Annu. Rev. Polit. Sci. 1998.

1:139–65
Copyright © 1998 by Annual Reviews. All rights reserved

THE CAUSES OF WAR AND THE

CONDITIONS OF PEACE
Jack S. Levy
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Department of Political Science, Rutgers University, New Brunswick, New Jersey

08901-1568; e-mail: jacklevy@rci.rutgers.edu
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KEY WORDS: levels of analysis, balance of power theory, power transition theory,
interdependence and war, diversionary theory

ABSTRACT
I organize this review and assessment of the literature on the causes of war
around a levels-of-analysis framework and focus primarily on balance of
power theories, power transition theories, the relationship between eco-
nomic interdependence and war, diversionary theories of conflict, domestic
coalitional theories, and the nature of decision-making under risk and uncer-
tainty. I analyze several trends in the study of war that cut across different
theoretical perspectives. Although the field is characterized by enormous di-
versity and few lawlike propositions, it has made significant progress in the
past decade or two: Its theories are more rigorously formulated and more at-
tentive to the causal mechanisms that drive behavior, its research designs are
more carefully constructed to match the tested theories, and its scholars are
more methodologically self-conscious in the use of both quantitative and
qualitative methods.

INTRODUCTION
The nuclear revolution, the end of the Cold War, the rise of ethnonational con-
flicts, and the spread of global capitalism and democracy have led to consider-
able speculation about a turning point in the history of warfare. Some foresee
an “end of history” (Fukuyama 1992) and gradual obsolescence of war, or at
least of great power war (Mueller 1989), whereas others see an explosion of
low-intensity warfare and “clash of civilizations” (Huntington 1996). Each of
these perspectives rests on some critical assumptions and theoretical proposi-

139
1094-2939/98/0616-0139$08.00
 140 LEVY

tions about the causes of war. My aim in this review is to assess the state of the
art in our understanding of the causes of war.
Nearly 20 years ago two leading international relations scholars argued,
from different perspectives, that our systematic knowledge of international
conflict had progressed very little since Thucydides wrote his History of the
Peloponnesian War (Gilpin 1981, p. 227; Bueno de Mesquita 1981, p. 2). That
view was somewhat overstated at the time, because the field of international
relations had made significant progress since its emergence by the end of
World War II as an autonomous field of study, and it is certainly incorrect to-
day. We are more explicitly theoretical in our general orientation, more rigor-
ous in theory construction, more attentive to the match between theory and re-
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search design, more sophisticated in the use of statistical methods, and more
methodologically self-conscious in the use of qualitative methods.1
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However, we have few lawlike propositions, limited predictive capaci-

ties, and enormous divisions within the field.2 There is no consensus as to
what the causes of war are, what methodologies are most useful for discover-
ing and validating those causes, what general theories of world politics and
human behavior a theory of war might be subsumed within, what criteria
are appropriate for evaluating competing theories, or even whether it is possi-
ble to generalize about anything as complex and contextually dependent as
war.
This enormous diversity of theoretical, methodological, and epistemologi-
cal perspectives on the study of war complicates the task of providing a con-
cise assessment of the field. Because of the extensive coverage of my earlier
review (Levy 1989b), I focus here primarily on significant developments in the
last decade. This chapter begins with a theoretical overview, continues with a
selective review and critique of some of the leading theories of the causes of
war, and ends with a discussion of some general trends in the field.
The chapter’s space limitations prevent discussion of several important
new developments in the study of war and peace. I give limited attention to ap-
plications of game theoretic models (Powell 1990, Bueno de Mesquita & Lal-
man 1992, Morrow 1994, Fearon 1995) and say relatively little about cultural,
constructivist, postmodern, and feminist approaches to the study of war and
peace (Huntington 1996, Katzenstein 1996, Elshtain 1987). My citation of the
literature is also selective. For more complete reviews and citations see Levy

1 1This is a more optimistic assessment than the one I advanced in an earlier review (Levy
1989b). I suspect that the two abovementioned scholars are also more optimistic today.
2 2Gaddis (1992) charges that the failure of theorists to predict the end of the Cold War raises
questions as to the utility of social scientific models of international behavior, although promising
new methodologies for prediction have been developed (Bueno de Mesquita 1996).
 CAUSES OF WAR 141

(1989b), Kugler (1993), and Vasquez (1993). Some of the following builds on
Levy (1997b).

THEORETICAL PRELIMINARIES
The Dependent Variable
International relations theorists generally define war as large-scale organized
violence between political units (Levy 1983, pp. 50–53; Vasquez 1993, pp.
21–29). To differentiate war from lesser levels of violence, they generally fol-
low the Correlates of War Project’s operational requirement of a minimum
(and an annual minimum) of 1000 battle-related fatalities (Singer & Small
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1972). Peace, which is analytically distinct from justice, is usually defined as

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the absence of war.

Analysts traditionally distinguish international wars from civil wars, and
interstate wars from imperial, colonial, and other international wars that in-
volve non-state actors. Until recently they devoted a disproportionate amount
of attention to great power wars, including “hegemonic wars.”3 This great
power and Eurocentric bias in the study of war is decreasing, however, in re-
sponse to the end of the Cold War, the shift in warfare away from the great
powers, and the rise of “low-intensity wars” and “identity wars” (Holsti
1996).4
Despite this consensus on what we are trying to explain, the question of
what causes war can mean several different things (Suganami 1996). It can re-
fer, first of all, to the question of what makes war possible, to the permissive or
logically necessary conditions for war. These fundamental or primary causes
of war explain why war repeatedly occurs in international politics, why war
can occur at any moment. Thus scholars trace war to human nature, biological
instincts, frustration, fear and greed, the existence of weapons, and similar fac-
tors.
Peace, however, is far more common than war, though as a “non-event”
peace is difficult to measure. At the systemic level there may be more years
characterized by war than by peace, but in nearly all war years most states are
at peace, and at the dyadic level war is rare (Bremer 1992). This makes it hard

3 3Also known as “general” or “global” wars, these are long, destructive wars that have a
disproportionate impact on the structure and evolution of the international system (Levy 1985,
Rasler & Thompson 1994). This has led to a debate on whether we need separate theories to explain
big wars and small wars (Midlarsky 1990).
4 4Identity wars between ethnically or religiously defined communal groups that cut across state
boundaries raise questions about the traditional distinction between international and civil wars
(Small & Singer 1982), and also about the contemporary relevance of some traditional theories of
war.
 142 LEVY

