Humans seem to have a capacity for violent aggression as strong as that of chimpanzees and a capacity for gentleness and docility as strong as that of bonobos. “Compared with other primates, we practice exceptionally low levels of violence in our day-to-day-lives, yet we achieve exceptionally high rates of death from violence in our wars,” Wrangham writes. “That discrepancy is the goodness paradox.” Wrangham has been pondering this paradox in the twenty years since the publication of “Demonic Males,” and he has the first draft of an explanation. It is, literally, far-fetched, relying on observations from Siberia, the South Pacific, the Amazon, Tierra del Fuego, and other remote corners of the earth, as well as on the work of archaeologists, paleontologists, psychologists, biochemists, neurophysiologists, geneticists, and others. The clichés about science being a vast coöperative endeavor may actually be true.

Scientists classify aggression into reactive and proactive types. Reactive aggression is a response to a provocation or threat. It’s angry, impulsive, and associated with high levels of testosterone. Proactive aggression is calculating, premeditated, and strategic. Think of those TV Westerns where the hero deliberately provokes a slow-witted, hot-headed antagonist who goes for his gun while our hero pulls his hat down over his eyes and clobbers him over the head with a six-shooter. Our hero is proactively aggressive; the villain is reactively aggressive. Usually, reactive aggression is individual, while proactive aggression is institutional, taking such forms as war or capital punishment.

The types of aggression constitute one building block of Wrangham’s theory of moral origins. An equally important element of that theory is domestication, which turns out to be a crucial category for interpreting the human evolutionary past. For a long time, no theory of human domestication was thought to be necessary, even by Darwin, on the apparently self-evident ground that domestication requires someone to direct the process, like the breeder. Obviously, no one had done that to humans.

But while breeding, or artificial selection, requires an external agent, natural selection does not. If selection pressures work against aggressiveness, animals will self-domesticate. That humans have self-domesticated has grown increasingly obvious over the past half century. Even apart from increased docility—the primary index of domestication—humans show many signs of what has come to be recognized as the domestication syndrome: smaller bodies and brains, thinner bones, shorter faces, and reduced physical differences between males and females. Besides these anatomical markers, there are also behavioral and physiological ones, which involve fear response, playfulness, learning rates, sexual behavior, and hormone production, among others.

What these markers all have in common is paedomorphism (literally, “child shape”). In dogs, foxes, guinea pigs, and many other species, domesticated animals resemble the juvenile stage of the wild animals that they descended from. Humans evolved from our Homo ancestor several hundred thousand years ago, and there aren’t sufficient fossils to demonstrate paedomorphism directly. But there are plenty of Neanderthal fossils, and comparisons strongly suggest that present-day humans are, in many respects, juvenilized—that is, domesticated—versions of our remote ancestors.

Why did these changes happen? For an evolutionary biologist, that question is normally equivalent to asking, What adaptive purpose did they serve? In this case, however, the answer is unusual: none. A decades-long, painstaking experiment by two Russian geneticists working in Siberia showed that reduced brain size, thinner bones, and all of the other markers of domestication syndrome are merely incidental byproducts of a primary adaptation: reduced reactive aggression. In organisms selecting against such aggression, the migration of neural-crest cells—a special kind of cell that carries developmental instructions throughout the embryo and fetus—is delayed, resulting in smaller bodies, smaller brains, hormonal changes, and the rest.

Studies have been fairly clear on this. What has been unclear is why human communities selected against reactive aggression. For Wrangham, the answer is that group life requires a minimum of stability. No trait is more disruptive than reactive aggression, which fuels such behaviors as quests for dominance and demands for submission; arrogance, bullying, and random violence; and the monopolizing of food and females. That is a behavioral profile of the alpha male, the arch-reactive aggressor. Communities must either endure such pests or eliminate them. Once humans could communicate (the origin of language can’t be further narrowed down than three hundred thousand to five hundred thousand years ago, but empathy or “shared intentionality” appears to be independent of language and might be sufficient for communication), the die was cast. The origin of domestication, Wrangham proposes, was the group execution of alpha males. Civilization is founded on capital punishment—or, to give it its anthropological name, “coalitionary proactive aggression.”

The executioners were adult males, usually married. (One of alpha males’ most salient offenses was commandeering other men’s wives.) Over time, as alpha individuals were regularly killed and the gene for reactive aggression became less frequent in a population, the coalition of executioners became more stable. Their power was, in effect, absolute—anticipating Max Weber’s famous definition of the state: the agency with a recognized “monopoly of the legitimate use of physical force.” Staying on the right side of these executioners came to seem a matter of life and death (by murder or—what often amounted to the same thing—ostracism). Community members would have welcomed rules that told them which behaviors were dangerous. They would also have cultivated a reputation for beneficence, since antisocial behavior was the original sin. These developments may have given rise to two of the most distinctive features of human morality: our orientation to abstract standards of right and wrong, and our much greater degree of prosociality—altruism, coöperation, fairness, etc.—than is found in other primates.


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By making us reflect on the rightness of our actions, capital punishment gave birth to virtue. But in replacing the limited power of the alpha male with the unlimited power of executioners and eventually of the state, Wrangham writes, “coalitionary proactive aggression is responsible for execution, war, massacre, slavery, hazing, ritual sacrifice, torture, lynchings, gang wars, political purges, and similar abuses of power.” That is the book’s constitutive paradox. Planned, coördinated violence gave us a social order that made virtue adaptive. But that social order also made exploitation and oppression possible, either by the state or by favored or powerful subgroups. We are, Wrangham concludes, “the best and worst of species.”