Sapolsky is hardly a starry-eyed optimist. He doubts whether acts of large-scale violence will ever completely vanish. Yes, the threat of war between major powers has declined, he notes, but the ability of small groups or even individuals to wreak enormous havoc—with nuclear, chemical, or biological weapons, not to mention jumbo jets—has grown. “So at a certain level the danger has risen, if not the sheer incidence,” he says. Nonetheless, Sapolsky believes that “there is a great potential for dramatically decreasing the frequency of war and getting a lot better at intervention, termination, and reconciliation.”

De Waal, who met me at the Yerkes center after attending a disarmament workshop in Geneva, agrees that aggression is part of our nature. So too, he adds, are cooperation, conflict resolution, and reconciliation. For decades he has carefully documented how apes and monkeys avoid fights or quickly make up after them by sharing food, grooming each other, or even hugging and kissing.

These traits are especially pronounced in the ape species Pan paniscus. More commonly known as bonobos, they are darker-skinned and more slender than common chimpanzees and have markedly different lifestyles. “No deadly warfare,” de Waal says, “little hunting, no male dominance, and enormous amounts of sex.” Their promiscuity, he speculates, reduces violence both within and between bonobo troops, just as intermarriage does between human tribes. What may start out as a confrontation between two bonobo communities can turn into socializing, with sex between members, grooming, and play.

De Waal suspects that environmental factors contribute to the bonobos’ benign character; food is more abundant in their dense forest habitat than in the semi-open woodlands where chimpanzees live. Indeed, his experiments on captive primates have established the power of environmental factors. In one experiment, rhesus monkeys, which are ordinarily incorrigibly aggressive, grew up to be kinder and gentler when raised with mild-mannered stump-tailed monkeys.

De Waal has also reduced conflict among monkeys by increasing their interdependence and ensuring equal access to food. Applying these lessons to humans, de Waal sees promise in alliances, such as the European Union, that promote trade and travel and hence interdependence. “Foster economic ties,” he says, “and the reason for warfare, which is usually resources, will probably dissipate.”

The question that divides primate researchers—whether war is innate in us and in our hairier kin—has also challenged anthropologist Douglas Fry, whose interest goes back to his teenage years, when the Vietnam War was still raging. He recalls wondering, “Is this something we always have to live with, war after war after war?” His research, says Fry, who left the United States in 1995 to accept a position at Åbo Akademi University in Finland, has led him to reject this conclusion. “Warfare is not inevitable,” he insists in his book Beyond War, because humans “have a substantial capacity for dealing with conflicts nonviolently.”

Fry notes that the earliest widely accepted evidence of possible warfare is a mass grave of skeletons with smashed skulls and hack marks found near the Nile River; the grave dates back some 12,000 to 14,000 years. Such evidence accumulates from later periods as humans around the world abandoned a nomadic existence for a more settled one, leading eventually to the creation of agriculture and states. This evidence consists not only of mass graves but also of weapons clearly designed for fighting, fortified settlements, and rock art depicting battles.

Fry has also identified 74 “nonwarring cultures” that—while only a fraction of all known societies—nonetheless contradict the depiction of war as universal. His list includes nomadic hunter-gatherers such as the !Kung in Africa and Aborigines in Australia. These examples are crucial, Fry says, because our ancestors are thought to have lived as nomadic hunter-gatherers from the emergence of the Homo lineage just over 2 million years ago in Africa until the appearance of agriculture and permanent settlements about 12,000 years ago. That time span constitutes 99 percent of our history.

Lethal violence certainly occurred among those nomadic hunter-gatherers, Fry acknowledges, but for the most part it consisted not of genuine warfare but of fights between two men, often over a woman. These fights would sometimes precipitate feuds between friends and relatives of the initial antagonists, but members of the band had ways to avoid these feuds or cut them short. For example, Fry says, third parties might step between the rivals and say, “‘Let’s talk this out’ or ‘You guys wrestle, and the winner gets the woman.’”

Fry has sought to determine what distinguishes peaceful societies from more violent ones. One clue comes from his fieldwork among the Zapotec, peasant farmers descended from an ancient, warring civilization in Oaxaca, Mexico. There, Fry studied two Zapotec communities, which he labeled with the pseudonyms San Andreas and La Paz. San Andreas’s rates of male-on-male violence, spousal abuse, and child abuse are five times higher than those in La Paz. The reason, Fry suspects, is that women in La Paz have long contributed to the income of their families by making and selling pottery, thus earning the respect of the males.

Fry believes that empowering females may reduce the rate of violence committed within and by a nation. He notes that in Finland—which has a low rate of crime and violence compared with other developed countries—a majority of the cabinet ministers and more than 40 percent of the members of Parliament are women. “I don’t see this as a panacea,” Fry adds, recalling “iron lady” Margaret Thatcher, “but there are good reasons for having a balance of the more caring sex in government.”