to argue that human nature and related factors are causes of war. The point is
that these factors are constants and cannot explain the variations in war and
peace over time and space (Waltz 1959, Cashman 1993). If human nature var-
ies, as some argue, then it is the sources of that variation, not human nature it-
self, that explain behavior. Moreover, even if human nature or biologically
based instincts could explain aggressive behavior at the individual level, they
cannot explain when such aggression leads to domestic violence, when it leads
to scapegoating or other outlets, when it is resolved, and when it leads to inter-
national war.
The question of how to explain variations in war and peace is the second
meaning of the broader question of what causes war. Why does war occur at
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some times rather than other times, between some states rather than other
states, under some political leaders rather than others, in some historical and
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cultural contexts rather than others, and so on? This differs from still a third
question: How do we explain the origins of a particular war?
Most international relations scholars (and particularly those in North Amer-
ica) focus primarily on the second question, explaining variations in war and
peace. They leave the question of why war occurs at all to philosophers and bi-
ologists and leave the question of why a particular war occurs to historians.
This is not to pass judgment on the relative importance of the three questions,
only to say that they are different and that their investigation may require dif-
ferent theoretical orientations, different conceptions of causation and explana-
tion, and different methodologies.
One important exception to this focus on variations in war and peace is the
argument by Waltz (1979) and other neorealists that the fundamental cause of
war is the anarchic structure of the international system. Anarchy, defined as
the absence of a legitimate governmental authority to regulate disputes and en-
force agreements between states, “causes” war in the sense that there is no gov-
ernmental enforcement mechanism in the international system to prevent
wars.5 Although anarchy may provide one persuasive answer to the question
of the permissive causes of war, it is generally treated as a structural constant
and consequently it cannot account for variations in war and peace. Waltz
(1988, p. 620) seems to concede this point and argues, “Although neorealist
theory does not explain why particular wars are fought, it does explain war’s
dismal recurrence through the millennia.” Other neorealists also recognize this
limitation and have begun to incorporate other variables—including the polar-
ity of the system and the offensive/defensive balance—in order to explain
variations of war and peace in anarchic systems.

5 5This anarchic structure differentiates international politics from domestic politics in realist
thought. The existence of chaos and violence does not define anarchy but is instead the
hypothesized causal consequence of anarchy.
 CAUSES OF WAR 143

Another exception to the focus on variations in war and peace can be found
in some feminist theorizing about the outbreak of war, although most feminist
work on war focuses on the consequences of war, particularly for women,
rather than on the outbreak of war (Elshtain 1987, Enloe 1990, Peterson 1992,
Tickner 1992, Sylvester 1994). The argument is that the gendered nature of
states, cultures, and the world system contributes to the persistence of war in
world politics. This might provide an alternative (or supplement) to anarchy as
an answer to the first question of why violence and war repeatedly occur in in-
ternational politics, although the fact that peace is more common than war
makes it difficult to argue that patriarchy (or anarchy) causes war. Theories of
patriarchy might also help answer the second question of variations in war and
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peace, if they identified differences in the patriarchal structures and gender re-
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lations in different international and domestic political systems in different

historical contexts, and if they incorporated these differences into empirically
testable hypotheses about the outbreak of war. This is a promising research
agenda, and one that has engaged some anthropologists. Most current feminist
thinking in political science about the outbreak of war, however, treats gen-
dered systems and patriarchal structures in the same way that neorealists treat
anarchy—as a constant—and consequently it cannot explain variations in war
and peace.

The Levels-of-Analysis Framework

Ever since Waltz (1959) classified the causes of war in terms of their origins in
the individual, the nation-state, and the international system (which he labeled
first-, second-, and third-image explanations, respectively), international rela-
tions theorists have agreed on the utility of the levels-of-analysis framework as
an organizing device for the study of war and international behavior more gen-
erally. Some scholars have modified Waltz’s framework by collapsing the in-
dividual and nation-state levels to create a simplified dichotomy of nation (or
unit) level and system level (Singer 1961, Waltz 1979), while others have dis-
aggregated the nation-state level into distinct governmental and societal-level
factors (Jervis 1976, Rosenau 1980), a practice that I follow here.
Following Waltz (1959), most scholars use the levels of analysis as a frame-
work for classifying independent variables. This leads to such questions as
whether the causes of war derive primarily from the level of the international
system, national societies or bureaucracies, or individual decision-makers. Al-
though the question of which level is most important has stimulated useful de-
bate, until recently it distracted attention from the equally important issue of
how variables from different levels interact in the foreign policy process.
The levels-of-analysis concept is sometimes used differently, to refer to the
dependent variable, or to the type of entity whose behavior is to be explained.
 144 LEVY

In this sense the systemic level of analysis refers to explanations of patterns

and outcomes in the international system; the dyadic level refers to explana-
tions of the strategic interactions between two states; the national level refers
to explanations of state foreign policy behavior; the organizational level refers
to explanations of the behavior of organizations; and the individual level refers
to explanations of the preferences, beliefs, or choices of individuals.
The failure of scholars to be explicit about exactly how they are using the
levels-of-analysis concept is a source of confusion in the field. Some have er-
roneously interpreted Singer’s (1961, p. 92) comment that the various levels
“defy theoretical integration...[and] are not immediately combinable” to sug-
gest that analysts should not combine variables from different levels of analy-
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sis to explain foreign policy decisions or international outcomes. But Singer’s

statement makes sense only if the levels of analysis are conceptualized in
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terms of the dependent variable. This is the familiar micro-macro problem of

aggregation across levels. We cannot assume that correlations or causal con-
nections at one level of analysis are necessarily valid at another level. It is con-
ceivable, for example, that concentrations of power may promote peace at the
dyadic level but war at the system level.
This has important implications for theories of war and peace. Although it
is possible that individual- or national-level variables could be the primary
causes of war, these variables do not constitute a logically complete explana-
tion because they do not explain how individual beliefs and preferences are
translated into state decisions and actions or how the strategies or behaviors of
states interact to lead to war as a dyadic or systemic outcome. To the extent that
most wars involve the mutual and interactive decisions of two or more adver-
saries, an explanation for the outbreak of war logically requires the inclusion
of dyadic- or systemic-level variables. This does not necessarily mean, how-
ever, that these variables are the most important in terms of amount of varia-
tion explained.
The levels-of-analysis framework is not the only way to organize the litera-
ture on the causes of war or on international politics more generally. It has long
been fashionable for international relations theorists to frame debates in terms
of the so-called paradigm wars between realism and liberalism (Baldwin
1993). In contrast to the realist focus on the struggle for power and security in
an anarchic and conflictual Hobbesian world, the liberal tradition sees a more
benign Grotian international society or Lockean state of nature where anarchy
does not imply disorder. States have common as well as conflictual interests,
aim to maximize economic welfare as well as provide for security, and create
international institutions that help regulate conflict and promote cooperation
(Keohane 1989). Although the paradigmatic debate between realism and liber-
alism has imposed some order on a chaotic field, it distracts attention from sig-
nificant variations within each paradigm, variations that often lead to conflict-
 CAUSES OF WAR 145

ing hypotheses. It also detracts from the important task of systematically inte-
grating key components from each approach into a more complete and power-
ful theory. Furthermore, the “paradigm wars” ignore important hypotheses as-
sociated with the Marxist-Leninist tradition and saddle liberalism with charges
of the neglect of power that are better associated with the utopianism of Tho-
mas Paine and others (Walker 1998).
As a field, international relations needs to shift its attention from the level of
paradigms to the level of theories, focus on constructing theories and testing
them against the empirical evidence, and leave the question of whether a par-
ticular approach fits into a liberal or realist framework to the intellectual histo-
rians.
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THEORIES OF THE CAUSES OF WAR

