In the last issue of the Journal of Economic Perspetives, Golman et al. have an article about the preferences for believe consonance, where they explain the reasons for the potency of small differences: "Some of the most vociferous disagreements occur between people who—at least from an outsider’s perspective—would seem to have very similar beliefs. In the studies just cited examining the source of armed conflicts in the world, for example, almost half of these conflicts were between different sects of groups within the same broad religious tradition. Drawing attention to the nastiness of disputes between people holding nearly identical views, Sigmund Freud referred in The Taboo of Virginity (1917 ) to the “narcissism of small differences,” commenting that “it is precisely the differences in people who are otherwise alike that form the basis of hostility between them.’’ The sociologist Pierre Bourdieu made a similar point in his treatise La Distinction (1979, English translation in 1984, p. 479), observing that “social identity lies in difference, and difference is asserted against what is closest, which represents the greatest threat.” Empirical research from social psychology and anthropology has documented the surprising potency of small differences. In a 1982 overview article in social psychology, Tajfel summarizes the results of three experimental studies that all find evidence for the importance of small differences for intergroup hostility (Turner 1978; Turner, Brown, and Tajfel 1979; Brown, as reported in Brown and Turner 1981). The studies find that groups with similar values display more intergroup discrimination in competitive situations than groups with dissimilar values. They also show that group members are more ready to sacrifice self-interest for the collective benefit of the in-group when they are dealing with outgroups that are more similar to the in-group. Further evidence of the potency of small differences comes from research by psychologists on “horizontal hostility.” In a series of surveys, White and Langer (1999) and White, Schmitt, and Langer (2006) find that members of minority groups express more unfavorable attitudes about members of other minority groups than about members of majority groups. In particular, people express more hostility toward other minority groups when the other minority groups are more mainstream than their own group. The pattern of horizontal hostility is also evident from a study of members of political parties in Greece by White, Schmitt, and Langer (2006). The authors asked eight party members from each of the four main parties to give a 10-point rating for the social traits of honesty, intelligence, fiscal responsibility, and attractiveness of hypothetical candidates from different parties. Again they find strongly negative evaluations of potential members of similar, but more-mainstream, parties.
In real conflicts, the most comprehensive and systematic investigation of the importance of small differences was undertaken by the Dutch anthropologist Anton Blok (1998, 2001), who drew on existing datasets and empirical findings on the basis of which he concluded that “the fiercest battles often take place between people who have a lot in common” (Blok 1998). In the civil wars in the former Yugoslavia, for example, the most severe fighting took place in the regions that had the smallest differences in ethnic and religious composition between groups and the highest incidences of mixed groups and intermarriages (Blok 2001; Hayden 1996). The differences that divide the fighting parties in many other conflicts are also minor: for example, between the Uzbek minority and the Kyrgyz majority in the conflict in Kyrgyzstan; between Indians and Pakistanis in the conflict in Punjab; between the Greeks and the Turks in the conflict in Cyprus; and between Tutsis and Hutus in Rwanda. The historian Gerard Punier (1995) argues, in his book The Rwanda Crisis, that the genocide in 1994 happened after a period in which economic and social differences between Hutus and Tutsis had narrowed. He discusses how the two groups had long lived side by side, had been involved in intermarriages, and how they neither have had separate homelands, languages, or religions. In all these conflicts, subtle differences in beliefs are often the major distinguishing feature, and in some cases the only difference, between the fighting parties. Hatred and suspicion based on these belief differences seem to increase in intensity the more similar the groups are on other dimensions."