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date: 24 March 2017

Is Democracy a Cause of Peace?

Summary and Keywords

Essentially all scholars agree that the levels of violent conflict, especially wars, within democratic pairs of states are significantly lower than levels of violent conflict within other pairs of states. However, debate rages as to whether this observed correlation is causal or spurious. Does democracy actually cause peace? Answering this question is critical for both scholarly and policy debates.

Critics have lodged two sets of arguments proposing that the observed correlation between democracy and peace does not mean that democracy causes peace. First, some claim that the peace observed among democracies is not caused by regime type, but rather by other factors such as national interest, economic factors, and gender norms. These critics often present statistical analyses in which inclusion of these or other factors render the democracy independent variable to be statistically insignificant, leading them to draw the conclusion that democracy does not cause peace.

The second critique claims that there is a causal relationship between democracy and peace, but peace causes democracy and not the reverse. Peaceful international environments permit democracy to emerge, and conflictual international environments impede democracy. Though peace causes democracy, democracy does not cause peace.

Careful examination of the theoretical claims of these critiques and especially the pertinent empirical scholarship produces two general conclusions. First, there is enough evidence to conclude that democracy does cause peace at least between democracies, that the observed correlation between democracy and peace is not spurious. Second, this conclusion notwithstanding, the critiques do make important contributions, in the sense that they demonstrate that several factors (including democracy) cause peace, that there may be some qualifications or limitations to the scope of the democratic peace, and that causality among factors like democracy and peace is likely bidirectional, part of a larger dynamic system.

Keywords: democratic peace, conflict, domestic politics, international relations, causation, empirical international relations theory

The Democratic Peace Debate

One of the most indisputable, nontrivial, observed patterns in international relations is that democracies almost never fight each other. Few dispute the existence of the empirical association of democracies not fighting each other, especially not fighting high-intensity conflicts such as wars. There is, however, great contention, over whether or not democracy causes peace. Some argue that democracy does in fact cause peace, while others argue that the observed democracy-peace correlation is either spurious or that causal arrow is reversed, as peace causes democracy but democracy does not cause peace.

Whether or not democracy causes peace is an issue of more than scholarly interest. For decades, international actors have sometimes supported democratization because of the belief that making states more democratic would cause them to be more peaceful, at least with each other. Conversely, policy critics have argued that because democracy does not cause peace, it is a fool’s errand to attempt to spread democracy, because of the costs and risks of trying to democratize other states.

This essay considers whether democracy causes peace. It proceeds in three sections. First, it describes the empirical pattern that democracies almost never fight each other, and presents the principal theoretical arguments as to why democracy might cause peace. It then considers two clusters of critiques of the proposition that democracy causes peace, that the correlation is spurious and that peace is actually caused by factors other than democracy; and that democracy and peace are causally related, but that peace causes democracy, and not the reverse.

The Correlation of Democracy and Peace

Though democracies sometimes become embroiled in conflicts with non-democracies, they almost never fight each other (Russett & Oneal, 2001). Since the emergence of modern democracy in the early 19th century, two mature democracies have never experienced intense violent conflict with each other, incurring at least 1,000 battle dead. The Correlates of War project and other data sets have long classified a conflict as a war if it experiences at least 1,000 battle dead (Reiter et al., 2016). On the rare occasions when two democracies have entered militarized disputes with each other, as in the 1898 Fashoda Crisis or the 1970s “Cod Wars” between Britain and Iceland, they essentially always settle the conflict short of war.

There have been close calls of democracies nearly fighting wars against each other. Mature democracies have sometimes fought repressive states with some democratic elements, such as the United States fighting Spain in the 1898 Spanish-American War (Ray, 1995). Democracies sometimes end up as members of opposing coalitions, though in those cases the opposing democracies avoid fighting each other. Democratic Finland fought alongside the Axis in World War II, but experienced no combat with any democratic members of the Allies. In the 1948 War of Israeli Independence, a somewhat democratic Lebanon found itself allied with Arab states against the new Israeli democracy, but Lebanon carefully avoided direct clashes with Israel (Morris, 2008, pp. 344, 348). In the 2006 Israel-Hezbollah conflict, though Israel launched strikes against Hezbollah forces based in Lebanese territory, Israel did not declare war on democratic Lebanon and generally avoided attacking Lebanese forces directly. Lebanon also mostly avoided attacking Israel forces. Probably the closest instance of two democracies going to war with each other was the 1999 Kargil War between India and Pakistan, though that conflict experienced less than 1,000 battle dead (Reiter et al., 2016). More systematic studies have also found that pairs of democratic states are less likely to experience less intense violent, international conflicts than other pairs of states (Russett & Oneal, 2001; Rousseau et al., 1996), though there is debate over whether jointly democratic pairs of states are less likely to experience non-war disputes as compared with all other pairs of states, or just with pairs of states that include a democracy and a non-democracy (Bennett & Stam, 2004).