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Systemic-Level Theories
The realist tradition has dominated the study of war since Thucydides, and in-
cludes Machiavellians, Hobbesians, classical balance of power theorists,
Waltzian neorealists, and hegemonic transition theorists.6 Although different
realist theories often generate conflicting predictions, they share a core of
common assumptions: The key actors in world politics are sovereign states
that act rationally to advance their security, power, and wealth in a conflictual
international system that lacks a legitimate governmental authority to regulate
conflicts or enforce agreements.
For realists, wars can occur not only because some states prefer war to
peace, but also because of unintended consequences of actions by those who
prefer peace to war and are more interested in preserving their position than in
enhancing it.7 Even defensively motivated efforts by states to provide for their
own security through armaments, alliances, and deterrent threats are often per-
ceived as threatening and lead to counteractions and conflict spirals that are
difficult to reverse. This is the “security dilemma”—the possibility that a
state’s actions to provide for its security may result in a decrease in the security
of all states, including itself (Jervis 1978).
Realists do not assume that international relations are always conflictual,
and they have recently focused on the question of the conditions for coopera-
tion under anarchy, often through the use of iterated Prisoner’s Dilemma mod-

6 6On the variations in realist theory see Keohane (1986) , Brown et al (1995), Baldwin (1993),
and Doyle (1997).
7 7This parallels the distinction between “aggressive realists,” who believe that the international
system induces conflict and aggression, and “defensive realists,” who argue that the system is more
forgiving and that defensive strategies are adequate to provide security (Snyder 1991, pp. 11–12;
Grieco 1990, pp. 36–40).
 146 LEVY

els (Jervis 1978, Axelrod 1984, Grieco 1990). The iterated models are more
appropriate for most situations than single-play Prisoner’s Dilemma models,
and the iterated Prisoner’s Dilemma framework has generated some useful hy-
potheses regarding the rather restrictive conditions under which cooperation is
likely. The assumption that the same game is repeated over and over is often
problematic, however, particularly for security issues, because one play of the
game (preemption, for example) can significantly change power relationships
and affect payoffs in the next iteration. Nor is it clear that unilateral defection
is always preferred to mutual cooperation; stag hunt models, in which mutual
cooperation is both sides’ first preference, may be more applicable in many
situations (e.g. where each side fears both war and preemption by the other).
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There has been a tendency to apply Prisoner’s Dilemma models to many situa-
tions in which the assumptions of the model are not satisfied.
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The core proposition of realist theory is that the distribution of power,

throughout the system or within a dyad, is the primary factor shaping interna-
tional outcomes. But different versions of realist theory generate different,
sometimes contradictory explanations and predictions of foreign policy
choices and international outcomes, based on different assumptions about the
motivations of states, the nature of power, and the identity and boundaries of
the system. Given the variety of realist propositions and predictions, it is possi-
ble to interpret nearly any foreign policy behavior or international outcome as
consistent with some version of realism. This is not particularly helpful, for if
theories are not falsifiable they have little explanatory power. We should treat
realism as a paradigm rather than as a theory, and focus instead on specific
theories that generate more determinant, testable propositions.
The key division in the realist literature on war is not the standard one be-
tween classical realism and neorealism8, but rather between balance of power
theory and hegemonic theory. Balance of power theory includes numerous
variations on the classical realism of Morgenthau (1967) and the more system-
ic structural realism of Waltz. Hegemonic theory is a structural theory that in-

8 8These two realist traditions differ in their underlying assumptions regarding the fundamental
source of international conflict. Classical realists emphasize both the inherently aggressive
propensities of human nature and the absence of a higher authority in the international system.
Waltz (1979) and his followers eliminate human nature as an explanatory concept, give primacy to
the anarchic structure of the international system, and attempt to construct a structural-systemic
theory of international politics.
Waltz (1979) retains the classical realists’ assumption of the primacy of states, reinforces their
assumption of rationality, shifts from Morgenthau’s (1967) idea that states try to maximize power
as an end in itself to the notion that power is a means to the maximization of security, abandons the
traditionalists’ concerns to develop a theory of statecraft or foreign policy, and argues emphatically
for a purely systemic theory of international politics that explains international outcomes and not
the strategies or actions of states. Most neorealists, however, conceive of realism as incorporating
theories of both foreign policy and international politics (Posen 1984, Walt 1987).
 CAUSES OF WAR 147

corporates power transition theory and hegemonic stability theory and that
downplays the importance of anarchy.
Balance of power theory posits the avoidance of hegemony as the primary
goal of states and the maintenance of an equilibrium of power in the system as
the primary instrumental goal. The theory predicts that states, and particularly
great powers, will balance against those states that constitute the primary
threats to their interests and particularly against any state that threatens to se-
cure a hegemonic position.9 Balance of power theorists argue that the balanc-
ing mechanism—which includes both external alliances and internal military
buildups—almost always successfully avoids hegemony, either because po-
tential hegemons are deterred by their anticipation of a military coalition form-
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ing against them or because they are defeated in war after deterrence fails. In
balance of power theory, serious threats of hegemony are a sufficient condition
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for the formation of a blocking coalition, which leads either to the withdrawal
of the threatening power or to a hegemonic war.
Balance of power theory is more concerned with explaining national strate-
gies, the formation of blocking coalitions, the avoidance of hegemony, and the
stability of the system than the origins of wars, which are underdetermined. All
balance of power theorists agree that some form of equilibrium of military ca-
pabilities increases the stability of the system (generally defined as the relative
absence of major wars), and that movements toward unipolarity are destabiliz-
ing because they trigger blocking coalitions and (usually) a hegemonic war to
restore equilibrium. There is a major debate, however, between classical real-
ists, who argue that stability is further supported by the presence of a multipo-
lar distribution of power and a “flexible” alliance system (Morgenthau 1967,
Gulick 1955), and neorealists, who argue that bipolarity is more stable than
multipolarity (Waltz 1979, Mearsheimer 1990).
Although neorealists rely heavily on polarity as a key explanatory variable,
they do so with very little supporting evidence. They overgeneralize from the
Cold War experience, where bipolarity is confounded with the existence of nu-
clear weapons and other key variables, and fail to demonstrate the validity of
their arguments with respect to earlier historical eras. Although bipolarity is less
common than multipolarity, it has occurred before, as illustrated by the
Athens-Sparta rivalry in the fifth century BC and the Habsburg-Valois rivalry in
the early sixteenth century, both of which witnessed numerous wars. Neoreal-
ists also ignore a number of quantitative studies that suggest that bipolarity is
no less war-prone than multipolarity, that wars occur under a variety of struc-

9 9Walt (1987) emphasizes balancing against threats rather than the traditional view of balancing
against power. There is a large literature on the question of whether states bandwagon with threats
or balance against them, and the conditions under which various kinds of states do each (Vasquez
1997).
 148 LEVY

