The roots of this behaviour can be traced back to the shared ancestor of humans, chimpanzees and bonobos, which had strong tendencies to form social dominance hierarchies and to fight, and strong tendencies for alpha peacemakers to stop fights. As well-armed humans were becoming culturally modern, they were living in mobile egalitarian hunting bands that lacked such strong peace makers and lethal retaliation had free play. This continued with tribal agriculturalists who were equally egalitarian, but they tended to live in patrilineal communities, with the males staying put at marriage, and people with such fraternal interest groups developed elaborate rules for feuding. State formation finally brought centralized social control sufficient to put an end to feuding, but self-help killing still continues in certain contexts in modern society. (Published Abstract)