Ethnic conflict seems to be common in all countries of the world where people are divided into separate ethnic groups, that may have a racial, national, linguistic, tribal, religious or caste basis. In this article it is proposed that a significant part of the universality of ethnic conflict can be explained by our evolved predisposition to ethnic nepotism, which can be regarded as an extended form of nepotism. Evolutionary theories of inclusive fitness and kin selection explain the origin and universality of nepotism. The members of an ethnic group tend to favour their group members over non-members because they are more related to their group members than to outsiders. This disposition to favour kin over non-kin becomes important in social life and politics when people and groups of people have to compete for scarce resources. Two hypotheses on political consequences of ethnic nepotism are presented: (1) significant ethnic division tends to lead to ethnic interest conflict in all societies and (2) the more a society is ethnically divided, the more political and other interest conflict tend to become channelled into ethnic lines. These two hypotheses are tested by empirical evidence for 183 contemporary states. The hypothetical concepts 'ethnic division' and 'ethnic conflict' are operationalized into empirical variables. The results support the two hypotheses. Deviating cases underline the importance of other relevant factors behind ethnic conflict and violence.