Though there is strong consensus about the “dyadic” democratic peace, that democracies do not fight each other, there is more debate about the existence of other possible patterns of democratic peace. Many dispute the existence of the “monadic” democratic peace, that democracies are more likely to be peaceful in their relations with all states (Russett & Oneal, 2001; for an early statement of the monadic democratic peace, see Rummel, 1979). There is also debate over the existence of a “systemic” democratic peace, whether making the international system or even a region more democratic will make the system or region more peaceful (Gleditsch, 2002; Mitchell et al., 1999).

Scholars have outlined two clusters of explanations as to why democracies ought to be more peaceful in their relationships with each other (Russett, 1993; for other summaries of the democratic peace literature, see Ray, 1995; Reiter, 2012b). First, democratic political institutions nurture peace between democracies. There are a few variants of the institutional explanation of the democratic peace. Perhaps the most general account, first described by Immanuel Kant in the 19th century, is that because democratic leaders are elected, they know that pursuing unpopular policies will increase their likelihood of being removed from office. Wars are unpopular because of their costs in blood and taxes, and as a result elected leaders attempt to avoid wars. Dictators, in contrast, are confident that they can use the tools of repression to stay in power even in the face of popular discontent, and are more likely to go to war, knowing that they can more easily stay in power. One variant of the institutionalist hypothesis is that democratic political institutions impose higher audience costs on elected leaders who back down in crisis, and the prospect of higher audience costs in turn helps democracies avoid wars with each other (Fearon, 1994). Another variant proposes that the ability of democracies to mobilize their economies more deeply during war time deters democracies from attacking each other (Bueno de Mesquita et al., 2003). A third variant is that the diffusion of power within democratic governments, such as the separation of powers, slows the abilities of democracies to make war (Reiter & Tillman, 2002; Russett, 1993).

A second explanation focuses on norms. Democratic political culture emphasizes nonviolent means of conflict resolution, using tactics such as law and the courts, elections, and free speech to resolve disputes. These norms percolate into democratic foreign policy, encouraging democracies to being more willing to use foreign policy tools such as mediation, diplomacy, and international law to resolve international disputes. The political culture of non-democracies emphasizes violence, as domestic politics within dictatorships are characterized by tactics such as repression, mass and elite revolutions, and brutality. This political culture of violent conflict resolution then pushes non-democracies to be violent in their international relations, using coercion, threats, and force to resolve interstate disputes. This normative explanation of the democratic peace overlaps with a constructivist explanation, that democracies share a common identity and see themselves as comprising a community of like-minded states. These shared identities, especially in concert with norms of nonviolent conflict resolution, help democracies transcend self-interest, enjoy deeper levels of cooperation, and avoid violent conflicts with each other (Deutsch, 1957; Kahl, 1998/1999; Risse-Kappen, 1995).

Causation and the Democratic Peace

The central claim of the democratic peace proposition is that democracy causes peace. As noted, the focus in this essay will be on causal processes within the dyadic democratic peace. The dyadic democratic peace proposes that relations within a pair of democracies are more peaceful than relations within other kinds of pairs, such as pairs of autocracies or a democracy matched with an autocracy.

Whether democracy causes peace can be framed around this question: Is the democratic peace a law, as Levy (1989, p. 88) asserted, or is it a mere correlation? Risjord (2014, p. 212), a philosopher of social science, distinguished “laws from mere correlations” as follows: “1. Laws must be general, making no reference to particular objects, times, or places. 2. Laws must support counterfactual statements.” Importantly, philosophers of social science agree that a law can be probabilistic and need not be exceptionless, analogous to the existence of probabilistic laws in the biological sciences (Risjord, 2014, p. 213).

In using the democratic peace proposition as an example in his discussion of causation and scientific laws, Risjord (2014, p. 213) accepts that the democratic peace is a law: “If laws are general regularities that support counterfactuals, then the democratic peace is a law. It is a strong correlation that makes no mention of particular objects, places, or times. And it seems to support counterfactuals. American foreign policy in the latter 20th century has often aimed to reduce war by spreading democracy. This policy is supported by the idea that if a country becomes a democracy, it is less likely to declare war; if North Korea were a democracy it would be less hostile to South Korea. In this sense, then, the democratic peace supports counterfactuals and is entitled to prima facie status as a law.”1 Note that the idea of counterfactual separates causal from spurious relationships, as a spurious relationship would not support a counterfactual. The focus of this essay is to explore two critiques of the assertion that the democratic peace is causal, and therefore a law. That is, in the context of the Korea example, if the democratic peace is mere correlation and spurious rather than causal, then the democratization of North Korea would not lead to reduced hostility with South Korea.

