In the Highlands of New Guinea, rival clans have often fought wars lasting decades, in which each killing provokes another.

I haven't read it, but given the author and the subject I'm sure it is interesting.

[ Very interesting. Dovetails into the Matt Ridley book I just finished - The Origins of Virtue. If this sort of thing interests you, I recommend it... talks a great deal about the genetic and social imperatives behind cooperation or non-cooperation and the evolution (and optimization, less convincingly) of powerful, cooperating social groups.

Annals of Anthropology: Vengeance Is Ours: Reporting & Essays: The New Yorker