrelativeto this interval'sproductivity.(c) An increase in the endowmentof skilled labor both increases the marginalbenefit and reduces the marginal
cost of research,by reducingthe wage of skilled labor. (d) An increase in the
10
In this exampleb(n)-yir(,r5(N
rNn)

b(n)=

-yii-

- n))

and

c(n)

-(N-

n)/A.

n) - Ab(n)andTherefore

BecauseAb(nf) = Ac(n)=b'(n^)/c'(n^)

Butrr'G5(N - n))=6(N=Ay)'(-(Nn))c5(N-

(c(N-

r+An

-

=

-'(N

n), therefore
-n^))

(t(N

- n) and(N+ A 6(N- n) =a2(N(A(N-n)-c'(n)- n^) 16'(N

-)a-l

-

n^)

Therefore

1

a

r+An

From this and (3.4),

b (nr)/c (nr)=y-+in r as a 0.O
1 0 (luniformly_)(, )- n)/A.

MODEL OF GROWTH

335

arrivalparameterdecreasesboth the marginalcost and the marginalbenefit ofresearch, because on the one hand it results in more "effective" units ofresearch for any given level of employment,but on the other hand it also
increases the rate of creative destructionduringthe next interval.The former
effect dominates.
The above discussion of result (d) suggests an interesting implication ofcreativedestructionthat could arise if the arrivalparameterAwere permittedto
vary from one interval to the next. Suppose, for example, that with eachsuccessfulinnovationa new value of Awas drawnfrom the finite set {A1,..., Am}
accordingto a fixed transitionmatrix B. Transitioninto a high-Astate could
represent a fundamentalbreakthroughleading to a Schumpeterianwave ofinnovations,whereas transitionto a low-Astate could representthe exhaustionof a line of research.Then a stationaryequilibriumwould involvenot one levelof researchemploymentbut one for each state. Now consider the effects of aceterisparibus increase in A2. This parameter change might raise research
employmentin state 2, but it would tend to reduce research employmentin
other states, by increasingthe rationallyexpected rate of creative destruction
during the next interval. Furthermore,even though the parameter change
represents an unambiguousimprovementin the productivityof the researchtechnology, it might reduce the average level of research employment instationaryequilibrium.Indeed, Appendix 1 works out a numericalexample inwhich,in the limit, as A2becomes infinite,averageresearchemploymentfalls tozero.
The linear Cobb-Douglasexampleof (3.4) and (3.5) aboveyields an additional
comparative-staticsresult by parameterizingthe degree of market power enjoyed by an intermediatemonopolist. Specifically,1 - a is the Lerner (1934)
measure of monopoly power (price minus marginal cost divided by price),(1 - a) -1 is the elasticityof demandfaced by an intermediatemonopolist,and
1 - a is the fractionof equilibriumrevenuein the intermediatesector accruingto the monopolist, wt/(wt+ w xt). Thus, by all measures, the degree of market

power is measuredinverselyby the parametera.
Accordingto (3.4), an increasein the degree of marketpower(decreasein a)
increases the stationary-equilibriumamount of research n whenever n is
positive.Accordingto (3.5), givenfixedvalues of the parametersy, A, r, and N,
the stationary-equilibrium
amount of research will be positive if and only ifthere is at least some minimaldegree of marketpower;that is, if and only if a
is less than the criticalvalue
a*--

AyN
< 1.
AyN + 1

If the degree of marketpowerfalls short of this minimalvalue, then the flow ofmonopoly profits from the next innovationwould not be enough to induce
positive researchaimed at capturingthose rents even if they could be retained
forever,with no creativedestructionin the next interval.

336

PHILIPPE AGHION AND PETER HOWITT

3.C. Balanced Growth
Real output (i.e. the flow of consumptiongoods) in the economy during
interval t is
Yt=AtF(N(3.6)
which implies(3.7)- ni),

Yt+1= yyt

Thus the time path of the log of real output ln y(r) will be a random
step-functionstartingat ln yo = ln F(N - ni) + ln AO, with the size of each step
equal to the constantln y > 0, and with the time between each step {A1,A2' ... }a sequence of iid variablesexponentiallydistributedwith parameterA0(nI). This
statement and (3.3) fully specify the stochasticprocess drivingoutput, in termsof the parametersof the model.
Not surprisingly,this stochastic process is nonstationary.Suppose observations were made at discretepoints in time 1 unit apart.Then from (3.7),(3.8)
lny(-+ 1) =lny(r) +E(ir)(T=0,1,...
where 80r) is ln y times the numberof innovationsbetween r and r + 1. From
the above analysis
(E(0)

E(1)

In
ly'

In y

...

is a sequence of iid variablesdistributedPoisson with parameterAp(h). Thus
(3.8) can be written asln y(r + 1) = ln y(r) + Ap(n) ln y + e(Tr)(3.9)( = 0, 1
where e(X) G--() - A'p(nh)ln y. Note that e(X) is iid., with
(3.10) E(e('i)) = 0, vare(Ti) = Ap(nh)(ln y)2.
From (3.9) and (3.10), the discrete sequence of observationson the log of
output follows a randomwalk with constant positive drift. It also follows thatthe economy'saveragegrowthrate (AGR) and the varianceof the economy's
growthrate (VGR) are given by(3.11)AGR = Ap(n) ln y,

VGR = AP(h)(lny)2.

