CHAPTER ONE

To get my students to appreciate war’s knotty contradictions, I ask them to propose causes of war, which I then scrawl on the whiteboard. They typically nominate these candidates: innate male aggression, ambition, greed, the desire for freedom, religion, ethnic differences, capitalism, patriotism, overpopulation, poverty, inequality, the military–industrial complex, the media, stupidity, and boredom.

This exercise helps me make a few crucial points. First, war has no single cause, not even close. Nor does it have a single solution. And some factors that trigger conflict can also help suppress it. In other words, they are both causes and solutions. Religion, for example, has certainly motivated many warriors, from bloodthirsty Christian crusaders in the Middle Ages to the al Qaeda terrorists who attacked the U.S. on September 11, 2001. But religion has inspired many pacifist sects, such as the Quakers and Jains, as well as leaders of nonviolent social change like Gandhi and Martin Luther King (who, it seems worthwhile to note, were both aggressive, ambitious men). As for nationalism, some thinkers, notably Jean-Jacques Rousseau, saw the state as a cause of bloodshed, while others, such as Thomas Hobbes, viewed it as the cure. Marx blamed war on capitalism, but some modern economists see capitalism as our best hope for reducing poverty, and by extension social unrest.

Science represents another problem that is also a solution. Science contributes to conflict, even as it fortifies my optimism that war can end. Scientists help to produce new-and-improved methods for humans to destroy each other, from catapults and cannons to nuclear submarines and drones. Innovations in weaponry spur arms races that destabilize relations between nations, and often culminate in war (although some nuclear enthusiasts insist that without the deterrent effect of hydrogen bombs, the cold war would surely have become white hot).

Scientific theories can also mutate into divisive, deadly ideologies. Marx hoped his economic system would help humanity achieve a just, peaceful utopia; instead, communism inspired countless bloody insurgencies, one of which, in Russia, spawned a horrendously brutal, totalitarian regime. Similarly, the writings of Darwin and other nineteenth-century biologists yielded social Darwinism and eugenics, which culminated in Nazism. In the twentieth century, as I point out to my religion-averse students, secular and pseudo-scientific ideologies killed far more people than religion did. What makes a belief system lethal, I suggest, is its adherents’ conviction that it is absolutely true, so much so that they feel justified in eliminating non-believers.

Science also exacerbates the problem of war in more subtle ways by, for instance, emphasizing genetic contributions to human behavior and downplaying cultural factors. Of course, all organisms are products of genes as well as their environment and experiences, all of which interact in complex ways. The question remains: how much does nature contribute to a given behavior? The more deeply a behavior is rooted in our biology, the less choice we may have to change. Is war a choice, or are we hardwired for it?

AGAINST GENETIC DETERMINISM

After World War II, the debate over whether we are products primarily of nature or nurture tipped temporarily toward nurture. The Nazis had given biological ideas about human nature a bad name. But since the 1970s, researchers in sociobiology, evolutionary psychology, and behavioral genetics have become increasingly insistent that our genes—more than our upbringing, education, and culture—shape our behavior, including our tendency to fight wars. Other scientists objected to this trend on political as well as scientific grounds, arguing that biological explanations of war would discourage attempts to abolish it.

In the 1980s, David Adams, a psychologist at Wesleyan University, sought to document the ill effects of biological theories of war. Adams and a colleague asked 126 students at Wesleyan and other schools whether they thought “wars are inevitable because human beings are naturally aggressive.” Thirty-three percent of the respondents said “yes” to that statement, and 40 percent agreed with the statement that “war is intrinsic to human nature.” Students with these views were less likely to work for disarmament and peace. Surveys carried out in Finland during the same period produced similar results.

Adams was horrified at how many young people felt humanity was doomed to wage war. “These results,” he writes, “support the need for a worldwide educational campaign to dispel the myth that war is instinctive, intrinsic to human nature, or unavoidable because of an alleged biological basis.” In 1986, he and nineteen other scientists—including specialists in neuroscience, genetics, and other fields—sought to launch such a campaign. Meeting in Seville, Spain, under the auspices of the United Nations, the scientists drafted a statement that begins with five propositions:

1. It is scientifically incorrect to say that we have inherited a tendency to make war from our animal ancestors.

2. It is scientifically incorrect to say that war or any other violent behavior is genetically programmed into our human nature.

3. It is scientifically incorrect to say that in the course of human evolution there has been a selection for aggressive behavior more than for other kinds of behavior.