The anthropologist Richard Wrangham is one of several scientists at Harvard who pre­sent a much darker view of human nature than Fry does. In his 1996 book, Demonic Males: Apes and the Origins of Human Violence (co­authored with Dale Peterson), Wrangham argues that “chimpanzee-like violence preceded and paved the way for human war, making modern humans the dazed survivors of a continuous, 5-million-year habit of lethal aggression.” Natural selection has favored combative, power-hungry males, he contends, “because with extraordinary power males can achieve extraordinary reproduction.”

“I worked in the Congo,” Wrangham remarks drily when I call him in England, where he is en route to Africa to study chimpanzees. “It’s hard for me to feel that we’re a peaceful species when you have hundreds of thousands of people being killed there.” Wrangham says de Waal is exaggerating the significance of the bonobos, and he scoffs at Fry’s attempt to minimize warfare among hunter-gatherers by excluding “feuds.”

But like his more sanguine colleagues, Wrangham believes we can overcome our propensity for aggression. Primate violence is not blind and compulsive, he asserts, but rather calculating and responsive to circumstance. Chimpanzees fight “when they think they can get away with it,” he says, “but they don’t when they can’t. And that’s the lesson that I draw for humans.” Wrangham notes that male hunter-gatherers within the same band rarely kill each other; their high mortality rates result from conflict between groups.

Wrangham even agrees with Fry on how to decrease conflicts both between and within nations. He points out that as female education and economic opportunities rise, birthrates tend to fall. A stabilized population lessens demands on governmental and medical services and on natural resources; hence, the likelihood of social unrest also decreases. Ideally, Wrangham says, these trends will propel more women into government. “My little dream,” he confesses, is that all nations give equal decision-making power to two entities, “a House of Men and a House of Women.”

Like Wrangham, the archaeologist Steven LeBlanc is critical of scientists who emphasize the peaceful aspects of human nature. At Harvard’s Peabody Museum of Archaeology and Ethnology, where he serves as director of collections, LeBlanc points to a piece of carved wood hanging on his office wall. This, he notes, is a spear employed by Australian Aborigines (who, according to Fry, rarely or never waged war). A short, bearded, excitable man, LeBlanc accuses Fry of perpetuating “fairy tales” about levels of violence among hunter-gatherers and other pre-state people.

LeBlanc contends that researchers have unearthed evidence of warfare as far back as they have looked in human prehistory, and ethnographers have observed significant levels of violence among hunter-gatherers such as the !Kung. In his book Constant Battles: Why We Fight (with Katherine E. Register), he espouses a bleak, Malthusian view of human prehistory, in which war keeps breaking out as surging populations outstrip food supplies. Warfare, he writes, “has been the inevitable consequence of our ecological-demographic propensities.”

Still, when asked point-blank if humans can stop fighting wars, LeBlanc replies, “Yes, I think it’s completely possible.” He notes that many warlike societies—notably Nazi Germany and imperial Japan and even the Yanomami, a notoriously fierce Amazonian tribe—have embraced peace. “Under certain circumstances,” he says, warfare “stops on a dime” as a result of ecological or cultural change. Two keys to peace, he believes, are controlling population growth and finding cheap alternatives to fossil fuels. “I was just in Germany,” LeBlanc exults, “and there are windmills everywhere!”

Despite the signs of progress against our belligerent side, all these scientists emphasize that if war is not inevitable, neither is peace. Major obstacles include religious fundamentalism, which not only triggers conflicts but also contributes to the suppression of women; global warming, which might produce ecological crises that spur social unrest and violence; overpopulation, particularly when it produces a surplus of unmarried, unemployed young men; and the proliferation of weapons of mass destruction.

Moreover, all the solutions to war come with caveats. Sapolsky suggests that eliminating poverty, while an important goal in its own right, may not extinguish war in all regions. Among baboons, lions, and other animals, aggression sometimes “goes up during periods of plenty because you have the energy to waste on stupid stuff rather than just trying to figure out where your next meal is coming from.” De Waal raises concern about female empowerment. Studies of apes and humans, he says, have found that while females fight less frequently than males, when they do fight, they “hold grudges much longer.”

A crucial first step toward ending war is to reject fatalism, in ourselves and in our political leaders. That is the view of the Harvard biologist Edward O. Wilson, who is renowned for his conservation efforts as well as for his emphasis on the genetic underpinnings of social behavior. A rangy man with a raptor’s long, narrow nose and sharp-eyed gaze, Wilson has not budged from his long-standing position that the propensity for group aggression, including war, is deeply ingrained in our history and nature. He notes, however, that group aggression is highly “labile,” taking many different forms and even vanishing under certain circumstances.

He is therefore confident that we will find ways to cease making war on nature as well as on each other, but it is a race against time and human destructiveness. “I’m optimistic about saving a large part of biodiversity,” he says, “but how much depends on what we do right now. And I think that once we face the problems underlying the origins of tribalism and religious extremism—face them frankly and look for the roots—then we’ll find a solution to those, too, in terms of an informed international negotiation system.” Wilson pauses and adds, “We have no option but optimism.”