Combining(3.11)with Proposition1 allowsone to sign the impactof parameter changes on the averagegrowthrate. Increasesin the arrivalparameter,thesize of innovations,the size of skilled labor endowment, and (in the CobbDouglas example)the degree of marketpower all raise AGR. Increasesin therate of interestlower it. The parameterchangeshave the same qualitativeeffect
The effect ofon VGR as on AGR. The effects are intuitiveand straightforward.
market power, combined with the finding that a minimal degree of market
power is needed before growthis even possible, underlinesthe importanceof
imperfectcompetitionfor the growthprocess.

337

MODEL OF GROWTH

The example of Appendix 1 shows, however,that in a more general setting
where the arrivalparameterA can varyfrom state to state, it is not alwaystruethat an unambiguousimprovementin the productivityof the researchtechnology will increasethe economy'saveragegrowthrate. Instead, an increasein thearrival parameter in one state can discourage research in other states by
increasingthe rationallyexpected rate of creativedestructionto such an extent
that the economy'saveragegrowthrate falls.
4. WELFARE PROPERTIES OF THE STATIONARY

EQUILIBRIUM

This section compares the laissez-faire average growth rate derived above
with the AGR that would be chosen by a social plannerwhose objectivewas tomaximizethe expected present value of consumptiony(r). Since every innovation raises y(r) by the same factor y, the optimalpolicy consistsof a fixed levelof research.Expectedwelfare is
000

(4.1)

U

0

e-rr EH(t,t=O- n) dT,T)AtF(

where H(t, r) equals the probabilitythat there will be exactly t innovationsup
to time T. Given that the innovationprocess is Poisson with parameterApo(n),we have(4.2)

H(t, r) = (A9(n)T)te-A@(n)Tt

From (4.1) and (4.2),
AOF( N-n)Equation (4.3) identifies U as the initial flow of output AOF(x) discounted at
the rate r - A(p(n)(y - 1). This "social discount rate" is less than the rate ofinterest r because the stream of output will be growing over time. Morespecifically, the social discount rate is the rate at which each risk-neutralindividualin the economywould capitalizea streamthat was perpetuallysubject
to increases by the factor (y - 1) with a Poisson arrivalrate of A*p(n),andconstantotherwise.
The socially optimal level of research n* maximizes U. The first-orderconditionfor an interiormaximumis
44)

F'(N -n*)Aspt(n*)(y -1)F(N
rr-Ap(n*)(y

-n*)
-1)

(If no solution exists to (4.4) then n* = 0.) This level of researchwould produce.
an average growth rate of Ap(n*)lny. Accordinglylaissez-faireproduces anaverage growth rate more (less) than optimal if n > (<)n*. Which way theseinequalitiesgo can be checked by comparing(4.4) with the equation determin-

338

PHILIPPE AGHION AND PETER HOWITT

ing the stationaryequilibriumlevel of research ni:(N -fi)

3

Ap'(fi)N -n))
yi=(4(
r + AP(n)There are four differencesbetween (4.4) and (3.3). The first is that the social
discount rate r - A<p(n)(y- 1) appears in (4.4) instead of the "private discountrate" r + A<p(n).The social rate is less than the rate of interest, whereas theprivaterate is greater.This differencecorrespondsto the intertemporalspillover
effect discussed in Section 2. The social planner takes into account that thebenefit to the next innovationwill continue forever, whereas the private researchfirmattachesno weightto the benefitsthat accruebeyondthe succeeding
innovation."1

The second differenceis that total output F(N - n*) appears on the righthand side of (4.4) instead of the flow of profitsi(J(N - ni))that appearsin themarginalbenefit in (3.3). This is the "appropriability"
effect familiarfrom thepatent-raceliterature.
The third difference is that the factor (y - 1) in (4.4) replaces y in themarginalbenefit of (3.3). This correspondsto a "business-stealing"effect. The
private researchfirm does not internalizethe loss to the previousmonopolist
caused by an innovation.In contrast,the social plannertakes into accountthat
an innovationdestroysthe social returnfrom the previousinnovation.12
The fourth difference is that the marginalproduct F'(N - n*) appears in(4.4) in place of the wage J(N - ni) in the marginalcost in (3.3). This is a"monopoly-distortion"effect which does not appear in the partial-equilibrium
patent-race literature. Specifically, the social cost of research employment
exceeds the private cost because in laissez-fairethe alternativeuser of skilledresearchlabor is a monopolist.
11Two additionalspilloverscould easily be included.First, researcherscould benefit from theflowof others'research,so that an individualfirm'sarrivalratewouldbe a constant-returns
functionof its own and others' research.Second, there could be an exogenousPoisson arrivalrate , ofimitationsthat costlesslycircumventthe patent laws and clone the existingintermediategood. Bothwould have the effect of loweringAGR. Also, as shownin Aghionand Howitt(1988),the inclusionof A wouldintroduceanothersourceof cyclesin the economy,since each imitationwouldmaketheintermediateindustryperfectlycompetitive,whichwouldraisemanufacturing
employment,until thenext innovationarrives.
12 In the patent-raceliteraturethe business-stealingeffect is usually derived in a symmetricmodel with no incumbent,in which all researchfirmsenjoysome positivesurplusbecause there isno free entry.An exampleis Mortensen(1982),who identifiesthe business-stealingeffect with thecomment:"Wastefulcompetitionarisesbecausenone of the contestantstakes accountof the loss ofthe prospectthat others sufferwhen the former'sdiscoveryends the game"(p. 970). In the present
paper the loss accruesnot to the other researchfirms,whose value remainsequal to zero, by freeentry,even after an innovationby anotherfirm,but to the incumbentmonopolistwho, because of
the replacementeffect, has chosen not to participatein the patent race. As Tirole (1988, p. 399)
notes, there is anothernegativeexternalitythatwouldoccurif the researchtechnologyhad memory.
Specifically,a firmmightengagein researchin orderto reducethe probabilitythat its rivalswill win
the race. This effect is absentfromthe presentmodel,in whichone firm'sresearchhas no effect on
the others'probabilitiesof innovating.