Proving that democracy causes peace has both scholarly and policy stakes. On the scholarly side, democratic peace theory describes a causal rather than correlational relationship, meaning that tests of the theory need to demonstrate causation rather than mere correlation. The democratic peace proposition is itself a centuries-old idea central to international relations, at the heart of one of the most prominent and well-established theories in international relations, Kantian liberalism (Russett & Oneal, 2001). Other theories of international relations refute the assumptions of democratic peace theory in this manner. Realism, for example, proposes that regime type has no effect on peace or war, because the brutally competitive nature of the international system forces all states, democratic and non-democratic, to behave similarly (see below for further discussion).

The policy stakes are also high. The policy implications of the democratic peace are that because democracy causes peace, actors interested in peace should take actions to spread democracy. Many important foreign policy decisions over the last century have been informed by the belief that democracy causes peace. This was an important part of President Woodrow Wilson’s 1917 public argument as to why the United States should enter World War I, that Prussian autocracy was a fundamental cause of the war, and the United States needed to help the Allies win the war to democratize Germany and create the foundations for a stable peace: “The world must be made safe for democracy. Its peace must be planted upon the tested foundations of political liberty.”2 The idea that democracy causes peace was part of the motivation to pursue the unconditional surrender of the Axis powers in World War II; that autocracy in Japan, Italy, and Germany were taproot causes of World War II; and that the United States needed to achieve the unconditional surrender of these states to permit the United States to democratize them, democratization in turn being a critical condition for creating a stable postwar world order (Reiter, 2009). President Clinton openly stated his belief in the democratic peace, and this belief in turn informed a number of his policies, including the 1995 intervention in Haiti and the expansion of NATO to Eastern Europe (Reiter, 2001). Especially after 2003, President George W. Bush justified the Iraq War with the democratic peace proposition, arguing that democratizing Iraq would in turn help stabilize the Middle East. In his 2005 inaugural address, President Bush stated unequivocally, “The best hope for peace in our world is the expansion of freedom in all the world.”3 During the Obama administration, some hoped that the Arab Spring, if it succeeded in ushering in stable democratic regimes, might permit the stabilization of the Middle East (Strauss, 2012).

The policy implications of whether democracy causes peace persist. Policy-makers would like to know whether the 2010s deterioration in U.S.-Russian relations was caused by the collapse of democracy in Russia under the rule of Vladimir Putin. Pakistan is enjoying an encouraging spell of democracy, and this might create an opening for the alleviation of the India-Pakistan rivalry. Perhaps Iran’s initial steps toward democracy, such as the election of a moderate president in 2013, might eventually permit improved relations with democratic rivals such as Israel and the United States. Some have debated whether or not a Chinese transition to democracy in the 21st century would substantially reduce the probability of war with democracies such as Japan and the United States (Friedman & McCormick, 2015).

Though most scholars concede that a pair of democracies is less likely to experience violent conflict than other pairs of states, some critique the inference that this observed pattern implies a causal relationship. These critics have made two major sets of causal critiques of the inference that the observed correlation of democracy and peace provides support for the hypothesis that democracy causes peace.4 The following sections describe and discuss these critiques.

Is the Democratic Peace Correlation Spurious?

The Spuriousness Critiques

The first set of critiques is that the observed correlation between dyadic democracy and peace is spurious. More informally, the critique is that the observed peace between democracies is caused by factors other than democracy, and not by democracy itself. More rigorously, consonant with the definition of a law provided above, the observed peace between democracies would not support the counterfactual that taking a pair of democracies and rendering one of them non-democratic would make their relationship less peaceful. Whether the democratic peace is spurious or causal is not merely a semantic quibble. Scientists across the social and natural sciences maintain a deep interest in determining whether an observed correlation is causal or merely spurious. Identifying causation is critically important in translating scientific findings into policy recommendations, in areas such as dietary guidelines, poverty reduction, education, fighting disease, and others.

Scholarship making the claim that the democratic peace is spurious frequently takes the following form. On the theoretical side, an alternative explanation for the causes of peace is provided. On the empirical side, a critique will present a previously published multivariate regression analysis showing support for the dyadic democratic peace, and then show that adding to this regression analysis an additional independent variable that measures the new, alternative explanation will cause the dyadic democratic peace variable to become statistically insignificant. Adding the new variable is justified from a methodological point of view as a means of improving the model by reducing what is referred to as “omitted variable bias.” The critique then draws the inference that because the inclusion of this additional variable (or variables) renders the democracy variable statistically insignificant, the initial result was flawed because of omitted variable bias. In turn, the inference is that the initial observed correlation between democracy and peace is spurious rather than causal, and that as a causal hypothesis the democratic peace proposition is not supported. A further implication is that because the democracy-peace relationship is spurious rather than causal, policy-makers should avoid concluding that spreading democracy will in turn cause the world to be more peaceful.5

