Sex and Violence

By Simon LeVay. Posted March 31 2009

In this article I make five points: i) Males are disposed to commit more individual acts of aggression, including sexual aggression, than are females. ii) Although influenced by cultural forces, this sex difference is due largely to evolved biological factors. iii) War and other forms of intergroup aggression reflect in large part the collective, directed aggressive impulses of numerous individual males. iv) Although wars can be partially restrained by social institutions and interventions, ending war will require re-engineering the human species to reduce male aggressiveness. v) That such re-engineering is possible is suggested by the recent evolution of the relatively peacable bonobos from aggressive chimpanzee-like ancestors. (I thank Kerry Sieh for stimulating discussions that lead to the writing of this article, but he doesn't necessarily share the opinions expressed here.)

In 2003 an international group of population geneticists, led by Chris Tyler-Smith of Oxford University, announced a remarkable finding. They had sampled Y chromosome DNA from over 2,000 men belonging to 26 different ethnic groups in Asia. From their analysis of this DNA, the researchers concluded that one in twelve of all the men who live in a broad swathe of Central Asia between the Caspian Sea and the Pacific Ocean—about 16 million out of the 190 million men in this region—belong to a single lineage that originated in Mongolia about one thousand years ago. This lineage, the researchers proposed, included Genghis Khan, the founder of the Mongol Empire, and his male relatives.[i]

The Mongol rulers were men of an extraordinary violent disposition.[ii] Genghis himself, when he was only thirteen, got into a dispute with his brother over a bird and a minnow that he had caught, and shot him to death with arrows. By the end of his own life a half-century later, he was responsible for the deaths of uncountable numbers of people—most of them the inhabitants of cities captured by the Mongols. One example suffices—that of Merv, the Silk Road city in present-day Turkmenistan, which was then one of the largest centers of population in the world. When Merv, filled to bursting with refugees from Genghis’ advancing forces, surrendered in January of 1221, the inhabitants were led out onto the adjacent desert, where their throats were cut. Later, a cleric led a team that spent 13 days counting the bodies: they totaled over one million three hundred thousand.

Often, when cities were captured, the women were spared—spared death, that is. Instead, they were raped and taken as concubines or sold into slavery. Always, by Mongol custom, the most beautiful women was offered to the leaders for their personal use—to Genghis, his sons, grandsons, or whichever of his male relatives were in the field on that particular campaign.

The Mongol empire was, in effect, a ruthless gene-propagating machine, probably the most efficient that the world has ever known. Small wonder, then, that the Khans now have sixteen million descendants—and that’s only counting the males, and only those males who are descended from the Khans through an uninterrupted male line. Quite likely, at least a drop or two of Genghis’s blood run in the veins of the majority of present-day Asians, male or female.

Yet if Genghis Khan was a man on a mission to spread his genes, he seems to have made one serious error. In their DNA study, Chris Tyler-Smith’s group tested men very widely across Asia, but they didn’t test men in Russia, even though a large part of what is now Russia once lay within the Mongol Empire. (This portion of the empire was known as the Golden Horde.) That deficiency was made up by a group of Russian and Polish geneticists whose results were made public in 2007.[iii] They came up with a shocker: aside from ethnic Mongolians and groups living close to Mongolia, Genghis Khan’s Y chromosome wasn’t present in Russia at all.

How could it be that the Khans spread their genes all over the rest of the Mongol empire, but failed to do so in Russia? The scientists didn’t put forward any explanation, but a careful reading of the history books offers one. The Golden Horde was founded and ruled by Batu, son of Genghis’s eldest son Jochi. Some unusual events surrounded Jochi’s birth—events that are retold in the Secret History of the Mongols, an anonymous account that dates in part to the time of Genghis and was probably influenced or approved by the emperor himself.

Genghis married his primary wife Börte when he was sixteen. A few years later, before the couple had any children, Börte was abducted by the Merkits, a tribe of Mongols who lived far to the north. (They abducted her in revenge for a similar crime that had been perpetrated in the reverse direction a generation earlier: Genghis’ father had abducted a Merkit girl.) When the Merkit raiders returned to their homeland they handed Börte over to a man named Chilger Bote, the younger brother of the Merkit man, now deceased, whose wife had been stolen earlier. A kind of rough justice, if you like, especially for a culture that viewed women as property.