MODEL OF GROWTH

339

The intertemporal-spilloverand appropriabilityeffects tend to make thelaissez-faireaveragegrowthrate less than optimal,whereas the business-stealing and monopoly-distortioneffects tend to make it greater than optimal.
Because these effects conflictwith each other, the laissez-faireaveragegrowthrate may be more or less than optimal. This can be seen in the linear
Cobb-Douglasexample,where n* and n satisfy
1

(4.5)

- (N-n*)A(y-1)
r-An*(
1=

Ay
(3.4)

1=

1 -a)
--

(N

n)

r+An

In this example, the appropriabilityand monopoly-distortioneffects are combined in the presence of the factor (1 - a) in (3.4); together they tend to makethe laissez-faireAGR less than optimal.This combinedeffect togetherwith theeffect dominateswhen the size of innovationsy is large,
intertemporal-spilloverin which case13n < n*. However,when there is muchmonopolypower (a close
to zero) and innovationsare not too large, the business-stealingeffect dominates, in which case'4 n > n*.
5. NONDRASTICINNOVATIONS

Until this point the analysishas assumedthat innovationsare drastic;that theintermediatemonopolistis not constrainedby potential competitionfrom owners of previouspatents.The presentsection showsthat the analysisof stationary
equilibriacan be generalizedto the case where innovationsare nondrastic.
Innovationsare nondrasticif and only if the previousincumbentcould make apositive profit when the current one was chargingthe price pt =A,F'(1(o,))
which yields an unconstrainedmaximumto the current incumbent'sprofit. If
innovationsare nondrastic,the currentincumbentsets the maximumprice thatgives the previousincumbentnonpositiveprofits,and satisfiesall the demandat
that price, leavingnone to the previousincumbent.
The previous incumbent could make a positive profit if and only if acompetitiveproducerof consumptiongoods could produceat an averagecost ofless than unity by combining unskilled labor with the previous incumbent's
good, buyingthe latter at a price equal to its averagecost of productionwt;that
13From(4.5), as y rises to the upper limit 1 + r/AN, n* approachesN while, from (3.4), n' isboundedbelow N.
14If 1/a> 1 + r/AN, then n>
for all , whereas if ay l+oar/AN, then n*=O. These
inequalitiesare compatiblewith the conditionderivedbelow for innovationsto be drastic,namelythat y > aa,

as can be verified with the example: a = 1/2, y = r2, r/AN = 2(W2 - 1).

340

PHILIPPE AGHION AND PETER HOWITT

is, if and only if the condition(5.1)

C(w,w,W) <Atwere to hold with strict inequality, where wm is the equilibriumwage ofunskilled labor and C is the unit-cost function associatedwith the production
function F.
In equilibriumall the unskilledlaboris combinedwith the currentincumbent's
intermediategood. Thus the unskilledwage must satisfythe competitiveequilibrium condition:(5.2)

C(wtm, pt) =At.

It follows that innovationsare nondrasticif and only if (5.1) holds with strict
inequalitywhen w/msatisfies(5.2) togetherwith pt =At F'(i(ct)). It also follows
that if innovationsare nondrasticthen Pt and w/m satisfy (5.2) and (5.1) withequality.
In the Cobb-Douglasexample,where the unconstrainedoptimalprice for thecurrentincumbentis wt/a, innovationsare nondrasticif and only if(5.3)

y < a-,

in which case15
Pt =

(5.)

laW

t,

7t=

(y

/a -wtxtg

xt = (Y 1/aZwta) 1/(a-1)

The rest of the analysisof this section will focus on stationaryequilibriawith
positive growthin the linear Cobb-Douglasexample.
The analysisassumesthat the monopolistchooses to do no research,as in thecase of drasticinnovations.This implicitlyplaces a lower bound on the size ofinnovations,because it requires the efficiency effect to be smaller than thereplacementeffect. Appendix2 shows that the condition(5.5)

y1/aY(1 + y-1)

> y-1 + min {y2/a,a-1}

is sufficientfor the monopolist to do no research. Note that this condition issatisfiedwhen y is close to the value a - at which innovationsbecome drastic.
If there is positive researchduringinterval t, then, as before,(5.6)=