Scholars have made a number of arguments about the spuriousness of the democratic peace, that is, they have pointed to a series of different variables that if included in multivariate regressions render the democracy-peace correlation statistically insignificant. The oldest and perhaps most central proposition of this type is the realist argument that common national interests rather than joint democracy explain peace. As indicated above, realism proposes that international relations are fundamentally driven by national interests, and not by domestic politics or institutions. Further, realism places no faith in the ability of public opinion coupled with democratic institutions to be a force for peace, because public opinion is not necessarily rational or peaceful; and because elected and other leaders can circumvent the constraints of public opinion through secrecy and other forms of manipulation (e.g., Mearsheimer, 2011; Rosato, 2003; Schuessler, 2015). Historically, the collapse of the international order in the interwar period made realist critics such as E. H. Carr, Walter Lippmann, and Hans Morgenthau deeply skeptical of Wilson’s vision that the spread of democracy could support global peace. Waltz (1959; 1993, p. 78) from the 1950s through the 1990s was also critical of the Kantian hope that democracy would bolster peace, proposing that the brutally competitive nature of the anarchic international system forces different types of political regimes to adopt converging foreign policies in order to survive. Realists in turn proposed that any observed correlation between democracy and peace must be spurious, and in turn that the observed peace between democracies was caused by commonalities in interest and/or by a functioning balance of power rather than by regime type (see Layne, 1994; Mearsheimer, 2014; Rosato, 2003).

Several quantitative studies have endeavored to demonstrate that decisions for war and peace are caused by realist factors such as national interests and the balance of power, and not by regime type. In the 1990s, realist critics took note that the first wave of rigorous quantitative democratic peace studies focused on the 1950–1985 time period, suggesting that especially during this Cold War period democracies were unwilling to fight each other not because of institutions or norms, but because North American, East Asian, West European, and South Pacific democracies needed to balance together against a common Communist threat. A variant of this argument is that peace among democracies during the Cold War was maintained by American hegemony, that a democratic America managed conflict among states within the democratic, anti-Communist bloc to solidify its global power position.

These studies took different approaches to demonstrating this point. Gowa (1999) argued that the democratic peace was a temporal phenomenon; that pairs of democracies were indeed less likely to become involved in militarized interstate disputes or wars after 1945; were less likely than other pairs of states to become involved in MIDs but not wars from 1919–1938; and were as likely to become involved in wars and MIDs before World War I. That is, she measured the presence of common interest indirectly by comparing political eras, arguing that democracies shared common interests after 1945, confronting the Communist threat, and therefore unsurprisingly were less likely to fight each other. Before World War II, she argued, when there were fewer common interests among democracies, the observed correlation between democracy and peace disappears.

Gartzke (1998) took a more direct approach toward testing the same theoretical supposition. He also proposed that common interest rather than joint democracy was the true cause of the observed peace between democracies, especially in the post-1945 period. Rather than comparing eras as Gowa did, he analyzed the post-1945 period, but included in his regression analysis a variable of common interest, measuring how similar were the United Nations General Assembly voting patterns of two states. He found that this variable was statistically significantly related to dyadic peace, and that inclusion of this variable rendered the joint democracy variable statistically insignificant as an explanation of dyadic peace. Some observers have also suggested that the observed peace between democracies is caused by geographic factors rather than regime type (Worley, 2012).

An additional cut on the national interests argument is that conflicts are caused by interstate disputes over contested issues, like territory, and not by regime type. Gibler (2012) focused on territorial disagreements between states. He proposed that territorial disagreements are the fundamental cause of conflict between states, and that inclusion of variables that measure the stability of borders, and therefore the absence of territorial disagreement, rendered the joint democracy independent variable to be statistically insignificant as a cause of peace.

A second cluster of spuriousness critiques focuses on economic rather than political factors. A perhaps more limited version of this critique is that there is a peace among democracies, but only in the developed world and not in developing areas such as sub-Saharan Africa (Henderson, 2008). A more ambitious form of this critique is that development and markets are the true causes of peace, and that democracy is uncorrelated with peace when these factors are accounted for. There are some variants of this observation. Gartzke (2007) focused on higher levels of economic development, proposing that more developed states enjoy lower marginal gain from winning a war over economic assets, and in turn are less likely to become embroiled in war. Conceptually, there is a related strand of research that war has become obsolete as states’ economies have become more advanced and rely more on trade and the global market (Rosecrance, 1986). This point is also related to the more popular assessments of a “McDonald’s Peace,” the observation that countries with McDonald’s restaurants have never fought, McDonald’s being a sign of development (Friedman, 2000, ch. 21), or the “greens peace,” the observation that nations in which golf is sufficiently popular (again, a sign of development) never fight each other (Plotz, 2000). Mousseau (forthcoming; 2009) made a different argument, proposing that only some forms of economic development nurture peace. He proposed that market-based societies place a cultural emphasis on contracts and the law. In turn, this cultural emphasis on law percolates into foreign policy preferences, pushing such states to prefer nonviolent means of conflict resolution. Mousseau proposed that inclusion of a variable measuring this emphasis on contracts and law, what he termed to be “contractualism,” renders the joint democracy variable statistically insignificant.