tural conditions, and that polarity is not a primary causal factor in the outbreak
of war (Sabrosky 1985).
An important alternative to balance of power theory is power transition the-
ory, a form of hegemonic theory that shares realist assumptions but empha-
sizes the existence of order within a hierarchical system (Organski & Kugler
1980, Gilpin 1981, Thompson 1988). Hegemons commonly arise and use their
strength to create a set of political and economic structures and norms of be-
havior that enhance the stability of the system while advancing their own secu-
rity. Differential rates of growth, the costs of imperial overextension, and the
development of vested domestic interests lead to the rise and fall of hegemons,
and the probability of a major war is greatest at the point when the declining
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leader is being overtaken by the rising challenger. Either the challenger initi-
ates a war to bring its benefits from the system into line with its rising military
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power or the declining leader initiates a “preventive war” to block the rising
challenger while the chance is still available (Levy 1989b, pp. 253–54). The
resulting hegemonic war usually generates a new hegemonic power, and the ir-
regular hegemonic cycle begins anew. Power transition theorists disagree
somewhat on the precise identity of hegemonic wars and the particular causal
dynamics from which they arise; for comparisons of the wars and critiques of
the theories see Levy (1985) and Vasquez (1993).
Although many of the theoretical analyses of power transition theory focus
on transitions between the dominant state in the system and a challenger, and
include conceptions of a broader international system and hierarchy, some ap-
plications of power transition theory are dyadic in nature and apply in principle
to any two states in the system. The dyadic-level “power preponderance” hy-
pothesis, which holds that war is least likely when one state has a preponder-
ance of power over another and is most likely when there is an equality of
power, has received widespread support in the empirical literature (Kugler &
Lemke 1996).
Balance of power theory and power transition theory appear to be diametri-
cally opposed; the former argues that hegemony never occurs and that concen-
trations of power are destabilizing, and the latter argues the opposite. It is im-
portant to note that traditional balance of power theory has a strong Eurocen-
tric bias and implicitly conceives of power in terms of land-based military
power, whereas most applications of power/hegemonic transition theory focus
on global power and wealth (Levy 1985). These different conceptualizations
suggest the possibility that these two theories are not necessarily inconsistent;
it is conceivable, for example, that the European system has been most stable
under a balance of military power whereas the global system is most stable in
the presence of a single dominant military and economic power.
This raises the relatively unexplored question of interaction effects between
international systems at different levels. Rasler & Thompson (1994), for ex-
 CAUSES OF WAR 149

ample, found that an increasing concentration of military power at the regional

level often contributes to large-scale regional wars, but that these regional
wars escalate to global wars only under conditions of a deconcentration of na-
val power and economic wealth at the global level. A similar question con-
cerns the interaction effects between dyadic relationships and their systemic
context. Geller (1993) found that dyadic power transitions are correlated with
war under conditions of decreasing systemic concentration but not under con-
ditions of increasing systemic concentration, where no dyadic transition war
has occurred. These results suggest that the investigation of the interactive dy-
namics of nested systems is an important question for further research.
The relationship between economic interdependence and war is an old
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question that has attracted new attention in the past few years. Montesquieu
[1977 (1748)] stated that “peace is the natural effect of trade,” and liberal eco-
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nomic theorists since Smith [1937 (1776)] and Ricardo [1977 (1817)] have ar-
gued that capitalist economic systems and the free exchange of goods in an in-
ternational market economy are the best guarantors of peace. Proponents ad-
vance a number of interrelated theoretical arguments in support of this propo-
sition. The most compelling argument is that trade generates economic advan-
tages for both parties, and the anticipation that war will disrupt trade and result
in a loss or reduction of the welfare gains from trade or a deterioration in the
terms of trade deters political leaders from war against key trading partners
(Polachek 1980).10
Realists, including mercantilists and economic nationalists, advance a
number of objections to the liberal economic theory of war. They often argue
that the effect of trade on war is small relative to that of military and diplomatic
considerations. They also question the liberal assumption that trade is always
more efficient than military coercion in expanding markets and investment op-
portunities and in promoting state wealth. Although this assumption may be
true in the contemporary system, at least for advanced industrial states, it is not
universal; mercantilists correctly argue that military force has been an instru-
ment to promote state wealth as well as power in certain historical eras (Viner
1948, Rosecrance 1986).
Realists downplay or reject the hypothesized deterrent and pacifying ef-
fects of the anticipated loss of welfare gains from trade. They argue that politi-
cal leaders are less influenced by the possibility of absolute gains from trade
than by concerns about relative gains, by the fear that the adversary will gain

10 10Theories of “hegemonic stability” focus on the stability of the world political economy, but
the implicit assumption is that a stable, liberal world economy promotes prosperity and peace.
Kindleberger (1973) discusses the role of the depression and closed trading blocs in the processes
leading to World War II, and Gilpin (1981) develops some of the theoretical linkages between a
stable world economy and international peace.
 150 LEVY

more from trade and convert those gains into further gains, political influence,
and military power (Waltz 1979, Grieco 1990, Huntington 1993). Concerns
over relative gains, bolstered by resource scarcities and domestic pressures,
can lead to economic competitions and rivalries that under certain conditions
can escalate to war (Choucri & North 1975).
In addition, gains from trade create dependence on one’s trading partner.
This dependence is often asymmetrical, and one party may be tempted to use
economic coercion to exploit the adversary’s vulnerabilities and influence its
behavior regarding security as well as economic issues. These tendencies are
reinforced by demands for protectionist measures from domestic economic
groups that are especially vulnerable to external developments, particularly in
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bad economic times, and by leaders’ temptations to bolster their domestic sup-
port through hardline foreign policies. These can lead to retaliatory actions,
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conflict spirals, and war.

Whether the incentives for the gains from trade dominate the incentives for
coercion or protection based on economic asymmetries, and whether the latter
escalate to trade wars and militarized conflicts, are empirical questions that
analysts have only recently begun to analyze systematically. Although many
find that trade is associated with peace (Polachek 1980, Oneal & Russett
1997), the relationship is modest in strength and is sensitive to measurement
techniques and time periods analyzed. Some find that trade is associated with
conflict (Barbieri 1996).11
Although liberals and realists disagree on the effects of trade on war, they
appear to agree that trade and other forms of economic interchange between
societies will cease or be drastically reduced once states are at war with each
other. The liberal argument that trade advances peace is based on the premise
that war eliminates or seriously reduces trade, and the realist emphasis on rela-
tive gains suggests that once war breaks out at least one of the belligerents will
terminate trade in order to prevent its adversary from using the gains from
trade to increase its relative military or economic power. Contrary to both lib-
eral and realist expectations, however, there are numerous historical cases of
trade between enemies during wartime, and a preliminary quantitative study
suggests that war frequently does little to depress the volume of trade between
adversaries (Barbieri & Levy 1997).
If validated by further research, this finding suggests a need for revisions in
theories of interdependence, war, and peace. These theories are framed almost
entirely at the systemic level and ignore the potentially important role of do-
mestic variables. Self-interested domestic groups often press for the continua-

11 11Scholars on both sides of the debate recognize, however, that these tests of the trade-conflict
relationship are technically misspecified; conflict can also affect trade, and the simultaneous and
reciprocal effects of trade on war and war on trade need to be incorporated into a single model.
 CAUSES OF WAR 151

tion of trade with the enemy, and governments often need tax revenues from
trade and the general support of business groups in order to finance the war ef-
fort. Theories of interdependence and peace need to incorporate domestic vari-
ables into their hypotheses, refine their arguments regarding the deterrent ef-
fects of trade, and demonstrate these effects empirically. They also need to
conceptualize relative gains at the systemic as well as the dyadic level; states
often hesitate to terminate trade with the enemy for fear that they will lose that
trade to a third party, who may be a greater economic or military rival.
The preceding reference to domestic variables applies to systemic-level
theories more generally. Although the incorporation of domestic variables is
certainly consistent with a broadened conception of liberal theory, it is less
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compatible with realist theories, which trace the sources of state behavior and
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international patterns to systemic-level structures of power. Although struc-

tural realist theories properly emphasize the constraints on action provided by
the distribution of power at the systemic and dyadic levels, they fail to give
adequate attention to state preferences and to the impact of domestic institu-
tions, cultures, and informational considerations on both the preferences and
the constraints of states. These omissions have led all but the most committed
neorealists to conclude that systemic-level theories are theoretically incom-
plete and empirically inadequate, in that they leave too much of the variation in
the outbreak and expansion of war unexplained. These conclusions have
brought on increasing challenges to realism and other systemic-level theories,
as well as a shift to the societal level of analysis.