A56

15+

w(Pn' )

r + ASD(nt+ I

In the Cobb-Douglasexample,the unit-costfunctionis
c wM, p)-(1(X-a)-apa(WM)I-a

It follows from this and (5.2) that if the incumbentchargedthe unconstrainedprofit-maximizing
price a-1lw,, the unskilledwage would be wfm= (1 - a)a 2a/(1-a)A t(W/A t)-a/(l-a). Puttingthis
into (5.1) yields the condition y< a-a. Treating (5.1) as an equality and solving it and (5.2)
simultaneouslyfor (wM,p,) yields Pt = yl/aWt

341

MODEL OF GROWTH

In a stationaryequilibriumwith positive growth,(5.4) and (5.6) imply1)(N-n)(5.7)
1 =Ay(yl/a(5*7) 1 = r + An^

Equation(5.7) defines the stationaryequilibriumlevel of researchn. It is thesame as the equation (3.4) that applies in the drastic case, except that themarkupyl/a in (5.7) replacesthe markupa1 in (3.4). The comparative-statics
resultsof Proposition1 applyto the solutionof (5.7). In addition,the stationary
equilibriumlevel of research defined by (5.7) is increased by an increase inmarket power (a decrease in a) as in the drastic case. Thus in the linear
Cobb-Douglasexampleall the comparativestaticsresultsderivedfor the case of
drasticinnovationsare valid also when innovationsare nondrastic.
Comparisonof (5.7) and (4.5) shows that the same welfare effects analyzedin
Section 4 operate in the case where innovationsare nondrastic,again with theresult that research and growth under laissez-fairemay be more or less than
optimal.16

As is customaryin the patent-raceliteraturethis analysishas ruled out thepossibilitythat the currentand previousincumbentmight contractto share thehighermonopolyprofitsthat could be earned if the previousincumbentagreed
never to compete. For example,the previousincumbentmight sell its patent tothe currentone; in the extremecase where the previousincumbentalwayshadno bargainingpower in negotiation with the current one, competition frompreviousvintagesof the intermediategoodwouldneverconstrainthe monopolist,
and the above analysisof drasticinnovationswould applyno matterhow small
the innovationswere.
6. ENDOGENOUS

SIZE OF INNOVATIONSThis section generalizes the analysis of stationary equilibria by allowing
researchfirmsto choose not only the frequencybut also the size of innovations.
It showsthat underlaissez-faire,innovationswill be too smallif they are drastic.
In the nondrasticcase, the tendency to make innovationstoo small is at least
partly mitigated by the incentive for innovators to move away from theircompetitivefringe,which they can do by increasingthe size of innovations.
Assume that the arrivalrate of innovationsto a firm employingthe factor
combination(z, s) and aimingfor innovationsof size y is A4(z, s)v(y), wherev'(y) < 0; the bigger the innovation, the harder it is to discover. Assumev"(y) < 0; the marginalcost (in terms of lower arrivalrate) of aimingfor larger
innovationsincreaseswith the size of innovations.Then the product yv(y) is aconcavefunctionof y.
16 Supposey = C2 and a = 1/2. To get hn< n* let r approach(V's-

1)AN fromabove;then n*

approaches N whereas ni is bounded below N. To get n > n* let r = 2(V2 - 1)AN; then n* = 0 andn > 0. In either case y = a-, but the exampleis robustto a smalldecreasein y that wouldsatisfy

the necessaryand sufficientcondition(5.3) for innovationsto be nondrasticwithoutviolatingthe
sufficientcondition(5.5) for the monopolistto do no research.

342

PHILIPPE AGHION AND PETER HOWITr

The analysis focuses again on stationary equilibria with positive growth.
Considerfirst the case of drasticinnovations.By the same logic as before, thepayoffto the t + 1st innovatoris
(6.1)

+
At1+=

( A)V(A)value of y. If the t + 1st innovationhas
where 5 is the stationary-equilibrium
size y, not necessarily equal to A,then A+ 1 - yAt and Vt+ 1 = yVt. Therefore

the expected flow of profitsto the researchfirmin interval t isAA(z, s)v(y)y7Vt-wtz-ws.
(6.2)
choice of y also maxiThe firm takes Vt as given. Thus its profit-maximizing
mizes the product v(y)y. Because this product is a concave function of y,therefore 5 is definedby the condition17(6.3)
v(9) + Av'(9) = 0.
The first-ordercondition for profit maximizationwith respect to skilled labor,
togetherwith (6.1) producesan expressionanalogousto (3.3):
Ji(N -fn)(6.4)

ApA'(n)v(A)A( dN -A))r + Ap(n)v()

The comparativestaticsanalysisof Section 3 carriesthroughunchanged,since 5
is determinedby (6.3) independentlyof all parametersthat do not enter thefunction v, with the obvious exception that it is no longer permissible toinvestigatethe effects of a change in y.
As in Section 4, the expected presentvalue of consumptionequals~~ AOF(N-n) -1
U=(6.5)~rAip(n) v(y) (y

(6.5)

U=

where the denominatoris the social discountrate. Therefore,independentlyof
the choice of n, the social planner will choose y so as to maximize theexpressionv(y)(y - 1). The sociallyoptimalvalue y* is then definedby(6.6)v(y*) + y*v'(y*)- v'(y*) = 0.