A third critique focuses on gender. One of the central questions asked in the study of gender and politics is the relationship between gender and war, with many arguing that biological sex and/or cultural constructions of gender are critical factors affecting the onset of political violence. Further, some scholars have used gendered perspectives to critique the proposition that democracy causes peace. Allison (2001) used Kant’s framework of perpetual peace to propose that the key cause of international peace is a feminine perspective on interpersonal care rather than joint democracy. Wisotzki (2015) suggested that gender equality encourages both democracy and peace, though she stopped short of proposing that there was no causal relationship between democracy and peace. Hudson and colleagues (2012) used new data on the physical security of women and political violence, finding that lower physical security of women makes political violence more likely, and that the inclusion of gender equality in the analysis renders democracy an insignificant determinant of peace. Notably, Hudson and colleagues’ unit of analysis is the state rather than a pair of states, and their measure of violence incorporates many types of violence, including intrastate violence. One of Hudson’s coauthors in earlier studies of the causes of interstate violence found in multivariate analyses mixed evidence that both measures of gender inequality and democracy were statistically significant causes of peace (Caprioli, 2000; Caprioli & Boyer, 2001).

Another possible critique, not quite leveled explicitly by any critics, is that common culture and common identity, rather than democracy, cause peace. This is perhaps an implication of Huntington’s (1996) “Clash of Civilizations” thesis: that differences in civilization or culture rather than regime type determine conflict between states.


To the great benefit of the broader field, these democratic peace critiques have enjoyed intensive scholarly debate, with both supporters and critics of the democratic peace successfully pushing each other to refine and improve their theoretical arguments and research designs. Regarding the critique that the democratic peace was purely a Cold War phenomenon, Russett and Oneal (2001), Maoz (1998), and Thompson and Tucker (1997) demonstrated that democracies were less likely to fight each other in the interwar and pre-World War I periods as well as in the post-1945 period. Russett (1993) also presented evidence of a democratic peace in ancient Greece and in pre-modern societies, and Park (2013) demonstrated that the democratic peace existed in the post-Cold War period, as well. Cederman (2001) took a different angle in addressing this question of the democratic peace being confined to the post-1945 time period. He agreed that the peaceful tendencies of democracies had strengthened over time, but he proposed that such a dynamic reflects the kinds of macrohistorical learning process that Kant himself predicted would happen.

Supporters of the democratic peace have also published analyses showing that inclusion of UN voting records does not render the democracy variable statistically insignificant (Oneal & Russett, 1999), these claims in turn attracting response (Gartzke, 2000). The proposal that capitalism rather than democracy causes peace has also attracted critiques, mostly focusing on issues of research design to show that inclusion of capitalism variables does not render democracy variables insignificant (Choi, 2011; Dafoe, 2011). Regarding the possibility of common culture or civilization rather than democracy causing peace, observational data and survey experimental studies have found that the inclusion of culture or civilization as an independent variable does not moot the effects of joint democracy (Bolks & Stoll, 2003; Henderson, 1998; Johns & Davies, 2012; Lacina & Lee, 2013). Regarding whether inclusion of factors such as trade or geography render the democratic peace to be insignificant, Russett and Oneal (2001) openly claim that both joint democracy and bilateral trade cause peace, as part of the “Kantian triangle.” They demonstrate that inclusion of trade and geography variables does not render the democratic peace relationship insignificant. The observation that contractualism and not democracy causes peace has been critiqued (Dafoe et al., 2013), and that critique has in turn attracted response (Mousseau, forthcoming).

The proposal that at least during the Cold War American hegemony rather than democracy itself fostered peace between democracies has also attracted scholarly debate. On a simple level, studies finding evidence of democratic peace routinely include dyadic alliance as an independent variable, including in the post-1945 period (Russett & Oneal, 2001). A broader question is whether or not the United States at least during the Cold War used its power to maintain both democracy and peace within its sphere of influence. In general, the United States supported a variety of anti-Communist states, including democracies like Japan and France and non-democracies like South Vietnam and South Korea (Reiter, 2001).

The critique that conflict is caused by territorial dispute rather than regime type has also experienced rigorous debate (e.g., Gibler, 2014; Park & Colaresi, 2014). Huth and Allee (2002) found that democracy played an important role in affecting whether or not territorial rivalries escalated to violence. Other studies looked at the related issue of rivalry between states, territorial dispute being one type of rivalry. Hensel and colleagues (2000; see also Rasler & Thompson, 2005) found that democratic dyads are less likely to experience rivalries; and that among rivals, the presence of democracy makes the onset of violent conflict less likely.

The gender critique has also enjoyed some scholarly exchange. The most aggressive gender-based critique of the democratic peace (Hudson et al., 2012) uses a monadic design, and a dependent variable inclusive of a wide array of forms of violence including intrastate and interstate conflict. As noted, some other work that focuses on interstate conflict has included gender as an independent variable, and still shown that democracy has a pacifying effect. Using a dyadic research design, Regan and Paskeviciuti (2003) found that both gender and joint democracy affect the likelihood of interstate violence.

