Why are humans so kind, yet so cruel?

Out of all the days humans have spent fighting wars, Dec. 24, 1914, stands out as particularly subversive. At scattered points along the Western Front in Belgium and France, fighting paused, and German and British forces soldiers began singing Christmas carols.

Then, in defiance of the well-heeled generals at the rear, the mostly working-class combatants on both sides laid down their rifles and tentatively emerged from their trenches. Tales of soccer matches are probably exaggerated, but the soldiers did exchange cigarettes and other trinkets and posed for photos together.

The so-called Christmas Truce is notable for juxtaposing our species’ extremes of kindness and aggression, and it illustrates an age-old question about human nature. How can the same species that will readily snap photos and trade gifts with their supposed enemies also readily slaughter each other by the millions?

“The origins of war and its relationship to questions of human nature is an old debate that goes back to the Enlightenment,” says Brian Ferguson, the director of the peace and conflict studies graduate program at Rutgers University in Newark, N.J.

The apparent contradiction in human nature has served as a mainstay in philosophy from ancient Confucian philosophy to the European Enlightenment up through today. A book by a prominent Harvard anthropologist offers a fresh approach to answering these questions. 

British primatologist Richard Wrangham offers a somewhat unsettling view that a certain kind of violence may have enabled the rise of human kindness. His hypothesis draws on findings in evolutionary biology, anthropology, primatology, moral psychology, and is sparking debate among those disciplines’ practitioners.

Our dual nature

“Humans are very extreme in the direction of the frequency of killing and intergroup aggression,” Professor Wrangham, author of “The Goodness Paradox: The Strange Relationship Between Virtue and Violence in Human Evolution,” published in the United States in January. “And at the same time, we are extremely an outlier in the fact that our interactions within groups are so extraordinarily docile.”

Even compared to bonobos, the peaceable cousins of chimpanzees, typical daily interactions between humans occur with very little physical violence, Professor Wrangham notes. He and his colleagues suggested in 2012 that, compared to chimps, bonobos show the same shortened skull, smaller teeth, and more gracile features that dogs do compared with wolves, guinea pigs with wild cavies, and domesticated foxes with wild foxes. Bonobos’ tranquility, Wrangham and his colleagues suggest, is the product of self-domestication.

The idea that humans are a domesticated species is as old as Aristotle, and was also considered, but ultimately dismissed, by Charles Darwin. But the evidence today supports the idea more strongly than ever. If you compare the skulls of modern humans with those from the mid-Pleistocene epoch, you’d see that our faces are smaller, flatter, and more juvenile, just like domestic animals compared with their wild counterparts. What’s more, humans have been found to share some of the same genes associated with domestication, ones that are absent in the genomes of our extinct relatives.

A grim explanation

But if it’s true that humans are self-domesticated, how did that happen? And why are we the only domesticated mammal that wages war?

“The question that I’ve been interested in is whether war has always been practiced by humans – even before we were fully human – as an expression of some sort of innate predisposition or drive, or has war developed later?” says Ferguson. “Did war have beginnings that reflect the changing nature of society?”

Wrangham argues that the roots of modern warfare lie in the distinction between “reactive aggression” and “proactive aggression.” An example of reactive aggression might occur when you surprise your cat with a pat on the belly. Proactive aggression is when your cat surprises you by quietly sneaking up and pouncing on your foot. Both involve you getting attacked, but the mechanisms producing the behavior are different.

When you frame it like this, it’s clear that, compared with cats, chimpanzees, bonobos, and other mammals, humans score extremely low in levels of reactive aggression, and extremely high in levels of proactive aggression.

What happened in our past to select for these traits? Wrangham argues that the only plausible explanation is what he calls the “execution hypothesis”: Over several thousand generations of prehistoric human history, organized coalitions of males killed off reactively aggressive males, and, in doing so, shifted the course of human evolution.

“Part of how we've been able to become more peaceful is that we kill off the most violent of our leaders,” says Rose McDermott, a professor of international relations at Brown University in Providence, R.I. “Over long periods of time among large numbers of people, you end up with a more – slightly more – egalitarian system. We breed a kind of peacefulness, at least for the in-group.”

Professor McDermott says Wrangham’s book helps explain some of the “emotional undergirding” of principles such as deterrence, both in warfare and in politics.

“When you think about the relationship between leaders and followers,” she says, “in a democracy where we vote people out of office or much more finally with dictators where populations rise up and assassinate them, there exists a universal recognition that vengeance and the drive for vengeance exists.”

“I think really, really good people are capable of really, really bad things under the right circumstances,” says McDermott. “But if you eliminate the capacity to be really bad, maybe you also eliminate the capacity to be really good.”

Wrangham cites examples of such aggression in hunter-gatherer societies. His examples vary in time and place, from Inuits to Aboriginal Australians, but they follow a similar pattern. A group of males identify a domineering or overly aggressive male and wait for the right opportunity to kill him, without injuring themselves.

The emergence of language, particularly sophisticated language some 100,000 to 60,000 years ago, would have greatly aided such conspiracies. “Once that happens, then everything changes,” says Wrangham. “Individual subordinate males can come together and get rid of a dominant. That seems to me to be the ultimate explanation for why human groups – particularly clearly in small-scale societies – do not have alpha males.”

Texas State anthropologist Jill Pruetz, the director of the Fongoli Savanna Chimpanzee Project in Senegal, suggests that Wrangham’s perceptions of violence among chimpanzees may have been shaped by the particular East African subspecies that he observed, which are typically more violent than the West African chimps she works with. More broadly, she’s skeptical about how much the social arrangements of chimpanzees can inform those of humans living today. [Editor's note: An earlier version confused east with west. The Monitor regrets the error.]

“Richard puts out very provocative and interesting ideas,” she says. “But we have to be careful about applying what we know about chimpanzees and bonobos to living humans.”

Rising above nature

Societies have become, on balance, more tolerant and egalitarian in recent centuries. But the continued existence of war shows that our species never abandoned its propensity for proactive aggression. Proactive aggression probably first evolved with hunting behavior, then adapted itself to intergroup aggression, and then to the kinds of planned executions within groups Wrangham describes.

“Even though intergroup aggression absolutely seems to have been a very important part of our evolutionary past, it does not translate simply into complex warfare,” he says, especially when the leaders are behaving with greater reactive aggression than the troops. “The king can order his minions into battle, and his minions are very unhappy about doing it. And the fact that the minions are being ordered into battle is an evolutionary novelty.”

Professor Ferguson, the director of the peace and conflict studies program at Rutgers, argues that violence observed in so-called primitive societies needs to be viewed in the context of European expansionism and colonialism. Lethal violence among chimpanzees, he says, should also be viewed with similar caveats.

To Ferguson, who has read passages from Wrangham’s book but not the whole thing, insisting that humans are innately warlike can undermine peace. “There’s always going to be conflict,” he says. “But we can find ways of dealing with conflict that don’t involve violent, destructive, killing conflict.”

Near the end of the book, Wrangham makes clear that he opposes capital punishment, and that he is hopeful that warfare will decline as the world moves toward an ever smaller number of independent nation-states.

“The past was very rough,” he says. He then quotes Katharine Hepburn’s character in “The African Queen”: “Nature is what we are put in this world to rise above.”