Why South American tribes are warlike people ?

Napoleon Chagnon is a Professor Emeritus of Sociobiology at the University of California, Santa Barbara. For many decades he studied the Yanomamo, a warlike people who live in the forests of Brazil and Venezuela. He found that men who had killed in battle had 3 times as many children as those who had not. Since personality is heritable, this would be a mechanism for Yanomamo nature to evolve and become fiercer than usual.