By concavityof yv(y),18 5 < y*. Innovationsare too small under laissez-faire.
This result is another manifestationof the business-stealingeffect. The social
plannerchooses y so as to maximizethe arrivalrate multipliedby the net size(y - 1) of innovations, whereas the private research firm, which does notinternalizethe loss of the existingvintage of intermediategood, maximizesthearrivalrate times the gross size y.
17 Note that it is always possible to choose the function v so that the solution to (6.3) satisfies thecondition for innovations to be drastic in the Cobb-Douglas example: y > a-.
18 Since v' < 0, (6.6) implies that v(y)y is locally decreasing at y*, so that y* exceeds the point y
at which v(y)y is maximized.

343

MODEL OF GROWTH

The sociallyoptimal level of researchemploymentn* satisfies the conditionv (y*) (y* -1) F(N -n*)F'(N-n*)ACp'(n*)67)

r -Ap(n*)v(y*)(y*1)

Comparisonbetween (6.4) and (6.7) reveals the same welfare effects as in theanalysisof Section 4. In addition, the fact that y <-y* in itself makes n < n*.
This is because, as we have seen, v(9)(9 - 1) < v(y*)(y* - 1). So if the otherfour effectswere absent,and both n and n* were determinedby (6.7), the effecton researchemploymentwould be the same as if the laissez-faireeconomyhada smallerarrivalparameterA, which would reduce n below n*.
The economy'saveragegrowthrate A(n)v( )ln y is affectedby the fact that
innovations are too small under laissez-faire, although the direction of theoverall effect is ambiguous.The direct effect on ln is to decrease AGR. The
direct effect on the arrivalrate Apn v(y) is to increase AGR. The indirect
effect on the arrivalrate workingthrough (n) is to decrease AGR.
In the nondrasticcase, the above business-stealingeffect wherebyinnovations
are too smallunder laissez-faireis mitigatedby an additionaleffect, namelythat
privateinnovatorstend to increase the size of innovationsin order to increase
their profitmargins.This marginis independentof the size y in the drasticcase
but it increaseswith y in the nondrasticcase. (In the CobbDouglas examplethe
profitmarginis a-1 - 1 if the innovationis drasticand y'/a - 1 if nondrastic.)
The following example shows, however, that this additional "profit-margin"effect does not necessarilyoverturnour earlier result to the effect that innovations are too small.
A

Let P(n) = n, F(x) = /2 . From (5.3) innovationsare nondrastic
From (5.4), the payoff to the (t + 1)st innovator in a stationary

EXAMPLE:

if y < 12.

equilibriumis
(6.8)

Vt+1

(Y2_ 1)X(Ay)W,+1

r+ Av()_(N-x( A,y))where x(d, -y)= (2dy2)-2. In (6.8), y is the size of innovationto be chosen by
the innovatorduringinterval t, whereas y and di denote stationary-equilibrium
values which the innovatortakes as given. As in the drastic case, w +1 =yw .
Therefore must solve the equationAy = argmax-

V(y)(y2v(

A< argmaxv(y)(y2

-

{y)

1)x(

r+AV(y)(NX(w,,y)ywtay))Since dx/dy < 0, we havey1)x(o, y)y = 7

From the above analysiswe know thatargmax v(y)(y- 1) = Y*.

344

PHILIPPE AGHION AND PETER HOWITT=< y* is that (-21)x(d,y)yTherefore a sufficient condition for(y - 1)g(y) with g'(y) < 0. The latter is true,9 with g(y) (y + 1)y/4j2y4.
-

7. STRATEGIC MONOPSONYEFFECT

In this section the i'ntermediatefirm is assumed to take into account its
influence on the amountof currentresearchand therebyon the probabilityof
its replacement.In particular,by increasingits demand xt for skilled labor, themonopolistcan raise the wage rate that must also be paid to skilledworkersin
research. The effect is to reduce the equilibriumamount of research nt andconsequentlyto delay the arrivalof the (t + 1)st innovation.The monopolistwill
trade this gain off againstthe higherwages it must pay its own skilled labor.
The analysis focuses on stationary equilibria with positive growth. The
monopolistduringintervalt chooses xt to maximizethe expectedpresentvalueof profits:=-(AtF'(x)wt)xr+Ap(N-x)

subjectto
(* )

wt

A
Ap'(N-x)Vt+,

where (*) follows from (2.10). The magnitudes xt, nt, Vt/At, and wt/At are
constant,at the equilibriumvalues x, Ti,V, and -. Therefore x solves
(7.1)
(7.1)

V= max[ F'(x) - Ay~p'(N -x)V]x( r A1p(N-)V]x

The first-orderconditionis(7.2)

F'( x) +xF"(x) = AV[(y-1)p'(N-x)-xyp"(N-x)J.

From the constraint(*),
-

(7.3)

= AyqP'(N-x)V.

