God Fearing Atheist

Ive read alot of stuff showing that, in pre-modern and modern primitive societies, warfare is most likely to be engaged in by kin groups. Ive always never quite understood the evolutionary logic behind that.

Is it a kin selection strategy, spreading the benifits (which were/are almost universally resources, status and women themselves) around genetically similar individuals? Wouldnt this also mean that the costs would be centered on those same genes, making it particularly risky?

Is there some other reason?

Links and/or peer-reviewed studies would be a great help.

-GFA