4. It is scientifically incorrect to say that humans have a “violent brain.”

5. It is scientifically incorrect to say that war is caused by “instinct” or any single motivation.

The statement concludes “that biology does not condemn humanity to war, and that humanity can be freed from the bondage of biological pessimism… The same species who invented war is capable of inventing peace. The responsibility lies with each of us.” UNESCO, the United Nations Educational, Scientific and Cultural Organization, approved the Seville Statement in 1989, and has disseminated it ever since. The statement has been endorsed by dozens of educational, political, and scientific organizations, including the American Anthropological Association and the American Psychological Association.

The Seville Statement is an accurate reflection of research on warfare and other forms of violence. Since Adams carried out his poll in the mid-1980s, though, people young and old have become more pessimistic, if my surveys are any guide. One reason may be that many scientists—including some who share the anti-war sentiments of Adams—dismiss the Seville Statement as an inappropriate or quixotic attempt to impose a political agenda on science. A more important factor, I suspect, is the widespread acceptance of a dramatic theory of the origins of warfare, based primarily on observations not of humans, but of chimpanzees.

THE MYTH OF THE DEMONIC MALE

As recently as the early 1970s, most scientists considered humans to be the only primates that kill each other on a regular basis. This assumption was challenged by an event that took place at the Gombe Reserve in Tanzania, where Jane Goodall had been observing chimpanzees since 1960. Chimps, whose formal name is Pan troglodytes, typically form communities with as many as several hundred members. In 1974, a researcher at Gombe named Hillali Matama watched a band of seven male chimpanzees and a female from one community attack and kill a young male, whom Goodall had named Godi, from a neighboring group.

What made this case remarkable was not simply its violence. Although sometimes Goodall emphasized the sweetness of Pan troglodytes in her early writings, she and her colleagues had witnessed chimps, especially alpha males, biting and beating others within their group, sometimes viciously. But Godi’s death resulted from an attack of members of one chimpanzee community on a member of another community, which was unprecedented. Other researchers subsequently observed similar inter-group raids at Gombe and elsewhere.

What makes these episodes especially disturbing is that Homo sapiens is more closely related to Pan troglodytes than to any other species, and vice versa. Genetic analyses reveal that our lineages diverged less than 7 million years ago, and we still share 98 percent of each other’s genes. Chimpanzee genes overlap more with our genes than they do with those of gorillas, orangutans, or other apes (with the notable exception of bonobos, whom I will discuss shortly).

A British-born anthropologist, Richard Wrangham, was doing field research at Gombe at the time of the attack on Godi in 1974. The incident had a profound impact on him, and he has spent much of his career studying what he calls “coalitionary killing” among chimpanzees. His 1996 book Demonic Males, co-written with the journalist Dale Peterson, opens with Wrangham’s recollection of how explosive conflict between the Tutsis and Hutus, which eventually killed almost 1 million Rwandans, thwarted his attempt to do field research in an African rain forest. Wrangham draws an analogy between this horrific inter-group violence and that of chimpanzees. “Chimpanzee-like violence,” he writes, “preceded and paved the way for human war, making modern humans the dazed survivors of a continuous, 5-million-year habit of lethal aggression.”

Wrangham contends that this “habit” is innate, because throughout our evolutionary history, natural selection has favored males with a predisposition not just for aggression, but for lethal group violence. His most disturbing claim is that both male chimpanzees and male humans engage in group attacks not for any particular purpose—such as gaining mates or territory—but because they like it. Human males “enjoy the opportunity to go and beat up on the neighbors,” he says, as long as they have little risk of being beaten themselves. Wrangham calls human males “natural warriors.”

Wrangham is a charming, genial man. Far from being a warmonger, he is a dove who marched in protests against the U.S. invasion of Iraq in 2003. But he seems to take pleasure in rubbing peoples’ faces in the supposed ferocity of our species’ closest relatives. In 2009, while lecturing at a conference in Salt Lake City on the evolution of aggression, Wrangham showed a film in which five screaming chimpanzees savagely beat a member of a neighboring group. When the victim went limp, Wrangham threw his arms up, like a football referee signaling a touchdown, and yelled, “Yay, our side won!”

When I ask Wrangham if he thinks war will ever end, he pauses before answering. “I’m not so fatalistic that I’d say we can never do it,” he finally replies, “but I would just not want to minimize the obstacles.” Wishful thinking about peace, he suggests, can propagate war. If Bush officials had shared his perspective, he contends, they would never have invaded Iraq in 2003, because they would have realized that the attack would probably result in violence and anarchy rather than peace and democracy. “It was so predictable!” he exclaims.