Societal-Level Theories
After decades of giving less attention to domestic sources of international con-
flict than to systemic or individual factors, much of the action in the study of
war is now taking place at the domestic level. Although interest in Marxist-
Leninist theories has waned, there has been a tremendous growth of research
on the relationships between regime type, particularly democratic regimes,
and war (analyzed by Ray in this volume). Scholars have also devoted atten-
tion to the “diversionary” use of force for domestic political purposes, to the
impact of ethnonationalism on international conflict, and to other domestic
sources of international conflict. Although the literature on societal sources of
international conflict is currently characterized more by collections of loosely-
connected hypotheses than by well-developed theories, this work marks a sig-
nificant advance in the study of war. This section focuses on diversionary theo-
ries of war and on theories of logrolling and coalition formation.
Liberal and Marxist theorists suggest that mass public opinion is inherently
peaceful; if a state initiates a war it is usually because political leaders or the
capitalist class choose war over the desires and interests of the public, or per-
 152 LEVY

haps because of “false consciousness” on the part of the people. For example,
the institutionalist explanation for the democratic peace—which suggests that
the division of power within democratic states imposes institutional con-
straints on the use of force by political leaders—assumes that mass publics in
any regime are less inclined than are political leaders to use military force.
This may be true, but little empirical research has been directed to this ques-
tion. There are numerous historical cases in which the public appears all too
eager for war, from the American Civil War to the eve of World War I in
Europe to contemporary “identity wars.” In some cases this popular enthusi-
asm for war may push political leaders into adopting more aggressive and
risky policies than they would have preferred. President McKinley, facing an
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escalating crisis with Spain in the 1890s, “led his country unhesitatingly to-
ward a war which he did not want for a cause in which he did not believe” be-
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cause of pressures from the public and the press (May 1973). There is substan-
tial evidence that the outbreak of war, particularly victorious war, generally
leads to a “rally ‘round the flag” effect (Mueller 1973) that enhances popular
support for political leaders. Leaders often anticipate this “rally” effect and are
sometimes tempted to undertake risky foreign ventures or hardline foreign
policies in an attempt to bolster their internal political support.
This is the age-old “scapegoat hypothesis” or “diversionary theory of
war” (Levy 1989a). It is theoretically grounded in social identity theory and
the in-group/out-group hypothesis, which suggests that conflict with an out-
group increases the cohesion of a well-defined in-group (Coser 1956, J Stein
1996). Group leaders are aware of the cohesive effects of external conflict
and sometimes deliberately create or maintain external conflict to serve their
internal purposes. As Bodin argued, “the best way of preserving a state, and
guaranteeing it against sedition, rebellion, and civil war is to...find an enemy
against whom [the subjects] can make common cause” (quoted in Levy 1989a,
p. 259).
Empirical research on the scapegoat hypothesis has progressed through
several stages. Early work by Rummel and others used factor analysis to deter-
mine empirical associations between the incidence of internal conflict and the
incidence of external conflict. The finding that “foreign conflict behavior is
generally completely unrelated to domestic conflict behavior” (Rummel 1963,
p. 24) led to efforts to control for regime type and other variables that might af-
fect the relationship between internal and external conflict (Wilkenfeld 1973).
Although these controlled studies generated some positive findings, few of the
correlations indicated strong relationships and there was little consistency
across studies.
The absence of significant findings regarding the relationship between in-
ternal and external conflict contrasts sharply with evidence of external scape-
goating from historical and journalistic accounts and from a growing body of
 CAUSES OF WAR 153

case-study evidence. This discrepancy, together with the perceived inadequa-

cies of the Rummel paradigm (Levy 1989a),12 has led to a number of major re-
search projects over the last decade on the politically motivated use of force.
These studies have attempted to specify theoretical models that link domestic
politics with the external use of force and to identify empirically the internal
conditions under which the use of force is most likely.
Scholars have devoted particular attention to the timing of economic cycles
and electoral cycles (in democratic states) and to existing levels of political
leaders’ domestic political support. The basic hypothesis is that the political
insecurity of elites, and hence their propensity to use military force abroad,
should be greatest during periods before elections, during periods of poor eco-
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nomic performance, or at other times when domestic political support is low.13

These hypotheses find some confirmation in empirical tests over the Cold War
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period based on levels of inflation and unemployment (often combined into a

“misery index”) to tap poor economic performance (Ostrom & Job 1986, Rus-
sett 1989), although the effects of the electoral cycle have been questioned
(Gaubatz 1991). There has also been extensive empirical support for the hy-
pothesis that external conflict does trigger a substantial rally effect, which gen-
erally diminishes over time (Russett 1990).
Most of these studies have focused on democratic regimes, partly because
of the ease of measuring elite support levels but also because of the assumption
that the greater political accountability of leaders in democratic states makes
them more likely than authoritarian leaders to engage in external scapegoating.
Autocrats too must maintain their basis of support, however, even if among
more narrowly defined groups such as the military and economic leaders. Thus
some argue that scapegoating may be as common among autocratic as among
democratic leaders, and indeed some classic examples of scapegoating involve
nondemocratic regimes (Argentina in the Malvinas, Germany in World War I,
and Russia in the Russo-Japanese War, for example).
Whether the diversionary use of force is more common (or more effective)
in democratic or authoritarian regimes depends on the potential benefits of

12 12The limitation of these studies to the mid–Cold War period (and in many cases to the
1955–1960 period) raises serious problems as to external validity. More important, the research
designs on which these studies were based were not guided by a well-defined theoretical framework
that might facilitate the interpretation of the empirical findings. They did not clearly specify the
direction of the relationship or control for alternative explanations; consequently any positive
findings would be consistent with either the externalization of internal conflict or the internalization
of external conflict—including the possibility that internal conflict generates conditions of
weakness that create a tempting target for external adversaries.
13 13Some scholars suggest that moderate levels of political support are most conducive to
scapegoating, on the assumption that if support is low, scapegoating will either exacerbate internal
divisions or not create a large enough rally effect to make a difference.
 154 LEVY