From (7.2), (7.3), and the definitionof 6,(7.4)[y -1-y(N -)qp"(1)/AP'(O)j6(N-i)-(n).

level of research Ti is given by theIt follows that the stationary-equilibrium
analogueto (3.3):
1

6@(N-n)F1(7[

Ap'(i)

j y -1- y(N[-fi)qP"(ni)/P'(n)J______

r + AP(n)19 Because it compares y* with f, this analysis would apply even if the research firm ignored thelegative effect that its choice of y has on the value of an innovation by reducing the equilibriumevel of x,+1 and hence raising the rate of creative destruction next period.

MODEL OF GROWTH

345

where the function 7Fis defined as-nT) - (T)] (N - n-)rrT(ni)--F'(N(7.6)
Assume that the expression (N n)'p"(n)is nondecreasingin n. Then theleft-handside of (7.5) is increasingin n-.If the right-handside is still decreasing,then the solution to (7.5) is unique. It is straightforwardto verify that all thecomparativestaticsresultsin Proposition1 applyto this solution,and that in the
Cobb-Douglasexample the solution is an increasingfunction of the degree ofmarketpower.
Welfare analysisof stationaryequilibriais somewhataffectedby the strategic
monopsonyeffect, but the overall result remains,namely that the laissez-faire
averagegrowthrate may be more or less than optimal.Comparisonof (7.5) withbusiness-stealing,
appropriability,(4.4) revealsthe same intertemporal-spillover,and monopoly-distortioneffects as before, although the monopoly-distortion
effect will be quantitativelydifferentbecause Fr(n)# i+(J(N - n)). There is anadditionaleffect, however,from the presence of the termy - 1 - y( N -n)/'()Jon the left-hand side of (7.5). This additionaleffect is the "monopsony-distortion" effect.
In the linear case, where so"is zero, the constraint (*) indicates that theintermediatefirm'swage rate is independentof the amount of skilled labor it
effect inducesit to hire more skilledworkers
hires, so the monopsony-distortionin order to reduce the amount of current research, and hence the amount ofcreative destruction.The effect just cancels the business-stealingeffect, as canbe seen by multiplyingboth sides of (7.5) by (y - 1)/y. Thus in the linear
Cobb-Douglas example, research and growth are unambiguouslyless than"

optimal.

< 0 the monopsony-distortioneffect is ambiguIn the general case where Sp"
ous, because hiring more skilled labor increases the intermediatefirm'swage
rate at the same time that it reduces creative destruction.Because of theseconflictingtendencies it is straightforwardto constructexamples in which theeffect vanishes.More specifically,given any specioverallmonopsony-distortion
ficationof the model it is possible to perturbthe researchfunction Soin such a
way20that n and n* remain unchangedand the solution ni to (7.5) becomesequal to n/. Since n/can be more or less than n* it followsthat nican be more orless than n*.
The rest of the paper ignores the strategic monopsonyeffect, by assumingthat intermediatefirmstake as given the wage of skilled labor and the amount
20Justperturb'pin such a way that p(ni),q(n*), 9'(n'),and 9'(n*) remainunchanged,but e"(h)is made equal to -9'(ni)/y(N - n). This can be done without alteringthe sign of '' and '" on[0, N). Accordingto (3.3) and (4.4), ni and n* will be unchanged.This perturbationmakes thesecondfactoron the left-handside of (7.5) equal to unitywhen ni = n', and makes i,(ni) = 4Kr((Nni)).Since ni solves(3.3) it will now also solve (7.5).

346

PHILIPPE AGHION AND PETER HOWITTof research. This assumption is based on our belief that the effect is notimportant,because it derivesfrom the simplifyingassumptionthat there is only
one intermediatefirmin the economy.If there were manycompetingintermediate firms,as in the next section, each mightplausiblyregarditself as too smallto
affect the skilledwage or the amountof research.
8. MANY INTERMEDIATE GOODS

This section relaxes the simplifyingassumption of a single economy-widemonopoly in intermediate goods. Suppose instead that there are m different
intermediate sectors. Output of the consumption good is y, = ETl 1Ai,F(xi,),

where xi, denotes the flow of output of the ith intermediate good during
interval t, and where F has all the properties assumed above. (This requiresthat each sector have its own specializedbrandof unskilledlabor.)
FollowingShleifer (1986), suppose that innovationsarrivein differentsectors
in a deterministicorder.21Specifically,the innovatingsector is alwaysthe one
with the lowest productivityparameter Ait, Each innovatorbecomes a local
monopolist in that sector for a period of m successive innovations, and is
replaced by the last of those m innovations.Let At denote the productivity
parameterin the leading sector, where an innovationhas arrivedmost recently.
Assume that Ait = y(l-k)/mA when i is the kth most advancedsector. Then yis again the size of each innovationrelative to the previousvintage of good in
the same sector.
Let xk denote the stationary-equilibrium
employmentof skilled labor in theThenkth most advancedsector.
xk maximizesthe flow of profits:[ Aty(l k)/mFf

( Xk)

- Wt

I Xk

Therefore,W),

Xk =X(

where x is definedas above and w is againthe stationary-equilibrium
productivflow of profits in the kthity-adjustedwage, w,/At. The productivity-adjusted
leading sector is
vr k/At= y(1-k)/

m . (y (k l )/m,)

21The alternative of allowing innovations to be randomly distributed across sectors is analyzed in
an Appendix to an earlier version of this paper, available from the authors upon request. This
Appendix assumes a continuum of sectors and a continual flow of innovations. Whoever innovates at
date r is thereby allowed to enter a randomly chosen sector with the "leading technology" A(r),where A(ir) grows continuously at the exponential rate o-An. The possibility that the same sector
might receive two innovations in rapid succession, before the leading technology has advanced bymuch, implies that in stationary equilibrium some positive (and endogenous) fraction of innovations
will be nondrastic. The model yields all the comparative-statics results of Proposition 1 above,
except possibly for result (b).