Our best hope for creating a peaceful world, Wrangham suggests, is for women to gain more political power. “My little dream,” he tells me, is that all nations give equal decision-making power to two entities, which he called “a House of Men and a House of Women.” On the other hand, in Demonic Males, Wrangham blames males’ aggressive behavior in part on females’ sexual preferences. “In the real world,” he writes, “the tough guy finds himself besieged with female admirers, while the self-effacing friend sadly clutches his glass of Chablis at the fern bar alone.”

One might think that most people would dismiss the behavior of apes as irrelevant to debates about human war. But Demonic Males has impressed many readers, including some in positions of power. Hillary Clinton is reportedly a fan of the book. So is the political philosopher Francis Fukuyama, who served in the State Department under President George H.W. Bush and wrote The End of History and other widely discussed books. Whereas Wrangham suggests empowering women as a solution to war, Fukuyama sees it as a potential problem. In an article in the journal Foreign Affairs, Fukuyama expresses concern that “feminine” political leaders like Norway’s Gro BrundHand might not be tough enough to deal with demonic males like Saddam Hussein.

Wrangham’s theory has also been cited favorably by a host of recent books, including Sex and War by the physician Malcolm Potts and journalist Thomas Hayden, The Most Dangerous Animal by the philosopher David Livingstone Smith, and The Blank Slate by the Harvard psychologist Steven Pinker. Pinker is one of the most respected modern proponents of gene-centric theories of human behavior. “Chimpicide,” he writes, “raises the possibility that the forces of evolution, not just the idiosyncrasies of a particular culture, prepared us for violence.”

DEBUNKING THE MYTH

“Extraordinary claims,” the astronomer Carl Sagan liked to say, “require extraordinary evidence.” Wrangham’s assertion that lethal group violence dates back millions of years to our common ancestor with chimpanzees qualifies as an extraordinary claim, with profound significance for human affairs. But Wrangham’s evidence, far from extraordinary, is weak. Wrangham never adequately establishes that chimpanzees, let alone humans, are innately predisposed toward war.

Wrangham often presents the rate of coalitionary killing in terms of annual deaths per 100,000. In a 2004 paper, he estimates that “the median death rate from intergroup aggression among chimpanzees is 140 per 100,000, which rises to 356 per 100,000” if suspected cases are included. Wrangham contends that these death rates “are similar to those resulting from war among humans in traditional subsistence societies.” (The U.S. homicide rate in 2010, in contrast, was less than six per 100,000 people.)

Based on Wrangham’s statistics, one might conclude that researchers have witnessed hundreds or at least scores of coalitionary killings. In fact, as of 2004, researchers had directly witnessed only twelve deaths from lethal intergroup aggression. They “suspected”—based on the discovery of a mutilated corpse or other circumstantial evidence—another seventeen coalitionary killings. These observations are based on 215 total researcher-years of observations of chimpanzee communities at nine separate sites in Africa. That means a researcher watching a typical chimpanzee community will observe, directly or indirectly, only one group killing every seven years. The anthropologist Robert Sussman, who together with his colleague Joshua Marshack compiled these statistics, accuses Wrangham of adopting the “five o’clock news” approach to primate studies: “If it bleeds, it leads.” Wrangham’s relentless focus on violence, Sussman says, results in a distorted picture of chimps and humans.

Responding to Sussman’s critique, Wrangham concedes that chimpanzee coalitionary killings are “certainly rare.” He also acknowledges that “there are various sites where scientists have studied chimpanzees without any record of coalitionary killing or other kinds of violence.” He suggests that these nonviolent chimpanzees are not “habituated” to their human observers, or are isolated from other chimpanzee communities. But this response raises another question: could unusual environmental conditions be triggering inter-group chimpanzee violence, rather than a “natural” warrior instinct?

Recall that the first deadly gang attack was not witnessed until 1974, after Jane Goodall and others had spent more than a decade closely observing chimpanzees. Nor had previous primatologists—such as the legendary Robert Yerkes, who pioneered observations of chimpanzees in the early twentieth century and even lived with them in his home—witnessed any killings. Goodall, who began supplying bananas to chimpanzees in 1965, expressed concern that the food “was having a marked effect on the behavior of the chimps. They were beginning to move about in large groups more often than they had ever done in the old days. Worst of all, the adult males were becoming increasingly aggressive.”

Ian Tattersall, an anthropologist at the American Museum of Natural History, suggests that lethal chimpanzee violence may also be “related to population stress occasioned by human encroachment.” Over the past few decades, Africa’s human population has surged, and farmers, loggers, and poachers have invaded the habitats of chimpanzees, who can no longer roam freely in search of food.

