Tatu Vanhanen has a theory: ethnic conflict is due to ethnic 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. 
Now he reports robust data supporting this:
Marshalling data on 176 contemporary societies, Vanhanen finds that 66% of global variation in ethnic conflict is explained by heterogeneity. This is the sort of robust result that comes from using palpable biological variables. There are many other causes of ethnic conflict but none as strong as ethnic diversity. Per capita income, level of human development, and level of democratization explain only between 6% and 16% of variation.
If, as this seems to show, ethnic conflict has biological roots in ethnic nepotism, what does this mean for the future? I would make two points:
- Ethnic conflicts of interest will likely continue and increase where heterogeneity increases;
- But this does not mean that ethnic fighting will continue. It is possible that pacification can spread, so that ethnic conflicts become fights over the spoils rather than fights to the death.
In short, ethnic conflicts will likely continue, and perhaps increase. Ethnic wars need not.
 Tatu Vanhanen, Domestic Ethnic Conflict and Ethnic Nepotism: A Comparative Analysis Journal of Peace Research
January 1999 vol. 36 no. 1 55-73 doi: 10.1177/0022343399036001004