WAR AND RELATEDNESS
NBER WORKING PAPER SERIES
WAR AND RELATEDNESS
Working Paper 15095
NATIONAL BUREAU OF ECONOMIC RESEARCH
1050 Massachusetts Avenue
Cambridge, MA 02138
We develop a theory of interstate conflict in which the degree of genealogical relatedness between populations has a positive effect on their conflict propensities because more closely related populations, on average, tend to interact more and develop more disputes over sets of common issues. We examine the empirical relationship between the occurrence of interstate conflicts and the degree of relatedness between countries, showing that populations that are genetically closer are more prone to go to war with each other, even after controlling for a wide set of measures of geographic distance and other factors that affect conflict, including measures of trade and democracy.
Militarized conflicts have been among the most dramatic and costly events in human history, and at the center of an enormous historical and political literature for centuries.1 In recent years, economists
and political scientists have started to use formal theoretical tools and systematic empirical analyses to provide insights into the determinants of conflicts and wars. Great progress has been made in our understanding of the effects of economic and political factors - such as trade and democracy - on the likelihood of international conflict. Nonetheless, wars continue to be elusive phenomena, and fundamental questions about their roots remain open. A key question, which has not yet received a satisfactory empirical answer, is whether armed conflicts are more or less likely to emerge between populations that differ along cultural and historical dimensions, such as ethnicity, language, and religion.
In this paper we present a new theoretical approach, new data and new empirical findings shedding light on the determinants of international conflict. We use information about human genetic distance - a summary statistic of very long-run historical and cultural relatedness between populations - to explore the relationship between kinship and conflict. Genetic distance measures the difference in gene distributions between two populations, where the genes under considerations are neutral: they change randomly and independently of selection pressure, and thus do not affect traits that directly matter for survival and fitness.
Most random genetic change takes place regularly over time, as in a molecular clock (Kimura, 1968). Consequently, genetic distance measures the time since two populations have shared common ancestors - i.e., since they were the same population. In other words, divergence in neutral genes provides information about lines of descent: genetic distance is a summary measure of general relatedness between populations. Heuristically, the concept is analogous to relatedness between individuals: two siblings are more closely related than two cousins because they share more recent common ancestors - their parents rather than their grandparents. Since a very large number of characteristics - including cultural traits - are transmitted across generations over the long run, genetic distance provides a comprehensive measure of long-term cultural and historical distance across populations.
This paper’s main result is that, surprisingly, genetic distance reduces the risk of conflict. Populations that are more closely related are more likely to engage in interstate conflict and wars, even after controlling for a wide range of geographic measures, measures of linguistic and religious distance, and other factors that affect interstate conflict including trade and democracy. These findings are consistent with a simple theoretical framework in which the degree of genealogical relatedness between populations has a positive effect on their conflict propensities, because closely related populations, on average, tend to share common traits and preferences, to interact with each other more, and to care about a larger set of common issues. In principle, such a conflictgenerating effect could be offset by countervailing forces. More closely related populations could also have closer ideal points or could be better at coordinating on peaceful equilibria. However, in the data these other forces, if they exist, do not seem to be strong enough to counteract the main effect stemming from the greater set of common issues arising among genetically related populations. In a nutshell, from a long-term world-wide perspective, issues of war and peace are (unhappy) family matters.
This paper builds on a large and diverse literature. Broad questions about cultural distance, relatedness and conflict are probably as old as wars themselves, but have received increasing attention
following the recent debate over the clash of civilizations (Huntington, 1993) and surging concerns about ethnic conflict within and across countries. For instance, Maynes (1993, p. 5) writes: "Animosity
among ethnic groups is beginning to rival the spread of nuclear weapons as the most serious threat to peace that the world faces". Several commentators have wondered whether there may be a general tendency towards violent confrontation between populations that are culturally and ethnically distant. For example, Bremer (2000, p. 27), referring to evidence from social psychology, wonders whether "cultural differences [...] should lead to misunderstandings, stereotyping, clashes of values, and so forth, which in turn promote intercultural fights". This debate can partly be traced back to the sociologist William G. Sumner (1906), who formulated the primordialist view that ethnic dissimilarity between groups should be associated with war and plunder, while societies that are culturally related would tend to fight less with each other.
In contrast, others have emphasized instrumentalist views of ethnicity, implying that such differences should not be closely correlated with inter-group conflict (e.g., Merton, 1957). A related hypothesis, proposed but not tested by Gleditsch and Singer (1975), is that the paramount force in conflict is geographical contiguity, and that, controlling for contiguity, one would not find a significant correlation between cultural relatedness and interstate conflict (see Henderson, 1997, for a review of this debate). At the same time, the few scholars who have attempted to estimate the effects of common culture,
language or religion on international conflict have found little or no evidence that such variables are systematically associated with a lower probability of conflict.6 In their influential study on conflict
within states, Fearon and Laitin (2003) also found no evidence that ethnically diverse states would be more likely to experience civil conflict.
