“I have little fear walking up to a pig on a farm or my neighbor’s dog, but I wouldn’t dream of approaching a wild boar or a wolf in the same way. Over generations of breeding, farmers have reduced the aggressiveness of these and other animals by selecting for lower levels of testosterone and higher levels of serotonin.36 Correspondingly, many domesticated species have smaller faces. Intriguingly, some wild species also evolved reduced aggression, less territoriality, and more tolerance on their own through another kind of selection known as self-domestication. The best example are bonobos. Bonobos are the rarer, less well-known cousins of chimpanzees that live only in remote forests south of the Congo River in Africa. But unlike male chimpanzees and gorillas, male bonobos rarely engage in regular, ruthless, reactive violence. Whereas male chimpanzees frequently and fiercely attack each other to achieve dominance and regularly beat up females, male bonobos seldom fight.37 Bonobos also engage in much less proactive violence. Experts hypothesize that bonobos self-domesticated because females were able to form alliances that selected for cooperative, unaggressive males with lower levels of androgens and higher levels of serotonin.38 Tellingly, like humans, bonobos also have smaller browridges and smaller faces than chimpanzees.39 Many scientists are testing the idea that humans also self-domesticated.40 If so, I’d speculate this process involved two stages. The first reduction occurred early in the genus Homo through selection for increased cooperation with the origins of hunting and gathering. The second reduction might have occurred within our own species, Homo sapiens, as females selected for less reactively aggressive males.”