To revenge himself in turn for this insolent abduction, Genghis organized an army of about 12,000 men that crossed the mountains and attacked the Merkits, who were encamped to the south of Lake Baikal. The Merkits fled in disarray, and Genghis was able to recover Börte, along with a large number of Merkit women who served as rewards for Genghis’ helpers. Börte returned to her wifely duties, and a few months later the pair’s first son, Jochi, was born.

A few months later? How many months, exactly? We are not told, but we are told that there was widespread suspicion among the Mongols concerning Jochi’s paternity. Genghis insisted that Jochi was his son and treated him as such, but so great was the suspicion of Jochi’s illegitimacy that, years later when Genghis was Great Khan and was ready to nominate his successor, he was not allowed to choose Jochi. Instead he chose one of his younger sons, Őgedei.

Genghis did give Jochi sovereignty over Russia, but Jochi died early, and the Golden Horde was ruled by Jochi’s son Batu, who was enormously successful in expanding the empire. He followed or outdid Genghis’s example in terms of murder, rape, and pillage. Thus Batu very likely did leave his genetic mark on the Russian and Eastern European territory he conquered—but it wasn’t Genghis’ mark, because Batu wasn’t Genghis’ biological grandson, evidently, but the grandson of a Merkit nonentity called Chilger Bote. Thus Chilger became a biological father of the Russian nation without ever having to leave home, simply by virtue of having once impregnated the wife of the future Mongol emperor. [Note added 12/5/11: In a personal communication, Nikolay B. Pestov of the Shemyakin-Ovchinnikov Institute of Bioorganic Chemistry, Moskow, suggests an alternative explanation for the absence of Genghis Khan's Y chromosome in Russia. He states that the war of conquest was brief and that the mostly-forested Russian terrain was uninviting to the Mongolian nomads, so there was little gene flow from Mongolia to Russia.]

Genghis Khan’s Y chromosome is not the only example of its kind. Giocangga was a 16th-century Manchu warrior and patriarch of the Qing dynasty (the last Chinese dynasty), whose rulers demonstrated their wealth and power by taking innumerable wives and concubines. A type of Y chromosome possessed by 1.6 million present-day Chinese men can likely be traced back to Giocangga, according to a 2005 study by Tyler-Smith.[iv] The Frankish and Ottoman empires also seem to have left their genetic imprints in present day populations.[v] One is left wondering whether “Y chromosome Adam,” the 60,000-year old African who has bequeathed his Y chromosome to the entire male half of the human race, also belonged to a lineage of exceptionally violent and lustful men.

When thinking about these findings, two alternative possibilities come to mind. One possibility is that there was something biologically special about Genghis Khan’s Y chromosome that was adaptive in evolutionary terms—that is, something that helped its carriers to have more offspring. For example, the chromosome might have carried genes that conferred a heftier than usual dose of aggressiveness and lust, promoting the very behaviors that allowed the Khans to replace large segments of the population with their own descendants.

The alternative possibility is that that Genghis Khan’s Y chromosome was an “adaptively neutral” variant, which is to say that it neither promoted nor impaired its owners’ reproductive success. In this scenario, the Khans were aggressive and psychosexually over-endowed for reasons that had nothing to do with their Y chromosomes: the real reason lay in genes located on other chromosomes, or in entirely non-genetic causes, such as the fact that the Khans were brought up to value physical and sexual aggression and had ample opportunity to put these traits into practice. Their Y chromosome (and presumably their other chromosomes too, to some degree) simply benefited from this behavior: it got an extraordinary free boost into the future.

Tyler-Smith calls this latter mechanism “social selection,” and he believes that it is the correct explanation for the prevalence of Genghis’s Y chromosome today. The reason he gives for this conclusion is that the Y chromosomes only carry a very few genes, and those that do lie on this chromosome have specialized functions that would not be likely to promote the behaviors for which the Mongol rulers were famous. And though Tyler-Smith did not use the argument, the fact that Batu was as aggressive as Genghis without apparently possessing Genghis’s genes supports the social-selection model. Still, the question is not yet resolved, because some geneticists who study the sex chromosomes in mice have reported evidence that genes on the Y chromosome can indeed influence aggressive behavior.[vi]

Regardless of the exact mechanism, it seems clear that historically, a disposition to violence and non-consensual sex has helped many men propagate their genes. These instincts toward what we might call “extreme masculine behavior” helped their owners reproduce, but at a tragic cost for society as a whole. In this article I argue that we must deal with the problems posed by these basic instincts if civilization is to survive. And to deal with them, we must understand them.