These environmental factors seem to have altered the cultural behavior of certain chimpanzee communities, leading them to engage in lethal raids on others and even to attack humans. In a paper written with several colleagues, Wrangham himself has emphasized that chimpanzees display “significant cultural variation” in tool use, grooming, courtship, food preparation, and other behaviors. Why, then, does Wrangham insist that coalitionary killing—which even he calls “rare”—stems primarily from males’ innate urges?

To explain why coalitionary violence breaks out in certain times and places and not others, Wrangham proposes that male chimpanzees from one group attack outsiders whenever they have a clear advantage. The same is true of human males, Wrangham asserts. One obvious objection to this thesis is that many powerful societies refrain from attacking weaker ones. The U.S. could easily crush Canada but somehow restrains itself. Why? When I put this question to Wrangham, he replies that “there must also be a state of hostility between the two meeting groups.” But this explanation is circular. The question is: what caused the hostility in the first place?

Wrangham concedes that his imbalance-of-power hypothesis cannot explain the “enormous variation in the rates of intergroup killing among human societies.” When it comes to modern nations in particular, “the roles of specialized military units, hierarchical leadership, huge groups, diverse weaponry, elaborate alliance systems, and other features specific to state organization are significant complicating factors.” More simply put, culture matters—precisely the point made by Wrangham’s critics.

FRANS DE WAAL AND THE HIPPIE CHIMPS

To get a more rounded picture of chimpanzees and other primates, I visited Frans de Waal at the Yerkes Primate Research Center in Georgia, which houses chimpanzees and monkeys. We spoke in a watchtower overlooking a fenced-in compound where three male chimpanzees and a dozen females lolled, lazily nitpicking or sniffing each other. While not denying that chimpanzees can be violent—he has witnessed gruesome attacks himself—de Waal says that Wrangham has created a cartoonishly distorted picture of the species.

Over decades of observing chimpanzees, de Waal collected overwhelming evidence of chimpanzees’ generosity, empathy, and peace-making. Chimps often hug and kiss each other and share food both to avoid fights and to make up after them. If one chimp has been injured, others will console it by licking its wounds. Chimpanzees are capable of extraordinary altruism toward members of their communities. They cannot swim, and hence they can easily panic and drown, even in shallow water. Because chimps fear water, zoos often surround chimpanzee compounds with moats. Yet male and female chimpanzees have died after plunging into moats to rescue others who have fallen into the water.

De Waal acknowledges that the most common species of chimpanzee, Pan troglodytes, does not treat members of outside communities so kindly, but bonobos do. Discovered in the forests of the Congo in 1929, bonobos were initially viewed as a subspecies of chimpanzee before being classified as a separate type altogether, Pan paniscus. Genetic analyses suggest that the two chimp species diverged 1 or 2 million years ago. Sometimes called pygmy chimps, bonobos are darker-skinned and more slender than Pan troglodytes, but are otherwise about the same size. (The descriptors “paniscus,” which means diminutive, and “pygmy” are really misnomers.)

Only in the past few decades has the bonobos’ extraordinary social behavior come to light. Male bonobos do not dominate their communities as male Pan troglodytes do. Although male bonobos occasionally beat and bite each other, they are much less violent than Pan troglodytes males. Researchers have not witnessed a single killing either within or between bonobo communities. Even more remarkably, bonobos engage in homosexual as well as heterosexual acts, including mutual masturbation, oral sex, and tongue kissing. Males and females often copulate face-to-face, missionary style, which chimpanzees do only rarely. “The frontal orientation of the bonobo vulva and clitoris strongly suggest that the female genitalia are adapted for this position,” de Waal says.

Sex smoothes relations between as well as within communities. When bonobos from different groups meet, “females often rush to the other side to have sex with both males and other females,” de Waal writes. “Since it is hard to have sex and wage war at the same time, the scene rapidly turns into a sort of picnic.” This promiscuity reduces violence between bonobo communities just as intermarriage does between human tribes, de Waal suspects. Bonobos display empathy and altruism even toward other species. In one case, an adult male rescued a duckling being harassed by two young bonobos and released it back into its moat.

Some critics charge that the reputation of the “Hippie Chimps”—for making love, not war—has been overblown. Researchers recently observed bonobos killing monkeys for food, as Pan troglodytes does. But de Waal and others have repeatedly confirmed bonobos’ relatively benign behavior toward each other in captivity and the wild. Bonobos are just as closely related to us as their chimpanzee cousins, de Waal notes, and hence “exactly, equally relevant to this whole discussion about the origins of warfare.”

