Editor's Note: This post is the second in a four-part series of essays for Scientific American by primatologist Frans de Waal on human nature, based on his ongoing research. (The first post, on our sense of fairness, can be read here.) De Waal and other researchers appear in a series of Department of Expansion videos focusing on the same topic.

In the 1960s Jack Calhoun placed an expanding rat population in a crammed room and observed how the animals killed, sexually assaulted and, eventually, cannibalized one another. The magnetism of the crowd and the behavioral deviancy led Calhoun to coin the phrase "behavioral sink."

In no time popularizers were comparing politically motivated street riots with rat packs, inner cities to behavioral sinks, and urban areas to zoos. Warning that society was heading for either anarchy or dictatorship, Robert Ardrey, a popular science journalist and author of African Genesis, remarked in 1970 on the voluntary nature of human crowding and its ill effects. These views entered mainstream thinking: The negative impact of crowding became a central tenet of the voluminous literature on aggression.

In extrapolating from rodents to people, however, these writers were making a giant leap. Compare, for instance, the per capita murder rates with the number of people per square kilometer in different nations. If things were straightforward the two ought to vary in tandem, but there is in fact no statistically meaningful relation. Among free-market nations the U.S. is an anomaly by having the highest homicide rate despite a low population density. Some seek the explanation in U.S. gun laws, but this issue remains largely taboo.

To see how other primates respond to being packed together, we compared rhesus monkeys in crowded cages with those roaming free on Morgan Island in South Carolina. We also compared chimpanzees in indoor enclosures with those living on large forested islands. Nothing like the expected crowding effects could be found. If anything, primates become more sociable in captivity, grooming each other more—probably in an effort to counter the potential of conflict, which is greater the closer they live together. Primates are excellent at conflict resolution.

For the future of the world this means that crowding by itself is perhaps not the problem it is made it out to be. Resource distribution seems the real issue. This was already true for Calhoun's rats, the violence among them could be explained by concentrated food sources and competition. Also for humans, I would worry more about sustainability and resource distribution than population density.

For more on this topic, see: Coping with Crowding by Frans de Waal, Filippo Aureli and Peter Judge; May 2000; Scientific American.


Frans de Waal, PhD, is a Dutch-American primatologist known for his popular books, such as Chimpanzee Politics: Power and Sex among Apes (1982) and The Age of Empathy: Nature's Lessons for a Kinder Society (2009). He teaches at Emory University in Atlanta where he directs the Living Links Center at the Yerkes National Primate Research Center. He has been elected to the National Academy of Sciences and the Royal Netherlands Academy of Arts and Sciences.

The views expressed are those of the author and are not necessarily those of Scientific American.

Video credit: Department of Expansion