"The ultimate weapon of mass destruction—and perhaps of economic destruction—is the testosterone molecule," quipped Malcolm Potts at the February 11, 2009, discussion of his new book, Sex and War: How Biology Explains War and Offers a Path to Peace, which explores the pivotal question, "Why do human beings systematically and deliberately kill our own species?" Potts, the Bixby Professor of Population and Family Planning at the University of California, Berkeley, was joined by Science magazine's Ann Gibbons, a leading correspondent on human evolution, who examined whether aggressive human behaviors are evolving in response to changing social structures.

Testosterone: Risky Business

"In 1987, some anthropologists and sociologists made a statement at UNESCO that it is scientifically incorrect to say we've inherited a tendency to make war from our animal ancestors," said Potts. "I think that that is wrong." Evolutionary psychology suggests that humans have inherited certain predispositions that "help us adapt to find food, select mates, avoid danger, and compete for resources in a hostile world," said Potts. Men compete for women, so it is logical, from a reproductive standpoint, that men would take more risks than women, he argued.

In addition, "there's strong evidence that there is a genetic tendency for men in the prime of life to attack and kill their neighbors," Potts noted, while emphasizing that this does not mean that men are preordained to fight one another. "Such predispositions are extraordinarily flexible," and respond well to peaceful cultural norms.

The Pill Is Mightier Than the Sword

"Once we recognize our violent origins, then we need to ask not ‘why do wars break out?' but ‘why does peace break out?'" posed Potts. "Judged on the basis of same-species killing," the violent 20th century may have been the most peaceful in human history, he claimed.

"In the whole of recorded history, I cannot find a single example of women banding together spontaneously and then going out to attack a neighboring group," maintained Potts. He argued that increasing women's individual freedom and collective power in civil society and government is the best way to achieve a more peaceful world. More specifically, slowing population growth and promoting more balanced age structures by giving women access to family planning will contribute not just to their own autonomy, but also to long-term peace, he argued.

Evolving To Become Less Aggressive?

"Humans are capable of incredible acts of kindness but also despicable acts of terror," said Gibbons. "We murder, slaughter, barbeque, and even eat our own species, and we've been doing it for a long time." But it is difficult to determine whether this propensity for aggression is an ancient trait or has more recently evolved. "There are no other human species alive to show us different models for male aggression…so we have look at fossils, DNA, and our closest relatives—the chimpanzees and gorillas," Gibbons said.

Human aggression may be continuing to evolve. As Gibbons explained, "researchers, as they look at the human genome project—the HapMap Project—have discovered there are many, many genes that have come under natural selection that have evolved in the last 100,000 years, since modern humans spread out of Africa." Therefore, as warfare becomes less necessary to our daily survival, our species might evolve to become less aggressive. "Are we seeing, in our sexual selection, mates being chosen that are a little less aggressive?" Gibbons asked. "We are still evolving," she emphasized. "The story isn't over yet."

Drafted by Will Rogers and edited by Rachel Weisshaar.