In fact, de Waal contends, there are good reasons to believe that bonobos are “more representative of our primate background” than Pan troglodytes. As evidence, he cites recent studies of the extinct primate species Ardipithecus ramidus, which roamed Ethiopia 4.4 million years ago and is the oldest known human ancestor. Anthropologists discovered the first partial skeleton of “Ardi” in Ethiopia in the early 1990s, and over the next decade they uncovered the remains of nine more individuals, all over 4 million years old. Researchers initially classified Ardipithecus as a member of the genus Australopithecus before deciding that it deserved its own genus.

Ardi “falsifies” the claim that the behavior of our ancestors strongly resembled that of chimpanzees, the anthropologist Owen Lovejoy of Kent State University asserts in a paper published in the prestigious journal Science in 2009. Ardipithecus ramidus “reveals that the early” hominid evolutionary trajectory differed profoundly from those of our ape relatives.” Ardi has triggered a “tectonic shift” in views of human evolution, Lovejoy tells me in an interview. “We now know, especially in light of Ardipithecus, that hominids have always been far less aggressive” than chimpanzees.

Ardi’s brain was roughly the same size as a chimpanzee’s, but in other ways the species was quite different. Ardi’s physiology suggests that it could move easily through trees, like chimpanzees, but that it was better adapted for bipedality, or upright walking. Male and female Ardis were closer in size than male and female chimpanzees and so were presumably more likely to engage in pair-bonding. Most importantly, Ardipithecus lacked the dagger-like canines that chimpanzees and even bonobos (whose canines are smaller) employ as weapons. These traits persisted in later hominid species such as Australopithecus.

As recently as the 1960s, Australopithecenes, which inhabited southern and eastern Africa from 4 to 2 million years ago, were thought to be “killer apes” who hunted each other as well as other animals. This belief was based on the claim of the anthropologist Raymond Dart that cut marks on Aus-tralopithecene bones and holes in their skulls were evidence of within-species violence. Popularizations of Dart’s work may have inspired the famous “Dawn of Man” scene at the beginning of Stanley Kubrick’s 1968 film 2001: A Space Odyssey, in which one ape-man beats another to death with a bone. The killer ape theory was discredited after other researchers concluded that predators such as leopards caused the bone marks studied by Dart.

SAPOLSKY’S BABOONS

To my mind, the most significant meta-lesson to emerge from research on non-human primates is how malleable, or “plastic” (to use a trendy scientific buzzword), their behavior is. The idea that primate violence stems primarily from hardwired biological impulses reflects “an old way of thinking,” de Waal says, “that I don’t think fits the facts.” De Waal has demonstrated that shifts in environmental conditions can dramatically reduce primate aggression. For example, he has shown that chimpanzees become much less aggressive toward each other if placed in situations in which cooperation will help them obtain more food.

In another experiment, de Waal placed captive adolescent rhesus and stump-tailed macaques together to see how their behavior might change. Although the two species of monkey are genetically almost identical, their behavior is as different as that of chimps and bonobos. Rhesuses normally dwell in rigid hierarchies, in which dominant males grab most of the food, enforce their rule viciously, and rarely reconcile after fights. Stump-tails, in contrast, are much less aggressive, more egalitarian, and more likely to make up after fighting with lip-smacking, grooming, and other behaviors. Within a few months after rhesus and stump-tails of both sexes lived in captivity together, the rhesuses were behaving less aggressively and were reconciling after fights as often as stump-tails.

A still more striking example of primate malleability comes from an improbable source: baboons, who even more than rhesus monkeys and chimpanzees seem hardwired for aggression. The evolutionary biologist Edward O. Wilson once said that if baboons acquired nuclear weapons, “they would destroy the world in a week.” Troops, which include as many as one hundred members, are ruled by alpha males, who typically achieve high status by beating rivals into submission and thereafter hog food for themselves. Most males die as a result of fighting.

But even baboons, who seem incorrigibly nasty, can change. Since the early 1980s, the biologist Robert Sapolsky has traveled to Kenya to spy on a group of baboons that he calls “Forest Troop.” After a tourist lodge started dumping large amounts of food at a site near the habitat of Forest Troop, some members began foraging for food at the dump. Because they had to fight baboons from another troop camped even closer to the dump, only the toughest males of Forest Troop frequented the dump.