scapegoating, the probability of a diplomatic or military victory, the domestic

costs and risks, and the availability of alternative means of dealing with do-
mestic opposition. Democratic leaders who initiate unsuccessful wars are
thrown out of power much more frequently than are nondemocratic leaders
(Bueno de Mesquita & Siverson 1995), but the personal costs of being re-
moved from office may be greater for authoritarian leaders. Gelpi (1997) hy-
pothesizes that the option of domestic repression is less available to demo-
cratic leaders and finds that democratic states were more likely than authoritar-
ian regimes to initiate the use of force between 1946 and 1982. Another possi-
ble strategy for dealing with domestic oppositions is to secure additional re-
sources from external allies, either to distribute among the disaffected mem-
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bers of society (or at least among key support groups) or to enhance the means
of repression (see Barnett & Levy 1991 and Bronson 1997).
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Gelpi’s (1997) finding raises interesting questions for the democratic peace
and for international conflict more generally (Ray 1995, 1998). It challenges
the hypothesis that democracies are more peaceful than are nondemocratic
states and the assumption that wars between the two are generally initiated by
the latter. This finding also leads to the dyadic-level questions of how fre-
quently democratic scapegoating is directed at other democracies, and why the
ensuing militarized dispute almost never escalates to war.
The more general question, which has been neglected in the literature, is
what kinds of adversaries make good targets for diversionary action.14 Scape-
goating might be a particularly useful strategy for leaders of states engaged in
enduring rivalries, and its use might help explain the intensification of rival-
ries, but the literature on enduring rivalries says little about this (Diehl 1998).
Ethnic adversaries also make excellent scapegoats. Although students of eth-
nonationalism recognize the use of scapegoating by “ethnic entrepreneurs” to
maintain and increase their own domestic support, theorists of diversionary
behavior and theorists of ethnonationalism have made few attempts to build
systematically on one another’s conceptual frameworks or empirical knowl-
edge.
Most theoretical and empirical studies of the diversionary use of force
assume the existence of a unified political elite attempting to increase its sup-
port among a mass public that is susceptible to symbolic appeals to the inter-
ests and honor of the nation. Neither elites nor masses are necessarily unified,
however, and some research explores the consequences of these divisions. Po-
litical leaders have different kinds of constituencies in different types of politi-
cal systems, and in their diversionary use of force they may be more concerned
about their support among some groups than among the public as a whole.

14 14The little work that has been done on images of the enemy (Holsti 1967) has not been linked
to scapegoating.
 CAUSES OF WAR 155

Morgan & Bickers (1992), for example, find that the use of force by the United
States is more related to levels of partisan support than to overall levels of sup-
port.
Elites are also divided, and one faction may support a policy of external
scapegoating as a means of advancing its own interests in intra-elite bargain-
ing and struggle for power (Lebow 1981). Alternatively, external scapegoating
may be a means of unifying a divided elite. The Argentine plan to seize the
Malvinas by force, for example, was designed not only to recover public sup-
port for the junta but also to give a divided regime a mission around which it
could coalesce.
There is an interesting divergence between explanations of why political
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elites engage in external scapegoating and explanations of why scapegoating

works to enhance or maintain their domestic support. The literature suggests
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that leaders adopt scapegoating as a rational instrument of policy to advance

their interests, while publics respond on the basis of symbolic and emotional
appeals, as explained by the in-group/out-group hypothesis. While the psycho-
logical rather than interest-based response of publics is certainly plausible, it is
also possible to construct an interest-based or integrated explanation for the re-
sponse of the public to the external use of force.
Recent research has attempted to integrate the behaviors of both leaders
and publics into principal-agent and other rational choice models. In the ba-
sic model, the public and other key social groups are the constituents or prin-
cipal, which has a choice as to whether to retain its agent, the political leader-
ship. This choice is a function of the extent to which the principal’s interests
are advanced by the agent, which is often measured in terms of the success of
both economic policy and the use of force in foreign policy (though other
measures are possible). Constituents’ choices are made in the context of un-
certainty, given information asymmetries favoring the agent, and they must
infer the agent’s “type” (competent or not) from a combination of current for-
eign policy outcomes and their own prior probability estimates based on past
economic performance. Agents understand this “signaling game,” so that
even incompetent leaders may have the incentive to engage in risky foreign
policy behavior in order to give the impression of competence. Analysts solve
for the equilibrium and test it against the evidence (Richards et al 1993, Smith
1996).
These models help to explain “gambling for resurrection” through risky
foreign policies by poor leaders who expect to be removed by their constitu-
ents (Downs & Rocke 1995). Although some of the assumptions of these mod-
els are rather strong, and although the classification of agents as either compe-
tent or not regardless of issue area is troubling, these attempts to model diver-
sionary processes in terms of self-interested behavior of leaders and constitu-
encies are a promising area for future research.
 156 LEVY

It is important to note that these models of diversionary processes (both for-

mal and nonformal) are all models of foreign policy, not strategic interaction
(and thus are not technically models of war). This raises the question of how
external actors perceive and respond to diversionary actions. Do states recog-
nize when another’s behavior is driven by diversionary motives? If so, do they
treat these actions differently than they do other actions and respond to them in
different ways? In addition, do they anticipate when the adversary’s domestic
conditions make it particularly likely to engage in external scapegoating or
other hardline foreign policies, and do they adjust their own behavior accord-
ingly? Do they anticipate domestically induced preferences for conciliatory
behavior by their adversaries, and do they attempt to exploit the adversary un-
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der such conditions? There is little attention in the literature to the strategic di-
mension of diversionary behavior or of domestic sources of foreign policy
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more generally, though Morrow’s (1991) model of arms control negotiations

and Smith’s (1995) model of diversionary theory are important exceptions.
The incorporation of a strategic component into diversionary theory is an im-
portant task for future research.
These rationalist models of aggressive foreign policies by self-interested
elites faced with rational publics are also reflected in the traditional literature.
Lenin’s (1916) explanation of World War I as an attempt by the imperialist
classes “to divert the attention of the laboring masses from the domestic politi-
cal crisis,” for example, assumed rationality on the part of key domestic con-
stituencies as well as the ruling class. Imperialist expansion works, for a time
at least, by propping up the falling rate of profit and providing the ruling class
with extra resources that they can use to secure the support of the leadership of
key labor groups (in Levy 1989a, pp. 259, 280).
Marxist-Leninist models of imperialism, like models of scapegoating, im-
plicitly assume that external expansion and the use of force serve the interests
of the ruling class or elite but not those of society as a whole. If society bene-
fited from expansionist foreign policies then a rational unitary model of the na-
tional interest would suffice to explain behavior and a distinctive societal com-
ponent would not be necessary. The basic argument is that the benefits of ex-
pansion go to the ruling elite (whether Lenin’s monopoly capitalists or Schum-
peter’s military-feudal elites), who have concentrated interests, while the costs
of expansion are diffused throughout society in the form of taxation.
If the costs of aggressive foreign policies are too high—and we have
enough examples of “imperial overstretch” to suggest that they sometimes
are—it is not clear how elites maintain their power. As Snyder (1991) argued,
groups concentrated enough to benefit from overexpansion are too narrow to
control state policy, while those broad enough to control policy are too diffuse to
reap benefits from overexpansionist policies. Snyder (1991) developed an alter-
native model in which key internal groups have parochial interests that favor
 CAUSES OF WAR 157

different but limited forms of imperial expansion or military buildups. They

secure and share power through logrolled coalitions and pursue the foreign
policy interests advocated by each, so that the resulting policy is more aggres-
sive or expansionist than that desired by any single group and cannot be sup-
ported by existing societal resources. The coalition of iron and rye in Germany
a century ago is a classic example: The industrialists sought a more active role
for Germany in the world economy and an expansion of the navy to back it up,
while agrarian interests sought protection against Russian grain exports. The
former alienated Britain and the latter alienated Russia, and Germany lacked
the resources to deal simultaneously with both of these adversaries.
For Snyder, this rationalist model of logrolling is not sufficient to explain
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overexpansion or the stability of the ruling coalition. He argues that coalitions

reinforce their positions of power and rationalize their policies by exploiting
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their control over information through the propagation of self-serving myths

about their nation, its adversaries, and their history (Van Evera 1990–91).
They then present these myths as lessons of history. These dynamics of over-
expansion are most likely to arise in cartelized political systems and least
likely to occur in democratic systems, where diffuse interests and the absence
of information monopolies work against strategic myth-making.15

