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."

A baboon group, in short, is an unlikely breeding ground for pacifists. Nevertheless, there are some interesting exceptions. In recent years, for example, it has been recognized that a certain traditional style of chest-thumping evolutionary thinking is wrong. According to the standard logic, males compete with one another aggressively in order to achieve and maintain a high rank, which will in turn enable them to dominate reproduction and thus maximize the number of copies of their genes that are passed on to the next generation. But although aggression among baboons does indeed have something to do with attaining a high rank, it turns out to have virtually nothing to do with maintaining it. Dominant males rarely are particularly aggressive, and those that are typically are on their way out: the ones that need to use it are often about to lose it. Instead, maintaining dominance requires social intelligence and impulse control -- the ability to form prudent coalitions, show some tolerance of subordinates, and ignore most provocations.

Recent work, moreover, has demonstrated that females have something to say about which males get to pass on their genes. The traditional view was based on a "linear access" model of reproduction: if one female is in heat, the alpha male gets to mate with her; if two are in heat, the alpha male and the second-ranking male get their opportunity; and so on. Yet we now know that female baboons are pretty good at getting away from even champions of male-male competition if they want to and can sneak off instead with another male they actually desire. And who would that be? Typically, it is a male that has followed a different strategy of building affiliative relations with the female -- grooming her a lot, helping to take care of her kids, not beating her up. These nice-guy males seem to pass on at least as many copies of their genes as their more aggressive peers, not least because they can go like this for years, without the life-shortening burnout and injuries of the gladiators.

And so the crude picture of combat as the sole path to evolutionary success is wrong. The average male baboon does opt for the combative route, but there are important phases of his life when aggression is less important than social intelligence and restraint, and there are evolutionarily fruitful alternative courses of action.

Even within the bare-knuckle world of male-male aggression, we are now recognizing some surprising outposts of primate civility. For one thing, primates can make up after a fight. Such reconciliation was first described by Frans de Waal, of Emory University, in the early 1980s; it has now been observed in some 27 different species of primates, including male chimps, and it works as it is supposed to, reducing the odds of further aggression between the two ex-combatants. And various primates, including male baboons, will sometimes cooperate, for example by supporting one another in a fight. Coalitions can involve reciprocity and even induce what appears to be a sense of justice or fairness. In a remarkable study by de Waal and one of his students, capuchin monkeys were housed in adjacent cages. A monkey could obtain food on its own (by pulling a tray of food toward its cage) or with help from a neighbor (by pulling a heavier tray together); in the latter case, only one of the monkeys was given access to the food in question. The monkeys that collaborated proved more likely to share it with their neighbor.

Even more striking are lifelong patterns of cooperation among some male chimps, such as those that form bands of brothers. Among certain primate species, all the members of one gender will leave their home troop around puberty, thus avoiding the possibility of genetically deleterious inbreeding. Among chimps, the females leave home, and as a result, male chimps typically spend their lives in the company of close male relatives. Animal behaviorists steeped in game theory spend careers trying to figure out how reciprocal cooperation gets started among nonrelatives, but it is clear that stable reciprocity among relatives emerges readily.

Thus, even the violent primates engage in reconciliation and cooperation -- but only up to a point. For starters, as noted in regard to the bonobo, there would be nothing to reconcile without violence and conflict in the first place. Furthermore, reconciliation is not universal: female savanna baboons are good at it, for example, but males are not. Most important, even among species and genders that do reconcile, it is not an indiscriminate phenomenon: individuals are more likely to reconcile with those who can be useful to them. This was demonstrated in a brilliant study by Marina Cords, of Columbia University, in which the value of some relationships among a type of macaque monkey was artificially raised. Animals were again caged next to each other under conditions in which they could obtain food by themselves or through cooperation, and those pairs that developed the capacity for cooperation were three times as likely to reconcile after induced aggression as noncooperators. Tension-reducing reconciliation, in other words, is most likely to occur among animals who already are in the habit of cooperating and have an incentive to keep doing so.

Some deflating points emerge from the studies of cooperation as well, such as the fact that coalitions are notoriously unstable. In one troop of baboons I studied in the early 1980s, male-male coalitions lasted less than two days on average before collapsing, and most cases of such collapse involved one partner failing to reciprocate or, even more dramatically, defecting to the other side during a fight. Finally, and most discouraging, is the use to which most coalitions are put. In theory, cooperation could trump individualism in order to, say, improve food gathering or defend against predators. In practice, two baboons that cooperate typically do so in order to make a third miserable.

Goodall was the first to report the profoundly disquieting fact that bands of related male chimps carry out cooperative "border patrols" -- searching along the geographic boundary separating their group from another and attacking neighboring males they encounter, even to the point of killing other groups off entirely. In-group cooperation can thus usher in not peace and tranquility, but rather more efficient extermination.

So primate species with some of the most aggressive and stratified social systems have been seen to cooperate and resolve conflicts -- but not consistently, not necessarily for benign purposes, and not in a cumulative way that could lead to some fundamentally non-Hobbesian social outcomes. The lesson appears to be not that violent primates can transcend their natures, but merely that the natures of these species are subtler and more multifaceted than previously thought. At least that was the lesson until quite recently.

OLD PRIMATES AND NEW TRICKS