347

MODEL OF GROWTH

The productivity-adjusted
wage is the solution to the equilibriumconditionm+ Ex (Y(ki-)/m),

N=n

k==1

which can be written as
Ct=c(N-n),

where the function d^satisfiesAssumption1 above.
Let the technology of research be the same as before, with an arrival
parameterequal to mA. The length of each intervalis distributedexponentiallywith parametermAqp(n),and each local monopolylasts for m intervals.Let rt
denote the time of the tth innovation.Then the value of the tth innovationismkVt= E: E( e - r(Tt +k- I Td

k=1

mmA~p(n)k=1tr+mA(p(n)rt+kmA()1\r +mA~p(n)Jkl()(-/)A

r+mAp(n).)'and the condition for a positive stationary-equilibrium
level of researchis theanalogueto (3.3):(N-)

m

mAp'(n)

k
k=1

(8.)

( mA(n)k-1

r+mAp())

y1/m+[y(k-1)/m&(Nh^)]

r+mA(p(n)(In the linear Cobb-Douglasexamplethis conditionis the analogueto (3.4):m

kl/m'Y/(mA

EY(k-1)/m(a-1)

1 = mA(8.2)/

1-a! \(N -n)a

W-1An )

r + mAn^)k
~~~(

1

m.
E 7(k-1)/m(a-1)
1

Since the right-handside of (8.1) is a decreasing function22of n and.theleft-handside an increasingfunction,the solution to (8.1), if it exists, is unique.
If no solution exists, then the equilibriumlevel of research is zero. All thecomparative-staticsresults of Proposition1 apply to the solution of (8.1), withthe possible exception of (b), the effect of y, the size of innovations.In the
22 Note thatamdn k-i(mA(p(n))kl1rT(r+mAp(n))((-mAp'(n)E(T>

2 > .>.

m>(m)mpf))k%-2(r + mAp(n))

m?1(k - 1)(mAgo(n ))k2( 1km O+1-.

(f))k-1~

k

k= 1

k=2

where

-

(r + mAp(n))

k+ 1 )(r + mAp(n))
-Fk-1

- )

348

PHILIPPE AGHION AND PETER HOWIlT

linear Cobb-Douglasexample,however,it can be shown that (b) remainstrue,
providedthat the social discountrate (the denominatorof U below)is positive.23
In stationary equilibriumeach innovation raises the entire cross-sectional
profileof productivityparametersby the factor y'l/m. It thereforeraises GNP by
and raises the log of GNP by the factor (1/m)ln y. Since the Poisson
y l/marrivalrate of innovationis mAn, thereforethe economy'saveragegrowthrate
is AnInly, exactly as before. The variance of the growth rate is An(lny)2/m;
aggregation across many sectors reduces variabilitythrough a law of large
numbers.
By the same logic as in Section 4, social welfare is measuredby
m
E

U=A0

Y,(1-k)ImF(

Xk )

1

r - mAp(n) (y/m - 1)

A social planner would choose (x1,..., Xmn) to maximize U subject to theconstraint,
(8.3)

N-n

=xl +

+xm,

and n > 0. The first-orderconditionsfor an interiormaximumare(8.4)

F'(Xk)(8.5)

mApD'(n)(yl/m(k = 1,.. . m),=pY(k-l)/m-1)U = Ao,

where ,u is a Lagrangemultiplier.Let ,i(N - n) denote the value of ,u such thatthe solutions(x1,..., xm) to (8.4) solve (8.3). Then (8.5) can be expressedas:(8.6)

mAp(n>)

k=1(Y

)ArP

(n*)(Y/m 1))In the linear Cobb-Douglasexample,
m r((-1mAn

(8.7)

r-A
a7

-Y1/

-1

Comparisonof (8.6) with (8.1) reveals the same four effects as before. The
monopoly-distortion effect is still present because ,i(N - n) > d^(N- n). The
23
It sufficesto show that the right-handside of (8.2) is increasingin y. The ratio of sums in thisexpressioncan be regardedas the expectedvalue of the discreterandomvariable

Zk=k

[lA

1 + rImAn]/

An,

underthe truncatedgeometricdistributionwith parameteryl/m(a -1) < 1. The effecton this ratioofa marginalincrease in y is the sum of the effect on each Zk and the effect of changingthe
parameterof the distribution.The formeris positive.The latterwouldalso be positive,by first-orderstochasticdominance,if Zk were decreasingin k, which it is if the social discountrate is positive.

MODEL OF GROWTH

349

appropriabilityeffect applies sector-by-sector,and is amplifiedby the fact that
>-l/m

>

(

mAp(n)

r + mAp(n) J
if the social discount rate is positive. As before, research and growth underlaissez-faire may be more or less than optimal; in the linear Cobb-Douglas
example,nt< n* if y is large,24but n1> n*if a is small and y is not too large.25
9. CONCLUSION

The paper has presenteda model of economicgrowthbased on Schumpeter's