Individual-Level Theories
Individual-level theories assume (a) that external and internal structures and
social forces are not translated directly into foreign policy choices; (b) that key
decision-makers vary in their definitions of state interests, assessments of
threats to those interests, and/or beliefs as to the optimum strategies to achieve
those interests; and (c) that differences in the content of actors’ belief systems,
in the psychological processes through which they acquire information and
make judgments and decisions, and in their personalities and emotional states
are important intervening variables in explaining observed variation in state
behaviors with respect to issues of war and peace.
There is a substantial literature on political leaders’ “operational codes” and
belief systems, on the influence of “lessons of history” on their beliefs and pol-
icy preferences, and on the role of misperceptions and biases in information-
processing that affect crisis decision-making (George 1969; Jervis 1976, 1988;
Lebow 1981; Holsti 1967, 1989; Levy 1994). A central theme in many of these
models is that, because of cognitive limitations, decision-making is character-
ized by “bounded rationality” rather than the ideal-type rationality posited by

15 15Snyder (1991) tested his model through comparative case studies of overexpansion by many
of the great powers over the last century and a half. Some have criticized his research design for
“selecting on the dependent variable,” although his combined cross-sectional and longitudinal
design does provide substantial variation on both dependent and independent variables.
 158 LEVY

formal decision theory. I limit my attention here to prospect theory, which is

one of the most recent attempts to apply a social-psychological model to inter-
national relations but which shares some elements of more formal rational
choice models. Kahneman & Tversky (1979) developed this theory of individ-
ual choice under conditions of risk to explain experimental anomalies in ex-
pected utility theory.
Prospect theory assumes that people evaluate outcomes in terms of devia-
tions from a reference point or aspiration level rather than in terms of net-asset
levels. They generally overweight goods in their possession relative to compa-
rable goods they do not own (the endowment effect), overweight losses from
that reference point relative to equivalent gains (loss aversion), and make risk-
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averse choices among gains but risk-acceptant choices among losses. People
also overweight certain outcomes relative to merely probable ones and low-
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probability outcomes relative to other outcomes. This asymmetry of behavior

with respect to gains and losses means that the way people frame their refer-
ence point in any given choice problem is critical. It is also subjective, al-
though people tend to adjust psychologically to gains much more quickly than
to losses and thus to frame around new acquisitions but not around recent
losses.
These assumptions lead to a number of hypotheses about behavior with re-
spect to international relations (Levy 1997a). (a) There is a “reference point
bias” in behavior, a greater tendency to move toward the reference point than
one would predict on the basis of standard expected-value calculations. When
the reference point is the status quo, as it commonly is, there is a status quo bias
in decision-making. (b) State leaders take more risks to maintain their interna-
tional positions, reputations, and domestic political support than they do to en-
hance those positions. (c) After suffering losses (in territory, reputation, do-
mestic political support, etc), political leaders do not adapt to those losses and re-
normalize their reference points but instead take excessive risks (relative to ex-
pected value calculations) to recover those losses. After making gains, political
leaders adapt to them, renormalize their reference points, and take excessive
risks to defend those gains against subsequent losses. Thus if A loses territory to
B, A will take excessive risks to maintain her gains while B will take excessive
risks to recover her losses. (d) Because adaptation to losses tends to be slow,
sunk costs frequently influence decision-makers’ calculations and state behav-
ior.
With respect to strategic interaction, (e) the endowment effect leads actors
to overvalue the concessions they give relative to those they get in return, lead-
ing to a “concession aversion” or a bias against agreements (relative to an
expected-value calculus). (f) It is easier to deter an adversary from taking an
action than to compel him to terminate an action or to undo what he has already
done, and it is easier to deter an adversary from making gains than to deter her
 CAUSES OF WAR 159

from recovering losses. (g) It is easier for states to cooperate in the distribution
of gains than in the distribution of losses.
These are intriguing hypotheses that appear to resonate in the world of in-
ternational relations, but several conceptual and methodological problems
complicate the task of testing these hypotheses empirically. The problem of
identifying the actor’s reference point, particularly in the absence of a theory
of framing, makes it very difficult to rule out the alternative and more parsimo-
nious hypothesis that behavior is driven not by framing, loss aversion, and the
reflection effect in risk orientation, but rather by a standard expected-value
calculation (Levy 1997a).
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CONCLUSIONS: GENERAL TRENDS IN THE

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STUDY OF WAR
There are general trends in the study of the causes of war that cut across differ-
ent theoretical perspectives, some in response to real-world trends and others
to autonomous shifts in intellectual paradigms. One is a partial move away
from a long-standing great power focus toward a greater emphasis on small
state conflicts, particularly on civil wars and ethnonational conflicts. There has
also been a pronounced shift away from the systemic level (in terms of both in-
dependent and dependent variables), in part because of growing dissatisfaction
with the failure of structural systemic models to explain enough of the variance
in war and peace. This shift has led to rising interest in both dyadic-level be-
havior and societal-level explanatory variables.
At the dyadic level, in addition to long-standing research on dyadic power re-
lationships and power transitions, there are new research programs on enduring
rivalries, bargaining, territorial contiguity, trade, and other relationships. Some
of these have generated much stronger empirical findings than those for sys-
temic patterns or national-level behavior. Bremer (1992), for example, demon-
strates that the probability of war is 35 times higher for contiguous dyads than
for noncontiguous dyads for the 1816–1965 period, and Vasquez (1996)
shows that when war occurs, a strong tendency exists for contiguous states to
fight dyadic wars and for noncontiguous states to fight multilateral wars.16
Whatever the relationship between concentrations of military and eco-
nomic capabilities at the systemic level, there is substantial evidence that at the
dyadic level an equality of capabilities is significantly more likely to lead to
war than is than is a preponderance of power (Kugler & Lemke 1996); bargain-

16 16Kal Holsti (1991) demonstrates that territorial issues have been at stake in most wars since
1648, although it is not clear whether this reflects the physical opportunity for war between
contiguous states or incentives for war such as disputes over resources or disputes involving
ethnonational groups (Goertz & Diehl 1992, Vasquez 1993).
 160 LEVY

ing strategies based on reciprocity are more likely to lead to peaceful outcomes
than those based exclusively on coercive threats (Leng 1993); and “asymme-
tries of motivation” are at least as important as power differentials in determin-
ing outcomes of international disputes (George & Smoke 1974).
The shift to societal-level explanatory variables is a response to their long
neglect in the literature; to the decline of systemic imperatives arising from the
bipolar Cold War structure; to the increasing salience of smaller, politically
unstable states and ethnonational conflicts in the post–Cold War world; and to
the availability of good quantitative data on key variables (Gurr 1989). Interest
in the societal level has also been spurred by the striking finding that demo-
cratic states rarely if ever go to war against each other, which “comes as close
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as anything we have to an empirical law in international relations” (Levy

1989b, p. 270).
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There has also been an increasing recognition of the complexity of the

causes of war and an increasing willingness to attempt to model that complex-
ity. This recent interest in complexity, exemplified by Jervis’ System Effects:
Complexity in Social and Political Life (1997) and by Axelrod’s Complexity of
Cooperation (1997), involves more attention to multilevel explanatory frame-
works, dynamic processes, reciprocal causation, endogeneity problems, and
selection effects.
In the past decade most international relations theorists have moved beyond
their earlier preoccupation with explanations at a single level of analysis and
debates about which level provides the most powerful explanations. They now
give much more attention to interaction effects among variables at different
levels in the processes leading to war. Game-theoretic models, for example,
which six or seven years ago were applied almost exclusively to problems of
strategic interaction between states, now incorporate domestic structures and
processes as well.
Applications of game-theoretic models have themselves become more so-
phisticated and more complex.17 Prisoner’s Dilemma and related 2 × 2 games
were useful for the analysis of simple strategic situations, but the move to
extensive-form games, particularly sequential games with incomplete infor-
mation, marks a profound theoretical advance. The greater realism of the new
models has contributed to a strong revival of interest in game-theoretic ap-
proaches in recent years. These games incorporate the uncertainty that deci-
sion-makers routinely face, the sequence of choices and counter-choices that
generally characterize the outbreak of war, the problem of the credibility of
commitments in an anarchic world, and the dynamics of signaling.