Our results go further in casting doubts over primordialist theories, as we show not only that their predictions are falsified when applied to interstate conflict, but that the effect goes into the opposite direction. The negative effect of genetic distance holds when controlling for a vast range of geographic measures (contiguity, geodesic distance, latitudinal and longitudinal differences, and
other measures of geographic barriers), contrary to Gleditsch and Singer’s (1975) hypothesis that geographic proximity should be the predominant force in international conflict. It seems that
the paramount effect attributed by some scholars to geographic proximity may in part be due to its correlation with cultural and historical relatedness. Once genetic distance is taken into
account, geographic variables have smaller effects (although they remain significant).
The effect of genetic distance is even higher - and the effects of geography smaller - when we instrument for modern genetic distance using genetic distance between ancestor populations of current countries as of 1500, to account for measurement error and possible endogeneity issues due to post-1500 migrations. The effect of genetic distance is also robust when accounting for other measures of cultural similarity, such as religious and linguistic distance, and for differences in income per capita across countries. Interestingly, religious distance also reduces the likelihood of conflict. This would be hard to rationalize within a clash-of-civilizations view, but is consistent with the predictions of our common-issues model.
Interesting results also emerge when adding measures of trade and democracy, to capture the central predictions of liberal peace theory: extensive bilateral trade links and the extent of democracy among countries in a pair should reduce their propensity to go to war. Not only are the effect of relatedness robust to controlling for trade and democracy variables, but the effects of trade and democracy on conflict hold even after controlling for relatedness. We are therefore able to address one of the most important criticisms of the empirical work on this subject: observers who believe that culturally related countries fight less with each other have often questioned whether there is a direct causal link going from trade and democracy to lower conflict, on the ground that culturally more similar societies also tend to trade more with each other and to share more similar political arrangements (such as democratic regimes).
Following this reasoning, the observed low level of conflict might not be the direct effect of trade and democracy, but rather the outcome of deeper cultural similarities (for discussions of this debate see, for example, Schneider, Barbieri and Gleditsch, 2003). In contrast, our estimates provide strong evidence that the premise that closely related populations fight less with each other is incorrect, and hence cannot account for the pacifying effects of bilateral trade and democracy. In sum, our findings validate the liberal view concerning the pacifying effects of trade and democracy.
This paper is the first, to our knowledge, to study the relationship between genetic distance and the likelihood of international conflict and wars. It is part of a small but growing empirical literature on the connections between long-term relatedness and societal outcomes. In particular, while human genetic distance is not commonly used in the social sciences, recent work has pointed out to its usefulness and predictive power in economics and related areas. Spolaore and Wacziarg (2009) document the relation between genetic distance and differences in income per capita across countries, and provide an economic interpretation in terms of diffusion of economic development from the world technological frontier. Desmet et al. (2007) find a close relationship between genetic distance and cultural differences measured by the World Values Survey, which supports our interpretation of genetic distance as a broad measure of differences in inter generationally transmitted traits, including cultural characteristics.8 More broadly, our paper is related to the evolutionary literature on cultural transmission of traits and preferences (e.g., Cavalli-Sforza and Feldman, 1981; Boyd and Richerson, 1985; Richerson and Boyd, 2004; for economic analyses of cultural transmission, see for instance Bisin and Verdier, 2000, 2001).
In this paper, we examined the empirical relationship between the occurrence of international conflicts and the degree of relatedness between countries. We found that populations that are genetically closer are more prone to engage in militarized conflicts with each other, even after controlling for a wide set of measures of geographic distance, income differences, and other factors affecting conflict, including measures of bilateral and multilateral trade and differences in democracy levels. We also provided a theoretical model of conflict and relatedness that is consistent with these results. In the simplest version of our model, populations that share a more recent common history have had less time to diverge in preferences and characteristics that determine the set of common issues they care about, and over which they are prone to fight.
To our knowledge, this is the first paper that documents a link between genetic distance and international conflict, and provides an interpretation in terms of cultural and historical relatedness. As we have discussed in the introduction, our results provide strong evidence against the primordialist view that cultural and ethnic dissimilarity should breed war and plunder. More broadly, this paper is part of a growing literature in political economy focusing on the effects of long-term cultural and historical variables on political, economic and institutional outcomes, both theoretically and empirically. It would be interesting to link our approach to the extensive literature on ethnic fractionalization and polarization within countries (see Alesina et al. 2003, Fearon, 2003) and to study the effects of long-term genealogical relatedness across groups on civil conflicts and other intrastate outcomes. A positive relationship between relatedness and conflict within states would be consistent, for example, with the finding in Fearon and Laitin (2003) that ethnic fractionalization and civil wars are unrelated. Further research on this question should focus on reliable subnational data on inter-group relatedness.