In the mid-1980s, all these males died after contracting tuberculosis from contaminated meat at the dump. The epidemic initially left Forest Troop with about twice as many females as males, who were less pugnacious than those who had died. Conflict within the troop dropped dramatically; Sapolsky even observed males grooming each other, which he calls “nearly as unprecedented as baboons sprouting wings.” Most remarkable of all, this cultural sea change persisted for more than two decades, even after Forest Troop absorbed males from other nearby populations. “Forest Troop’s low aggression/high affiliation society constitutes nothing less than a multigenerational benign culture,” Sapolsky writes.

The research of Sapolsky, de Waal, and even Wrangham reveals that the aggressive behavior of our primate cousins, far from being hardwired, is surprisingly flexible and varied. Nurture matters. Yes, chimps, especially males, are prone to violent aggression, but “coalitionary killing” is rare, and seems to be a response to environmental and cultural conditions as much as it is biological. And that’s the point: it’s not clear-cut. You might even say that chimpanzees have choices, more than they are often given credit for. The plasticity of primate behavior also undercuts a crucial premise of the demonic male theory, which assumes that if chimps have been violent for the past few decades, their ancestors must have always been violent, too. We can’t be sure how chimps were behaving a century ago, let alone millions of years ago.

THE FIERCE YANOMAMÖ

Of course—I can hear critics saying—just because chimpanzee warfare is not innate doesn’t mean, necessarily, that human warfare isn’t. Our lineage, after all, diverged from that of chimpanzees and bonobos millions of years ago. That’s more than enough time for our ancestors to have acquired a genetic propensity for coalitionary killing. Just in the last million years, humans acquired the capacity for language, which most scientists now claim is programmed into our genes. So the question remains: is war in our genes? Could the Seville Statement apply to chimps and bonobos, but not to humans?

Many people have concluded as much, based in part on fieldwork by the anthropologist Napoleon Chagnon. In 1964, when he was a twenty-six-year-old graduate student, Chagnon trekked deep into the rain forests of the Amazon to study the Yanomamö, a polygynous, tribal people who obtain their food by hunting, gathering, and cultivating small gardens. The Yanomamö live in enclosed villages sheltering as many as three hundred people.

Chagnon hoped the Yanomamö, one of the few isolated indigenous people left in the world, would yield clues to human evolution. His 1968 account of his work, Yanomamö: The Fierce People, became one of the best-selling ethnographies ever. Early on, he recalls his first encounter with the Yanomamö:

I looked up and gasped when I saw a dozen burly, naked, sweaty, hideous men staring at us down the shafts of their drawn arrows! Immense wads of green tobacco were stuck between their lower teeth and lips, making them look even more hideous, and strands of dark-green slime dripped or hung from their nostrils—strands so long that they clung to their pectoral muscles or drizzled down their chins.

Later Chagnon learns that just before his arrival, these men were battling rivals in a neighboring village—hence their hostility toward the pale-skinned stranger. Yanomamö males, as described by Chagnon, are almost comically violent. To resolve disputes or even just for sport, men in the same village whack each other over the head with clubs until one combatant quits or is knocked cold. Men display the lumps and scars on their heads like medals.

Yanomamö men from different villages also raid and kill each other with spears and bows and arrows. Fights are triggered by accusations of sorcery (which the Yanomamö believe causes most illnesses) and, even more commonly, disputes over women. Sometimes a man seduces, rapes, or kidnaps a female from another village, triggering a feud in which men kill to avenge past killings and scarcely remember how the war had started. By Chagnon’s estimates, 30 percent of the male population dies violently.

Published at the Vietnam War’s bloody peak, Chagnon’s book seemed to undercut the proposition—often attributed to Rousseau—that prior to civilization humans were “noble savages” living in harmony with nature and each other. In 1988, Chagnon made headlines again with a paper published in Science. Yanomamö killers, he reported, have twice as many wives and three times as many children as non-killers, whom Chagnon—when I interviewed him for Scientific American—called “wimps.” This finding seemed to imply that natural selection favored innately aggressive, warlike men in human prehistory.

Critics have accused Chagnon of exaggerating and even indirectly provoking Yanomamö violence. First, the visits by Chagnon—or any outsiders—to the Yanomamö exposed them to pathogens to which they had little or no resistance. The Yanomamö blamed these illnesses on sorcery by their enemies, against whom they sought revenge. Second, Chagnon gave his favorite tribesmen machetes and other highly prized tools. Inevitably, Chagnon’s critics charged, Yanomamö men vied for Chagnon’s attention and gifts. In other words, they fought over access not to women—which Chagnon claims is the main cause of violence—but to Chagnon. Is it surprising that he recorded higher rates of violence than other anthropologists did?

