heh, I read the title and I was going to fly down to comments and say: "Have you heard of John Calhoun's 'Experiment 25'?"

I read it instead, and find that, indeed you have!

There are anthropologists who say that early humans weren't conflict free, just that it was such small scale it was usually only violence without killings between findividuals for food, women or territory. Wars and large-scale mass killings were unheard of until the first psychopath...err.."God-king" organized the group to kill for "the good of our children" or whatever excuse was employed.