Christopher Hitchens has a point--that ethnic groups with less differences will often have far more violence than groups with more distinctions between them. The classic exemplar of late would be Hutus and Tutsis as the genocidaires notoriously needed to look at people's id cards to figure out whether they should be killed or not.
Why is this the case? Easier to maintain an identity short of violence if it is distinct. But if it is not so distinct, violence may be used to create and reinforce the sense of difference, of us and them.
Anyhow, take a look at the Hitchens piece and figure out who you are very much alike but distinct from and then who are you very distinct from? Want to kill the former and not so much mind the latter?