In short, the variety of critiques arguing that inclusion of additional variables in multivariate regressions of observational data renders democracy variables to be insignificant have each enjoyed rigorous debate. A larger question to consider is whether there are other ways of testing causation beyond this approach of testing the possibility of spuriousness by adding variables to a regression. There is ongoing debate within political science and the social sciences more generally as to the general utility of whether the addition of more independent variables always makes a regression model “better” by generating a net reduction in bias (Achen, 2005; Clarke, 2005; Pearl, 2011). There is a broader debate as to the general utility of multivariate regression of observational data as a means of assessing causation, given that this approach, sometimes called a quasi-experiment, requires the nonrandom assignment of the treatment condition (the independent variable).

This is not to take a maximalist position that quasi-experiments add nothing, or that adding variables is never advised. It does suggest, however, considering other means of assessing causation, in addition to the conventional approach of seeing if adding plausible exogenous variables renders the democracy-peace correlation to be statistically insignificant. Scholars have explored other means of assessing causation in the democratic peace, and have amassed three other types of evidence that support the conclusion that democracy causes peace: evidence demonstrating support for other empirical patterns suggested by democratic peace theory; evidence produced using experimental methods; and evidence produced using case studies.

The first type of evidence explores for the existence of other empirical patterns predicted by democratic peace theory. If a theory predicts the existence of a variety of empirical patterns and these patterns are demonstrated through tests, we can be more confident in the validity of the theory, and in turn that observed correlations are causal and not just spurious. And, indeed, there is a wide array of quantitative empirical studies that provide support for various assumptions or implications of democratic peace theory, especially for institutionalist accounts of the democratic peace. Perhaps the central institutionalist explanation of the democratic peace proposes that elected leaders are motivated to avoid fighting wars, because the costs of wars will incite popular discontent in turn threatening their hold on power. Studies have demonstrated a number of empirical patterns consistent with this view. Democracies fight shorter wars (Reiter & Stam, 2002, ch. 7). Democracies suffer fewer casualties when they fight wars (Valentino et al., 2010), and when they fight, popular support for the leadership declines as casualties escalate (Mueller, 1973). The benefits of victorious wars may sometimes push democratic publics to accept the costs of war when they are confident of victory, and accordingly democracies almost never start wars they go on to lose (Reiter & Stam, 2002). During war, public support erodes as the perceived likelihood of victory declines (Gelpi et al., 2009). As the institutional explanation of the democratic peace would predict, variations of institutional and leadership form within democracies also affects conflict behavior, as in general more constrained states are less conflict prone (Reiter & Tillman, 2002). Consistent with the audience costs explanation, democracies can more effectively signal their resolve than at least some kinds of autocratic states (Schultz, 2001; Weeks, 2014). There are also some studies supporting elements of the normative explanation. For example, some studies have found that democracies are especially likely to use mediation or binding arbitration to resolve interstate disputes (Dixon, 1993; Raymond, 1994, 1996). In total, though there are certainly scholarly debates about some of these observed patterns,6 this collection of studies improves our confidence that democracy is causing peace in the manners described by democratic peace theory.

The second type of evidence uses experimental methods. Some have proposed that experimental methods enjoy critical advantages over the analysis of observational data in assessing causation. Experimental methods are able largely to skirt some of the biggest causal inference problems associated with quasi-experimental methods, such as biased samples and nonrandom assignments of treatment. That said, the limitation of experimental methods is that, especially in international relations, they can only be used to test some arguments, or some components of arguments. For example, regarding the democratic peace an experimenter cannot take a set of states and then randomly assign some to be democratic and others to be non-democratic.

That said, scholars have thus far been able to conduct survey and laboratory experiments that have tested some elements of the democratic peace. A number of surveys have found support for one of the core assertions of dyadic democratic peace theory: that citizens of democracies are significantly less likely to support the use of force against democracies as compared to using force against non-democracies (Geva et al., 1993; Johns & Davies 2012; Lacina & Lee, 2013; Mintz & Nehemia, 1993; Rousseau, 2005, pp. 219–232; Tomz & Weeks, 2013) Other experiments have tested elements of the audience costs variant of the democratic peace, showing that the public does inflict audience costs on leaders who back down in a crisis (Horowitz & Levendusky, 2012; Tomz, 2007; Trager & Vavreck, 2011).

