The Evolution of Human Aggression

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Everyone has experienced anger at one point in their lives and some of us — males mostly, going by statistics — have channeled that anger into violence, perhaps by throwing a punch during a hockey game or after too many beers at the bar.

Then there's aggression on a much more sinister scale, in the form of murder, wars and genocide. Trying to understand what fuels the different levels of human aggression, from fisticuffs to nation-on-nation battle, has long preoccupied human biologists.

Is there evolutionary reasoning that explains our aggressive tendencies?

This is the central question that anthropologists are now asking as they meet this week at the University of Utah to discuss violence and human evolution. Speakers at the conference, "The Evolution of Human Aggression: Lessons for Today's Conflicts," intend to explore how the long process of human evolution has shaped the various ways we display aggression in modern society.

Though it may seem easier to divide the debate into two camps — those who think evolution has made humans naturally peaceful and those who think we're more naturally prone to violence, the real answer probably lies somewhere in between, said conference organizer Elizabeth Cashdan, professor of anthropology at the University of Utah.

"There is plenty of evidence to support both claims: violence, reconciliation, and cooperation are all part of human nature," said Cashdan, who thinks these wide-ranging emotions all evolved because they benefitted humans in some way in the past.

Animal instincts Evolution can explain why humans exhibit aggression because it is a primal emotion like any other, experts say.

"Emotions (including revenge, spite, happiness, anger) must have evolved because most of the time they motivate fitness-enhancing behaviors, and that is surely true for humans as for other animals," said Cashdan.

Just as compassion for your offspring increases your genes' chance of survival, violent tendencies may have been similarly useful for some species, agreed biologist David Carrier, also of the University of Utah.

"Aggressive behavior has evolved in species in which it increases an individual's survival or reproduction and this depends on the specific environmental, social, reproductive, and historical circumstances of a species. Humans certainly rank among the most violent of species," Carrier said, adding that we also rank among the most altruistic and empathetic. In true nature-nurture fashion, though some kind of genetic preprogramming for violence may exist in humans as a result of our evolution, it is the specific environment that decides how, or whether, that biological determination is triggered, scientists say.

"Biologists speak of 'norms of reaction,' which are patterned responses to environmental circumstances. For example, some male insects are more likely to guard their mates when there are fewer females in the population, hence fewer other mating opportunities. Natural selection didn't just shape a fixed behavior, it shaped the norm of reaction — the nature of the response," said Cashdan.

In other words, though aggression for aggression's sake is rare, an intricate set of conditions could, conceivably, drive most people to violence.

Instead of competing for food, which has become relatively easy to attain in most parts of the world, today we compete for material resources, said Cashdan, and some individuals lack or lose that switch that tells us when enough is enough. Gang violence is a good example of competition for resources gone haywire, though while a gang member's desire for more things, money or partners causes problems now, it may have been the key to their survival 100,000 years ago.

Our emotions make us unique While human aggression is a naturally evolved phenomenon we have in common with other animals, the difference between human and animal violence comes down to the complexity of the emotion driving it, said Cashdan.

"Humans are unique in the complexity of their social relationships and their highly developed social intelligence. Revenge and spite are quintessential social emotions and so are not likely to be found in many, if any, other species," she said. Aggression in few animals goes beyond protecting one's territory, mates, offspring and food — there is some evidence that domestic dogs and chimpanzees do hold grudges, said Carrier — but human violence has evolved to stem from less typical sources.

"For example revenge killings, and the cultural institutions that support and restrain it, shape human aggression in new ways," said Cashdan. The intelligent reasoning that lets most of us override any innate desire to be violent also makes some people, such as parents that kill their children, as well as institutions justify violence illogically, experts say.

Worry over the future An understanding of the evolutionary roots of human aggression could help institutions make better policy decisions, according to experts.

"Evolution didn't just shape us to be violent, or peaceful, it shaped us to respond flexibly, adaptively, to different circumstances, and to risk violence when it made adaptive sense to do so. We need to understand what those circumstances are if we want to change things," said Cashdan.

Though conflicts like the ones that occurred in Rwanda and the former Yugoslavia in the 1990s may seem a distant memory, the tipping point between peace and that sort of violence is a finer line than we think, said Carrier.

"My personal opinion is that Western society, as a whole, is in mass denial about the magnitude of the problem that violence represents for the future. We are peace-loving and want to believe that the violence and transgressions of the past will not return, but recent history and current events illustrate how easy it is for humans to respond with interpersonal and intergroup violence," he said.

This will be especially important in places where key natural resources are becoming scarce, said Carrier, who warned that "if basic resources such as food and clean water become more limiting, as many scientists believe is likely to happen as a result of climate change and energy shortages, then the environmental and social drivers of violence may become more difficult to control."

Heather Whipps is a freelance writer with an anthropology degree from McGill University in Montreal, Canada. Her history column appears regularly on LiveScience. [History Column archive]

Heather Whipps
Heather Whipps writes about history, anthropology and health for Live Science. She received her Diploma of College Studies in Social Sciences from John Abbott College and a Bachelor of Arts in Anthropology from McGill University, both in Quebec. She has hiked with mountain gorillas in Rwanda, and is an avid athlete and watcher of sports, particularly her favorite ice hockey team, the Montreal Canadiens. Oh yeah, she hates papaya.