The human race domesticated itself over hundreds of thousands of years, with huner-gatherer societies punishing overly aggressive males using capital punishment if all else fails.
A few years ago, I stayed in Kenya with the conservationists Karl and Kathy Ammann, who kept a rescued chimpanzee named Mzee in their home. Even as a young adult, Mzee was generally well-behaved and trustworthy. Yet he could be impulsive. At one point, over breakfast, Mzee and I reached for the jug of orange juice at the same time. He grabbed my hand as I held the jug, and he squeezed. Ouch. “You first!” I squeaked. I was still rubbing my fingers back to life once he had finished his drink.
The truth is that even when chimpanzees know the rules perfectly well, they don’t always restrain their aggression. In the wild, their lives are full of violence. A day spent with wild chimpanzees gives you a good chance of seeing chases and hitting; every month, you are likely to see bloody wounds. Compared with even an unusually violent group of humans, chimpanzees are aggressive several hundred to a thousand times more often over the course of a year.
The greater peaceability of human societies comes from our nature. We can look each other in the eye. We don’t lose our tempers easily. We normally control our aggressive urges. In primates, one of the most potent stimuli for aggression is the presence of a strange individual. By contrast, Jerome Kagan, a pioneer in developmental psychology, reports that in his hundreds of observations of two-year-olds meeting unfamiliar children, he has never seen one strike out at the other. That willingness to interact peacefully with others, even strangers, is inborn.
What accounts for this human difference? The answer lies in the evolutionary pressures that selected against aggression, particularly in men. The cultural anthropologist Christopher Boehm has found that, in hunter-gatherer societies, a man who threatens others by having too violent a temper is treated in a consistent way. If the bully can’t be contained by the cajoling effects of ridicule or ostracism, the other men reach a consensus, make a plan and execute him. Over the eons, the long-term practice of killing unrepentant aggressors must have favored genes for more peaceful behavior.
No other mammal has the brainpower to organize capital punishment.