It is often argued that ethnic conflict is an extreme example of nepotism, and is genetically based. This may be so: in the EEA (the environment of evolutionary adaptedness), such conflict may have been fitness improving, and we may be descended from those who participated successfully in such conflicts. This would provide us with a “taste” for xenophobia. But this taste can be overcome relatively easily, as shown by the changes in behavior in the United States in the 50 years since racial segregation was outlawed. Moreover, in today's world, such conflict does not provide benefits. There are several reasons for this, but the most important (and one that is often overlooked, even by evolutionists) is the possibility of gains from trade in exchanges between ethnic groups. While ethnic relations in the EEA may have approximated a zero-sum game, today a prisoner's dilemma is a more appropriate model for interactions, so that there are significant gains from cooperation. If we want to reduce the amount of conflict in the world, it is probably better to rely on increasing gains from trade than on increased size of in-groups, since the latter strategy will reach a natural limit.
Edward C. McDonagh and Eugene S. Richards, Ethnic Relations in the United States