A third empirical means of demonstrating causation is to engage in process tracing through case studies. Scholars have presented several individual case studies of the democratic peace in events such as 19th-century American diplomatic crises, the 1898 Fashoda Crisis, the onset of World War II, the Spanish-American War, and many others (see Elman, 1997; Owen, 1997; Ray, 1995; Risse-Kappen, 1995; Rousseau, 2005; Schultz, 2001; for case studies presenting evidence against the democratic peace, see Layne, 1994). Some of these case studies demonstrate specific parts of the causal logic of the democratic peace, such as the ability of democracies to signal more effectively through invoking greater audience costs (Schultz, 2001), or the inability of elected leaders to manipulate public opinion or secretly drag their nations into wars the public would otherwise avoid (Reiter, 2012a). Perhaps the most striking case study of democratic peace dynamics is the pacification of Western Europe after World War II, democracy helping to dissolve immediately and completely one of the most violent interstate conflicts in modern history, the France-Germany rivalry (Russett & Oneal, 2001).

Causal Arrows: Does Peace Lead to Democracy, but Not Vice Versa?

The Causal Arrow Critique

A different cluster of arguments critiquing the claim that democracy causes peace focuses on the direction of the causal arrow, proposing that peace causes democracy, but that democracy does not cause peace. The central claim of the “peace causes democracy” element of this claim is that threatening international environments motivate governments to improve their abilities to balance against these threats, and that part of that response is to expand the power of the state and to weaken democratic political institutions. Lasswell (1941) described how a sense of international threat can encourage the emergence of the “garrison state,” a government that empowers military leaders, emphasizes the collective over the individual, and prioritizes coercion over bargaining, as part of a process that eventually destroys democracy. Sometimes states will take internal actions to improve their ability to confront these threats, and these internal actions can in turn bolster autocracy and undermine democracy. One course of action is to strengthen the state itself, recalling Tilly’s (1975, p. 42) claim that “war made the state, and the state made war.” Greater external threat may require a government in the short term to increase defense spending (Nordhaus et al., 2012) and revenue collection and total spending more broadly (Lektzian & Prins, 2008), expanding its control of the national economy.

A related point is that higher levels of external threat push states to reduce individual liberties, including political competition. Poe and Tate (1994; see also Davenport & Armstrong, 2004) demonstrated that participation in interstate wars was negatively correlated with a measure of the percentage of the adult population who vote and with respect for human rights. Conversely, autocracies may be more willing to undergo democratization in a peaceful international context as compared with a more threatening environment (Thompson, 1996).

Importantly, there are two variants of the peace causes democracy argument. The more moderate form proposes that the causal arrow runs both ways, that democracy causes peace and peace causes democracy (Crescenzi & Enterline 1999; Gleditsch, 2002; Midlarsky, 1995; Rasler & Thompson, 2005; Reuveny & Li, 2003; Thompson, 1996). Russett and Oneal (2001) framed this as part of a Kantian positive feedback loop in which peace, economic interdependence, democracy, and international organization all mutually and positively reinforce each other.

The more ambitious form of the argument contends that peace causes democracy but not the reverse. Authors making this claim present different types of quantitative tests to show support. James and colleagues (1999) is perhaps the most sophisticated, using a multiple equations model to demonstrate that peace causes democracy, but that the effect of democracy on peace is statistically significant but substantively negligible. Gibler (2012; Gibler & Tir, 2014) coupled his critique that border stability and not democracy causes peace with the observation that border stability in turn causes democracy. Mitchell and colleagues (1999) presented an interesting variant, albeit using a system level of analysis. Using Kalman filter analysis, they found that though democracy spreads peace, war itself can cause democracy, because democracies often win wars and victors in war pursue regime change.


As with the spuriousness critiques, the claim that peace causes democracy but that democracy does not cause peace has attracted scholarly debate. Regarding the garrison state thesis, some have proposed that levels of threat may cause states to grow stronger, but that need not come at the expense of undermining democracy (Friedberg, 2000; see also Zakaria, 1998). The James and colleagues (1999) article described above attracted scholarly exchanges (Oneal & Russett, 2000; James et al., 2000), though in a later article James himself (Enia & James, 2015) moved to the more moderate position of bidirectional causality, that peace and democracy both cause each other. Some scholars have been skeptical of the proposition that democracy causes peace, presenting quantitative evidence that peaceful international environments do not make democratic transitions more likely, or strengthen democratic regimes (Reiter, 2001). Pevehouse (2005) found only mixed evidence that regional conflict impeded democratization, and little evidence that regional conflict impeded democratic consolidation. Mousseau and Shi (1999) used interrupted time series models to make the point that peace did not cause democracy but democracy does cause peace. Kadera and colleagues (2003; see also Gleditsch, 2002) critiqued the view that the level of conflict in the system affects a democracy’s ability to survive, finding instead that democratic survival is more strongly affected by the number of other democracies in the system. Regarding the thesis that democracies spread democracy after winning wars, more recent analysis paints a different picture, finding that foreign imposed regime change often fails to implant either stability or democracy (Bueno de Mesquita & Downs, 2006; Peic & Reiter, 2011).