On the other hand, the Yanomamö were apparently killing each other well before Chagnon arrived on the scene. A memoir by a white woman kidnapped and raised by the Yanomamö in the 1930s details raids and even massacres of women and children bloodier than anything reported by Chagnon. I found Chagnon’s claims about the Yanomamö disturbing but persuasive, so much so that I wrote a positive account of his work for Scientific American.

WERE THE YANOMAMÖ TYPICAL?

This is the crucial question. Other studies—not just of living tribes but of ones that lived millennia ago—have suggested that the Yanomamö’s violent behavior was all too typical of pre-state societies. The archaeological evidence for prehistoric warfare consists primarily of skeletons with crushed skulls, hack marks, and projectile points embedded in them; rock art depicting battles with spears, clubs, and bows and arrows; and walls and other fortifications that provide protection against attacks. Archaeologists have found these signs of prehistoric group violence throughout the world.

This ethnographic and archaeological research is summarized by the anthropologist Lawrence Keeley in his 1996 book War Before Civilization: The Myth of the Peaceful Savage, which like the work of Wrangham and Chagnon has become a touchstone in the debate over war’s origins. In his book’s introduction, Keeley reveals that he once assumed that war began with civilization. His research forced him to accept that the vast majority of pre-state societies waged war. The few exceptions proved the rule: nonviolent societies tended to be either pacified by other, more powerful groups or so geographically isolated that they never encountered potential enemies.

Keeley’s most controversial claim is that war occurred even among the most primitive peoples known, nomadic hunter-gatherers, who traveled in search of their main sources of food: herds of animals and edible plants. Our ancestors were nomadic hunter-gatherers throughout the Paleolithic era, which began 2 million years ago. Then, about twelve thousand years ago, humans started cultivating crops and domesticating animals in permanent settlements. (Because they cultivate small gardens and live in semi-permanent villages, the Yanomamö represent a more advanced and, according to anthropologists such as Keeley, “modern” form of social organization than hunter-gatherers.)

Nomadic hunter-gatherers, also called foragers, are quite rare today. The best known are the !Kung, who live in the Kalahari desert in southern Africa. In her 1959 book The Harmless People, Elizabeth Marshall Thomas describes these bushmen as a gentle, jovial folk, “whose way of life had remained unchanged for thousands of years.” That noble-savage image was amplified by the 1981 film The Gods Must Be Crazy, which depicts the !Kung as innocents in the midst of an insane, violent, “civilized” world. But Keeley contends that in the nineteenth century the !Kung raided neighboring tribes before retreating to their current home in the Kalahari desert. Hunter-gatherers on other continents, he says, also battled: European colonizers of Australia observed Aborigine hunter-gatherers fighting more than two centuries ago, and ten-thousand-year-old rock drawings depict them throwing spears at each other.

Keeley rejects the claim of many previous anthropologists that tribal combat usually took the form of ritualized games with few casualties. Although warfare involved skirmishes and ambushes rather than pitched battles, the fighting sometimes culminated in massacres, in which one tribe wiped out all members of a rival group. And the fighting, he argues, was often chronic—over time the casualties piled up. War killed 25 percent of the population of prehistoric societies, Keeley estimates, about the same rate that Chagnon observed among the Yanomamö and more than ten times higher than the global casualty rate during the blood-soaked twentieth century. Do these data mean war is innate?

WAR’S RECENT ORIGIN

Chagnon and Keeley have persuaded me that people in simple, tribal societies occasionally waged war before the emergence of large, complex cultures. Hence we cannot blame war on civilization in general or western civilization in particular, as some scholars have. But neither Chagnon and Keeley nor any other researchers have established that war, in Barack Obama’s words, “appeared with the first man.” The Homo genus emerged about 2 million years ago and Homo sapiens about two hundred thousand years ago. But the oldest clear-cut relic of lethal group aggression is not millions or hundreds of thousands of years old. It is a 13,000-year-old grave site along the Nile River in the Jebel Sahaba region of Sudan. Excavated in the 1960s, the site contains fifty-nine skeletons, twenty-four of which bear marks of violence, such as embedded projectile points.

What’s more, the Jebel Sahaba site is an outlier. Most of the other evidence for warfare dates back no more than 10,000 years. The oldest known homicide victim—as opposed to war casualty—was a young man who lived 20,000 years ago along the Nile. The victim’s skeleton has stone projectile points embedded in it. Evidence of earlier lethal violence is ambiguous at best. One frequently cited example is a 50,000-year-old male Neanderthal, found in a cave in Iraq, whose ribs were apparently pierced by a sharp object. The anthropologist Erik Trinkaus, who discovered the skeleton, thinks that the marks probably stemmed from an accident. “You find a lot of evidence of bumps and bruises and broken bones among Neanderthals and other early humans, Trinkaus says. Most of these injuries probably resulted from “hunting large animals who object to being speared.”