process of creative destruction.Growth results exclusivelyfrom technologicalprogress, which in turn results from competition among research firms thatgenerate innovations.Each innovationconsistsof a new intermediategood thatcan be used to producefinal outputmore efficientlythan before. Researchfirms
are motivatedby the prospect of monopolyrents that can be capturedwhen asuccessfulinnovationis patented. But those rents in turn will be destroyedby
the next innovation,which will render obsolete the existingintermediategood.
It would be useful to generalizeand extend the analysisin severaldirections,such as assumingthat technologyis ultimatelybounded, thereby requiringthe
size of innovationseventuallyto fall. The model would gain richnessand realism
if capitalwere introduced,either physicalor humancapitalembodyingtechnicalchange,or R and D capitalthat affectsthe arrivalrate of innovations.Allowing
unemployment,by introducingsearch into the labor market, would facilitatestudy of the reciprocal interaction between-technological change and thebusiness cycle. All these extensionsseem feasible because of the simplicityandtransparencyof the basic model.
European Bank for Reconstructionand Development,122 Leadenhall St.,
LondonEC3 V4QL,UK
andThe
Universityof WesternOntario,London,Ontario
Economics,
Departmentof
N6A 5C2, Canada
receivedJune,1989;final revisionreceivedJuly,1991.
Manuscript

APPENDIX

1

AN EXAMPLE WITH A RANDOM ARRIVAL PARAMETER

Let A follow a two-stateMarkovprocess on the space {A1,A2}with all transitionprobabilities
wage
equal to 1/2. A stationaryequilibriumis an equilibriumin which the productivity-adjusted
rate dependsonly on the state of the world,not on time. Let AtVj be the value of makingthe tth
24
From(8.7), as yl/m
is boundedbelow N.
25

rises to the upperlimit 1 + r/mAN, n* approachesN while, from(8.2), ni

If 1/a > 1 + rIAN then ni> 0 for all y, whereasif yllm < 1 + arlmAN, then n* = 0.

350

PHILIPPE AGHION AND PETER HOWITT

innovationand movinginto state j. Assumethe linearcase of qp(n)= n. In anystate i, the marginal
expectedreturnto researchin intervalt is AiAt+?(Vl + V2)/2. This will equal the wage if positive
researchoccursin state i. If researchoccursin all states,the Vi'smustsatisfythe analogueto (2.12):
Vi

r + A i[N-y(Aiy(Vl+ V2)/2)](i = 1, 2).

Define ni= N - i(Aiy(Vl + V2)/2). Then the average level of research employment is
n = nlql + n2q2,

where qi is the asymptoticfractionof time spent in state i. It is easilyverifiedthatql = 1 -q2= A2n2/(Alnl+ A2n2).

To complete the example, take the Cobb-Douglas case (F(x) = xa), and suppose a = y

and r =N= Al = 1. Since k(co) = (co/la2)l/(aeach Vi can be rewrittenas:
V, = [16(Vl+ V2)]

-

{1 -

[4(Vi

=

1/2

= ((1 - a)/a)wA (w), the formula forand r(wo)

-)

+

1

2)]

and
V2 = [16A2(V1+ V2)]'- A2{1 -[4A2(V1+ V2)]

2.

When A2 = 1, the solution to these equations is V, = V2 = VT/8v'_, which implies n, = n2 ==1/3.
As A2 -* oc, the solution approaches V, = 1/4, V2=0, which implies n, =0, n2 = 1, and q1 =n2/(n2 + Alnl/A2) = 1; hence ni = 0.

The economy's average growth rate equals f ln y, where f is the asymptoticfrequencyof
innovations:
Al + A21]

f = Alnlql + A2n2q2-

Thus when A2 = 1, f ln y = (1/3) ln 2 > 0; and as A2 -+ oo,f In y approaches 0.

APPENDIX2DERIVATION OF CONDITION (5.5)

In the stationaryequilibriumdescribedin Section 5, the monopolisthas no incentive to doresearchif(A.1)

Wt> A( VtM+
1- Vt)

where()//a-

t1)wt(N-r+AR

is the value of the monopolist'scurrentpatent and[min (y 2/a,a 1) - 1]w
Vt+l=

A1(N-)~~r+Anwouldbe the grossvalue of the next innovationto the currentmonopolist,for whomthe innovation
would be drasticif y2 > a-', if next period the level of researchwas the stationary-equilibrium
value nR.In fact more than this level of researchwould be conductedif the monopolistwere to
innovate,becausethe monopolistcould then chargea markuphigherthan yl/a, so the valuewould
actuallybe less than Vtm+Y.
Substitutingthese expressionsfor Vt and Vtm+into (A.1) and using the351

MODEL OF GROWTH

fact that wt+ 1 = y wt produces

(A.2)

(

A[yi(y2/a,al)foll
minm date- y -from(A

(yl/a

a

-

_A

1)](N5.7)

r+An

Condition (5.5) follows immediately from (A.2) and (5.7).
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