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."

Soon afterward, tuberculosis, a disease that moves with devastating speed and severity in nonhuman primates, broke out in Garbage Dump Troop. Over the next year, most of its members died, as did all of the males from Forest Troop who had foraged at the dump.[See Footnote #1] The results were that Forest Troop was left with males who were less aggressive and more social than average and the troop now had double its previous female-to-male ratio.

The social consequences of these changes were dramatic. There remained a hierarchy among the Forest Troop males, but it was far looser than before: compared with other, more typical savanna baboon groups, high-ranking males rarely harassed subordinates and occasionally even relinquished contested resources to them. Aggression was less frequent, particularly against third parties. And rates of affiliative behaviors, such as males and females grooming each other or sitting together, soared. There were even instances, now and then, of adult males grooming each other -- a behavior nearly as unprecedented as baboons sprouting wings.

This unique social milieu did not arise merely as a function of the skewed sex ratio; other primatologists have occasionally reported on troops with similar ratios but without a comparable social atmosphere. What was key was not just the predominance of females, but the type of male that remained. The demographic disaster -- what evolutionary biologists term a "selective bottleneck" -- had produced a savanna baboon troop quite different from what most experts would have anticipated.

But the largest surprise did not come until some years later. Female savanna baboons spend their lives in the troop into which they are born, whereas males leave their birth troop around puberty; a troop's adult males have thus all grown up elsewhere and immigrated as adolescents. By the early 1990s, none of the original low aggression/high affiliation males of Forest Troop's tuberculosis period was still alive; all of the group's adult males had joined after the epidemic. Despite this, the troop's unique social milieu persisted -- as it does to this day, some 20 years after the selective bottleneck. In other words, adolescent males that enter Forest Troop after having grown up elsewhere wind up adopting the unique behavioral style of the resident males. As defined by both anthropologists and animal behaviorists, "culture" consists of local behavioral variations, occurring for nongenetic and nonecological reasons, that last beyond the time of their originators. Forest Troop's low aggression/high affiliation society constitutes nothing less than a multigenerational benign culture.

Continuous study of the troop has yielded some insights into how its culture is transmitted to newcomers. Genetics obviously plays no role, nor apparently does self-selection: adolescent males that transfer into the troop are no different from those that transfer into other troops, displaying on arrival similarly high rates of aggression and low rates of affiliation. Nor is there evidence that new males are taught to act in benign ways by the residents. One cannot rule out the possibility that some observational learning is occurring, but it is difficult to detect given that the distinctive feature of this culture is not the performance of a unique behavior but the performance of typical behaviors at atypically extreme rates.

To date, the most interesting hint about the mechanism of transmission is the way recently transferred males are treated by Forest Troop's resident females. In a typical savanna baboon troop, newly transferred adolescent males spend years slowly working their way into the social fabric; they are extremely low ranking -- ignored by females and noted by adult males only as convenient targets for aggression. In Forest Troop, by contrast, new male transfers are inundated with female attention soon after their arrival. Resident females first present themselves sexually to new males an average of 18 days after the males arrive, and they first groom the new males an average of 20 days after they arrive (normal savanna baboons introduce such behaviors after 63 and 78 days, respectively). Furthermore, these welcoming gestures occur more frequently in Forest Troop during the early post-transfer period, and there is four times as much grooming of males by females in Forest Troop as elsewhere. From almost the moment they arrive, in other words, new males find out that in Forest Troop, things are done differently.

At present, I think the most plausible explanation is that this troop's special culture is not passed on actively but simply emerges, facilitated by the actions of the resident members. Living in a group with half the typical number of males, and with the males being nice guys to boot, Forest Troop's females become more relaxed and less wary. As a result, they are more willing to take a chance and reach out socially to new arrivals, even if the new guys are typical jerky adolescents at first. The new males, in turn, finding themselves treated so well, eventually relax and adopt the behaviors of the troop's distinctive social milieu.

NATURAL BORN KILLERS?

Are there any lessons to be learned here that can be applied to human-on-human violence -- apart, that is, from the possible desirability of giving fatal cases of tuberculosis to aggressive people?

Any biological anthropologist opining about human behavior is required by long-established tradition to note that for 99 percent of human history, humans lived in small, stable bands of related hunter-gatherers. Game theorists have shown that a small, cohesive group is the perfect setting for the emergence of cooperation: the identities of the other participants are known, there are opportunities for multiple iterations of games (and thus the ability to punish cheaters), and there is open-book play (players can acquire reputations). And so, those hunter-gatherer bands were highly egalitarian. Empirical and experimental data have also shown the cooperative advantages of small groups at the opposite human extreme, namely in the corporate world.