Some readers may object to my concept of “masculinity.” Maybe St. Francis preaching to the sparrows is a more appropriate example of “extreme masculine behavior” than Genghis Khan’s murderous rampages? And haven’t some women committed grisly murders and sexual assaults, led armies into battle, or started wars?

The concepts of “masculinity” and femininity” only make sense if there exist psychological or behavioral traits that differ in a reliable, though not necessarily absolute way between the sexes. And although there are plenty of exceptions, the traits of physical aggressiveness and interest in casual, promiscuous, or coercive sex do indeed differ very significantly between the sexes, as has been demonstrated quantitatively by psychologists, social scientists, and criminologists.[vii] Preaching to sparrows, on the other hand, does not.

In the contemporary U.S., men are nearly ten times more likely to commit murder than are women, and nearly fifty times more likely to commit sexual assault. What’s more, the male superiority in these traits—if “superiority” is the right word—exists across all cultures and historical periods that have been studied, and (with regard to aggressiveness) emerges early in childhood.[viii] If you come across a study that reports equal aggressiveness by men and women, you will find that it does so by focusing on non-physical or indirect forms of hostility such as malicious gossip.

Why are men more aggressive than women, physically and sexually? Feminists have tended to lay the blame on the different ways that boys and girls are brought up, and the unconscious lessons they absorb from their families, television, and so on, especially with regard to sexually aggressive behavior. If you do an internet search on phrases like “boys are taught to” or “boys are socialized to” you will find hundreds of propositions of that kind. In the words of the sociologist Diana Russell, for example, “Males are trained from childhood to separate sexual desire from caring, respecting, liking or loving. One of the consequences of this training is that many men regard women as sexual objects, rather than as full human beings. … [This view] predisposes men to rape.”[ix]

Certainly, there are social forces that tend to bolster gender stereotypes and encourage sex differences in aggression and many other traits. To that extent, Russell is probably right. But what Russell and many other feminists ignore is the fact that there are also very strong social forces working in the opposite direction, reining in men’s tendency to violence.

Most obviously, the criminal justice system does so. In one study, 37 percent of male college students stated that there was some likelihood that they would commit rape, if they knew that they wouldn’t be caught.[x] Since this is far higher than the percentage of college students who actually commit rape, one can conclude that the socially imposed fear of punishment prevents most potential rapes. Conversely, when social controls weaken, as during wartime or when policing becomes ineffective, the rates of murder, rape and other violent crimes—nearly all committed by men—soar.[xi] During the 1969 Montreal police strike, for example, the frequency of bank robberies increased fifty-fold.

Socialization also works at a more subtle level to restrict male violence, by strengthening the internal policeman we call guilt. Socialization is presumably a large part of the reason why, in the study mentioned above, a sizeable majority of the male students said that they wouldn’t commit rape even if they knew that they wouldn’t be caught. Saying “bad boy!” to a misbehaving toddler, or “good boy!” to a well-behaved one, doesn’t magically imbue him with righteousness, but when repeated ten thousand times over a decade or so it may just do so.

Socialization doesn’t create that internal policeman necessarily, for much evidence points to the existence of an innate moral sense.[xii] Nevertheless, this innate sense appears to differ qualitatively between the sexes, being more justice-based in men and more caring-based in women. This difference already tends to justify or even promote some forms of violence in men’s minds, such as retaliation and preemptive aggression. “An eye for an eye” doesn’t require much socialization, but “turning the other cheek” certainly does.

My point is this: the effects of socialization on gendered traits like aggression are mixed, but the restraining effect of socialization is at least as strong as (and quite possibly much stronger than) the inciting effect. This makes me doubt that socialization is the original cause of sex differences in these traits. But if it isn’t socialization, what is it?

