A Natural History of Peace

Robert M. Sapolsky

From Foreign Affairs, January/February 2006

Summary:  Humans like to think that they are unique, but the study of other primates has called into question the exceptionalism of our species. So what does primatology have to say about war and peace? Contrary to what was believed just a few decades ago, humans are not "killer apes" destined for violent conflict, but can make their own history.

Robert M. Sapolsky is John A. and Cynthia Fry Gunn Professor of Biological Sciences and Professor of Neurology and Neurological Sciences at Stanford University. His most recent book is "Monkeyluv: And Other Essays on Our Lives as Animals."

THE NAKED APE

The evolutionary biologist Theodosius Dobzhansky once said, "All species are unique, but humans are uniquest." Humans have long taken pride in their specialness. But the study of other primates is rendering the concept of such human exceptionalism increasingly suspect.

Some of the retrenchment has been relatively palatable, such as with the workings of our bodies. Thus we now know that a baboon heart can be transplanted into a human body and work for a few weeks, and human blood types are coded in Rh factors named after the rhesus monkeys that possess similar blood variability.

More discomfiting is the continuum that has been demonstrated in the realm of cognition. We now know, for example, that other species invent tools and use them with dexterity and local cultural variation. Other primates display "semanticity" (the use of symbols to refer to objects and actions) in their communication in ways that would impress any linguist. And experiments have shown other primates to possess a "theory of mind," that is, the ability to recognize that different individuals can have different thoughts and knowledge.

Our purported uniqueness has been challenged most, however, with regard to our social life. Like the occasional human hermit, there are a few primates that are typically asocial (such as the orangutan). Apart from those, however, it turns out that one cannot understand a primate in isolation from its social group. Across the 150 or so species of primates, the larger the average social group, the larger the cortex relative to the rest of the brain. The fanciest part of the primate brain, in other words, seems to have been sculpted by evolution to enable us to gossip and groom, cooperate and cheat, and obsess about who is mating with whom. Humans, in short, are yet another primate with an intense and rich social life -- a fact that raises the question of whether primatology can teach us something about a rather important part of human sociality, war and peace.

It used to be thought that humans were the only savagely violent primate. "We are the only species that kills its own," one might have heard intoned portentously at the end of nature films several decades ago. That view fell by the wayside in the 1960s as it became clear that some other primates kill their fellows aplenty. Males kill; females kill. Some kill one another's infants with cold-blooded stratagems worthy of Richard III. Some use their toolmaking skills to fashion bigger and better cudgels. Some other primates even engage in what can only be called warfare -- organized, proactive group violence directed at other populations.

As field studies of primates expanded, what became most striking was the variation in social practices across species. Yes, some primate species have lives filled with violence, frequent and varied. But life among others is filled with communitarianism, egalitarianism, and cooperative child rearing.

Patterns emerged. In less aggressive species, such as gibbons or marmosets, groups tend to live in lush rain forests where food is plentiful and life is easy. Females and males tend to be the same size, and the males lack secondary sexual markers such as long, sharp canines or garish coloring. Couples mate for life, and males help substantially with child care. In violent species, on the other hand, such as baboons and rhesus monkeys, the opposite conditions prevail.

The most disquieting fact about the violent species was the apparent inevitability of their behavior. Certain species seemed simply to be the way they were, fixed products of the interplay of evolution and ecology, and that was that. And although human males might not be inflexibly polygamous or come with bright red butts and six-inch canines designed for tooth-to-tooth combat, it was clear that our species had at least as much in common with the violent primates as with the gentle ones. "In their nature" thus became "in our nature." This was the humans-as-killer-apes theory popularized by the writer Robert Ardrey, according to which humans have as much chance of becoming intrinsically peaceful as they have of growing prehensile tails.

That view always had little more scientific rigor than a Planet of the Apes movie, but it took a great deal of field research to figure out just what should supplant it. After decades' more work, the picture has become quite interesting. Some primate species, it turns out, are indeed simply violent or peaceful, with their behavior driven by their social structures and ecological settings. More important, however, some primate species can make peace despite violent traits that seem built into their natures. The challenge now is to figure out under what conditions that can happen, and whether humans can manage the trick themselves.