17 17Moreover, the shift in formal rational choice analysis from decision-theoretic models (Bueno
de Mesquita 1981) to game-theoretic models (Bueno de Mesquita & Lalman 1992) represents a
major paradigm shift.
 CAUSES OF WAR 161

The conceptualization of international relations and war as a sequence of

choices, which has long been common in the qualitative literature, is also re-
flected in much of the quantitative empirical literature. Early studies in the
“correlates of war” tradition looked for simple empirical associations between
systemic structures and the frequency and severity of war, reflecting the rather
static balance-of-power propositions that they were designed to test. This re-
search tradition has evolved such that it now conceptualizes international con-
flict as a process or series of steps (Vasquez 1993) from background condi-
tions to the occurrence and evolution of militarized disputes to the outbreak
and evolution of international war, which in turn affect background conditions
(Bremer 1992, Cusack 1996). This more dynamic conceptualization led to the
Annu. Rev. Polit. Sci. 1998.1:139-165. Downloaded from www.annualreviews.org

development of new data sets on militarized interstate disputes (Jones et al

1996), the Behavioral Correlates of War (Leng 1993), and international crisis
Access provided by 182.183.247.81 on 09/17/21. For personal use only.

behavior (Brecher et al 1988). These data sets have further facilitated analyses
of the sources, dynamics, and consequences of international dispute and crisis
behavior. The more dynamic character of theorizing about war is also reflected
in the literatures on long cycles, power transitions, enduring rivalries, learning,
and evolutionary processes.
The increasing complexity of theories of war and peace is reflected in the
recognition of the importance of unintended consequences, endogeneity ef-
fects, and selection effects, as well as in the construction of rational choice and
systems models that facilitate the analysis of such effects. In addition to plac-
ing greater emphasis on strategic behavior and interconnectedness of systems,
scholars increasingly acknowledge that earlier modeling of actors’ responses
to exogenous events, institutions, and other external shocks neglected the pos-
sibility that those events or institutions were themselves the endogenous result
of conscious strategic behavior. Many studies of deterrence, for example, con-
clude that the impact of military power differentials on crisis outcomes is often
modest, but ignore the fact that such power considerations may have deter-
mined whether an actor initiated the crisis in the first place (Levy 1989b, p.
243). Extensive-form games facilitate the modeling of these dynamics (Fearon
1994, Bueno de Mesquita 1996).
There has been significant progress in the empirical study of the causes of
war. Most empirical research is more theoretically driven than it was two dec-
ades ago. It is also characterized by a better match between theory and research
design. We find fewer “barefoot empiricist” fishing expeditions and fewer
idiographic single-country case studies. Large-n statistical studies now regu-
larly employ such methods as event-count models, logistic regression, survival
models, and interrupted time series, the assumptions of which better match the
underlying theory and the nature of the data.
Case-study research is also more methodologically sophisticated. Influ-
enced by George (1982) and King et al (1994), qualitatively oriented scholars
 162 LEVY

have made increasing efforts to deal with standard problems of scientific infer-
ence. Case selection is more likely to be driven by theoretical considerations
and by the need to maximize control over extraneous variables than by sub-
stantive interest alone. Case-study research has become more historical, but it
has also become more analytical and comparative. Graduate training programs
have increasingly begun to include courses on qualitative methods as well as
quantitative methods.
Another positive trend is toward the adoption of multi-method research de-
signs, either in a single study or in a broader research program. Two decades
ago, relatively few studies combined decision or game-theoretic models with
statistical tests, but this is the norm today. There are also more and more stud-
Annu. Rev. Polit. Sci. 1998.1:139-165. Downloaded from www.annualreviews.org

ies that combine case studies with statistical analyses. Although some regard
rational choice and case studies as antithetical, the combination of quantitative
Access provided by 182.183.247.81 on 09/17/21. For personal use only.

and qualitative research designs in the testing of rational choice theories al-
lows each method to compensate for the limitations of the other. The combina-
tion of large-n statistical studies, case studies, and game-theoretic models by
numerous scholars studying the interdemocratic peace and the diversionary
theory of war provide examples of the potential of a multi-method approach,
and the cumulative results have been far more convincing than those produced
by any single method.
Whatever one’s assessment of the state of the art in the study of war one or
two decades ago, there are considerable grounds for optimism today. Although
theoretical and empirical research in the field is more diverse and contentious,
almost everything has improved. Our theories are more imaginative, rigorous,
and relevant; our research designs are more closely matched to our theoretical
propositions and more sensitive to potential problems of inference; our data
sets are more numerous and more refined. Whether we have approached a
turning point in the history of warfare is still a matter of debate, but we have
clearly improved our understanding of the motivations, conditions, and pro-
cesses that contribute to war or peace.

Visit the Annual Reviews home page at

http://www.AnnualReviews.org.

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 Annual Review of Political Science
Volume 1, 1998

CONTENTS
Models of Government Formation, Michael Laver 1
Does Democracy Cause Peace?, James Lee Ray 27
Social Capital and Politics, Robert W. Jackman, Ross A. Miller 47
Compliance with International Agreements, Beth A. Simmons 75
The Intellectual Legacy of Leo Strauss (1899-1973), Nasser Behnegar 95

Federalism, Federal Political Systems, and Federations, Ronald L. Watts 117

The Causes of War and the Conditions of Peace, Jack S. Levy 139
Communication and Opinion, Donald R. Kinder 167
Social Science and Scientific Change: A Note on Thomas S. Kuhn's
199
Contribution, Nelson W. Polsby
Annu. Rev. Polit. Sci. 1998.1:139-165. Downloaded from www.annualreviews.org

The Party Family and Its Study, Peter Mair, Cas Mudde 211
Access provided by 182.183.247.81 on 09/17/21. For personal use only.

Reinventing Government, Frank J. Thompson, Norma M. Riccucci 231

Social Choice Theory, Game Theory, and Positive Political Theory,
259
David Austen-Smith, Jeffrey S. Banks
Domestic Politics, Foreign Policy, and Theories of International
289
Relations, James D. Fearon
Narrative in Political Science, Molly Patterson, Kristen Renwick Monroe 315
Gendering Research in Political Science, Joni Lovenduski 333
The Decline of Parties in the Minds of Citizens, Harold D. Clarke,
357
Marianne C. Stewart
Neglected Aspects of the Study of the Presidency, John Hart 379
Contending Theories of Contentious Politics and the Structure-Action
401
Problem of Social Order, Mark I. Lichbach
Justice and Fairness, Jack Knight 425
Reflections on British Elections and Their Study, David Butler 451