Researchers have found the remains of Homo sapiens, Neanderthals, and Homo erectus—including a 780,000-year-old skeleton in Spain and a 600,000-year-old specimen in Ethiopia—that show signs of having been butchered with stone tools. These marks are consistent with cannibalism, but not necessarily with homicide, let alone warfare. The “defleshing” marks may be evidence of ritualistic treatment of the dead or of starvation-driven cannibalism, the anthropologist Tim White says, but they “cannot be considered evidence” for lethal group violence.

Sarah Blaffer Hrdy, an anthropologist and authority on both primates and early humans, believes that our human and proto-human ancestors were at least occasionally violent. Given how often fights occur among virtually all primates, including humans, “we can be fairly certain that lethal aggression occasionally broke out” in the Paleolithic era, she says. “It would be amazing if it did not.” But Hrdy sees no persuasive evidence that war—which she defines as “organized aggression between groups with the intent of killing those in other groups”—is either ancient or innate.

Nomadic hunter-gatherers such as the !Kung “are known for being fiercely egalitarian and going to great lengths to downplay competition and forestall ruptures in the social fabric,” she says. If disputes erupted between different bands, they would be far more likely to move on rather than risk their lives by fighting. Warfare emerged, Hrdy contends, when humans abandoned their foraging ways and settled down. Unlike nomadic hunter-gatherers, sedentary societies had assets worth fighting over, such as fertile land, fishing grounds, stored grains, and herds of animals. “As groups grow larger, less personalized, and more formally organized,” Hrdy explains, “they would also be prone to shift from occasional violent disagreements between individuals to the groupwide aggression that we mistakenly take for granted as representative of humankind’s naturally warlike state.”

Defenders of the war-is-in-our-genes theory insist that absence of evidence of warfare deep in human prehistory does not equal evidence of absence, especially given the paucity of human and pre-human remains prior to ten thousand years ago. But archaeologists have found clear-cut relics of other complex cultural behaviors—big-game hunting, cooking, decoration of clothing, painting, music, religion, burials—dating back tens and even hundreds of thousands of years, so why not warfare? And if war is so hardwired, why does it break out only intermittently among pre-state societies as well as modern ones? Scientists believe that our capacity for language is innate, even hardwired, because all known human societies, the simplest and most complex, have employed language at all times. War, in contrast, comes and goes.

The sporadic nature of war leads even Keeley, whose work is often cited by proponents of genetic theories of warfare, to reject such theories. In a section of War Before Civilization titled “The Irrelevance of Biology,” he acknowledges the “incredible plasticity” of human social behavior, noting that extremely belligerent human societies can rapidly become peaceful, and vice versa. “To anthropologists,” he writes, “who have spent over a century exploring the huge variety of human behavior and its mutability, human biology looks less like destiny and more like its absence.”

Another surprising critic of genetic theories of warfare is Chagnon, chronicler of the Yanomamö. Chagnon often denounced critics of his work as left-wing, anti-war ideologues who denied any biological contributions to behavior, but he is far from being a genetic determinist. In his interviews with me, he consistently denied that Yanomamö males are compelled by their genes to wage war. Yanomamö males engage in raids and other violent behavior not because they are innately violent but because violent behavior is esteemed by their culture, Chagnon contends. Most village leaders resort to violence in a controlled manner, and males who cannot control their aggression generally do not live long enough to bear children. Many Yanomamö warriors have confessed to Chagnon that they loathe war and wish it could be abolished from their culture—and, in fact, in recent decades rates of violence have dropped dramatically as Yanomamö villages have, for better or worse, accepted mores of the outside world.

Chagnon once told me that if Yanomamö males were raised in a society that esteemed not violence but, say, farming skills, they would quickly conform to that system. I said he sounded like the evolutionary theorist Stephen Jay Gould, a fierce critic of genetic accounts of human violence and other behaviors. I meant to provoke Chagnon with the comparison, but to my surprise he did not object to it. “Steve Gould and I probably agree on a lot of things,” he replied.

Still, war could be said to be biological in one simplistic sense: the vast majority of warriors in history and prehistory—more than 99 percent, according to one estimate—have carried a Y chromosome. Some men also seem especially predisposed to violence, including war. In the next chapter, I’ll examine the possibility that innately bellicose men have helped perpetuate war since its emergence ten thousand years ago. I call this notion the “bad apples” theory of war.