There are important methodological dimensions of the debate around the causal arrow critique, mostly concerning the difficulties of constructing a research design that would definitively conclude that peace caused democracy but democracy did not cause peace. One issue concerns the level of analysis. The dyadic democratic peace focuses on the pair of states as its unit of analysis, whereas the peace causing democracy thesis focuses generally on the single state, that a peaceful international environment will affect a single state’s likelihood of becoming democratic. The difference in level of analysis presents difficulties in constructing an integrated estimation strategy that could test satisfactorily both causal arrows (Reuveny & Li, 2003, perhaps come closest to tackling this thorny problem).

Methodological issues aside, there is some theoretical inconsistency within the claim that peace causes democracy but not the reverse. The spuriousness critiques make theoretically straightforward claims: Peace is caused by factors other than regime type, such as national interests, and therefore democracy does not cause peace. In contrast, the causal arrow critique is less straightforward. It proposes that a threatening international environment undermines democracy because leaders perceive that democracy would impede a state’s ability to fight war, because a weak state would be insufficiently agile or powerful to mobilize quickly for war, and/or because individuals empowered by democratic political institutions might resist mobilization and/or a decision for war.7 And yet if these assumptions are true, then they are in turn of course reasons why democracies are more peaceful in their international relations, the heart of the democratic peace thesis. That is, if states or leaders shun democracy because they fear democratic systems are better suited for peaceful rather than violent international environments, then the implication is that democracies ought to be more peaceful in their foreign policies. Peace would cause democracy because democracy causes peace.

Toward Synthesis

What conclusions should be drawn from the spuriousness and causal arrow critiques, both with regards to academic scholarship and policy? Should political scientists recognize that the theory that democracy causes peace has been empirically disproven, meaning that the theory should be rejected? When considering whether or not to spread democracy, should policy-makers no longer consider the benefit that democratization might spread peace?

There is enough evidence to draw the conclusion that joint democracy does cause peace, and that the dyadic democratic peace is a law. None of the spuriousness critiques, though intriguing, have sufficiently withstood scholarly rebuttals to justify dismissing as spurious the very strong correlation between joint democracy and peace, especially given the experimental, case study, and other quantitative observational work that provide support for different elements of the democratic peace argument. That said, the spuriousness critiques suggest possible modifications of the law of the democratic peace, such as perhaps that the democratic peace could be weaker in less-developed regions. Regarding the causal arrow thesis, though there is evidence that peace may cause democracy as well as democracy causing peace, the evidence is much weaker that peace causes democracy but not vice versa. Further, the claim that peace causes democracy but not the reverse contains theoretical inconsistencies.

That said, it is of course conceivable that future studies may emerge that cast decisive doubt on the proposition that democracy causes peace. Data collection in international relations is never going to be as decisive in supporting or refuting theory as data collection in fields like physics or chemistry, where highly precise, often non-probabilistic theory permits point predictions that can be tested many times in controlled laboratory settings. It also will not be as decisive as data collection in the medical sciences, where theories are probabilistic but experiments can be conducted on thousands of subjects and repeated dozens of times. That said, the evidence that dyadic democracy causes peace is as strong as the evidence supporting essentially any theoretical proposition in international relations, other than relatively trivial propositions such as that adjacent states are more likely to fight each other than nonadjacent states. Echoing his 1989 assessment, Levy (2013, p. 587) remarked that in international relations “no one has identified a stronger empirical regularity” (Levy, 2013, p. 587). That is, if the dyadic democratic peace is not a law, it’s as close to a law that we have in international relations, and probably as close to a law as we are ever going to see.

Even accepting that neither cluster of critiques dislodges the conclusion that democracy cause peace, the inescapable conclusion is that we live in a complex, multi-causal world. As the democratic peace advocates themselves have long recognized, many factors beyond democracy cause peace (Russett & Oneal, 2001). Democracy and peace likely mutually cause each other. Further, as Kant envisioned, this variety of factors each cause each other. Development may cause peace, but democracy also causes development (Przeworski et al., 2000). Gender equality and democratization are likely tightly connected in complex ways. Future empirical work using observational, experimental, and case study methods should continue to unpack and describe this web of complex and important relationships.


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(1.) Italics in original.

(4.) There are, to be a sure, a variety of other critiques of the democratic peace proposition. For a summary, see Reiter (2012b).

(5.) Ward et al. (2007) used more advanced statistical methods to assess whether the observed democratic peace was spurious, peace being instead caused by other factors such as geography and other dependencies. Their approach suggested that democracy does still cause peace, but that the magnitude of the effect is lower than what had previously been suggested.

(6.) For example, on whether democracies win their wars, see Brown et al. (2011). On whether public support for war is driven by objective factors such as casualty rates, see Berinsky (2009). On whether elected leaders are more likely to lose power following defeat in war as compared with unelected leaders, see Chiozza and Goemans (2011).

(7.) Note that these specific claims are matters of dispute among scholars. The point here is that these are the specific points made by the causal arrow critique.