The most likely answer is: biology. And there are two kinds of biological answer—one that describes the factors operating within individuals, and one that describes the factors operating over evolutionary time.

When thinking about individuals, the first word that comes to mind is testosterone, the hormone that, in popular culture at least, is the physical embodiment of masculinity. Testosterone and similar hormones (collectively called androgens) help generate and sustain the sex drive in both men and women, but they are present at much higher levels in the blood of adult men than adult women. Castration of males (removal of the testes, thus eliminating the principal source of androgens) decreases or extinguishes their sex drive and makes them less aggressive and more docile. This is true both in humans and many animals.

Still, the effects of castration in humans take some time to show themselves—sometimes months or years—even though the reduction in testosterone levels are nearly instantaneous. This suggests that testosterone does not regulate the sex drive or aggressiveness on a short-term basis. In fact, a healthy man’s blood testosterone levels fluctuate wildly over the course of the day, but these fluctuations do not translate into equivalent hour-by-hour fluctuations in sexual or aggressive feelings or behavior. Rather, testosterone appears to play a long-term role in maintaining the activity of brain circuits that are involved in generating these feelings and behaviors.

Among men, high testosterone levels are correlated with an increased likelihood of engaging in criminal violence and other forms of aggression,[xiii] and male aggression and sexual coercion is most prevalent during young adulthood, when testosterone levels are high. But the correlation between testosterone levels and aggression is not terribly strong: it leaves a lot of the variation in aggressiveness unexplained. A more important causal factor may be the levels of testosterone during fetal life, when the brain circuits involved in aggression and sexuality are first assembling themselves. The higher levels of circulating testosterone in male fetuses compared with female fetuses is probably a large part of the reason why adult males are on average more aggressive and more interested in casual and coercive sex than females.

What’s more, there’s evidence that differences in testosterone levels between one male fetus and another are part of the reason why one adult man is more aggressive than another. So far, this statement is based on indirect evidence, such as the measurement of anatomical details—ratios of the lengths of different fingers, in particular—that are believed to reflect testosterone levels during fetal life. Men with finger-length ratios indicative of high fetal testosterone levels are more aggressive, on average, than men whose ratios are indicative of low fetal levels.[xiv] More direct studies are under way: the group led by Simon Baron-Cohen at Cambridge University has measured the levels of testosterone secreted into the amniotic fluid of a large number of fetuses, and is following the postnatal psychological development of the resulting children. They are testing the hypothesis that differences in fetal testosterone affect the development of gender-differentiated psychological characteristics including aggression.[xv]

Still, testosterone levels, whether in pre- or postnatal life, may not be the whole story; other biological factors also probably play a role. The brain neurotransmitter serotonin, for example, appears to help regulate aggressive behavior.[xvi] Violent criminals tend to have low serotonin levels, and increasing serotonin levels with drugs tends to reduce aggressive behavior, at least among mentally ill individuals.

Genes influence aggressive behavior. Based on twin studies, it appears that genes account for about half of the total variability in physical aggressiveness among men.[xvii] For the most part, the actual genes involved have not been identified. However, Dutch researchers achieved an important breakthrough in 1993, when they studied a family in which nine male members show severe impulsive aggression, including assault, attempted rape, and arson. The men are easily provoked into outbursts of rage, cursing, or violence by stressful events that most of us would cope with more calmly. The researchers showed that the affected men (but not the psychologically normal men in the same family) have a non-functional gene for an enzyme named monoamine oxidase type A or “MAOA.” This enzyme metabolizes certain neurotransmitters: serotonin, norepinephrine (noradrenaline), and dopamine. Male mice in which this same gene has been knocked out also show hyperaggressive behavior, a finding that strengthens the causal link between the gene and aggressiveness in the Dutch family.[xviii]

Most violent men do not have a non-functional MAOA gene. However, there is a common variant of the gene, possessed by about 10 percent of the population, in which the MAOA enzyme is somewhat less active than normal. This variant has the effect of exacerbating the so-called “cycle of violence”: boys who possess the less-active variant, and who have also been physically or sexually abused, are more likely to become violent or antisocial themselves when they grow up, compared with similarly abused boys who carry the fully active gene.[xix] This finding illustrates the complex interactions between genetic predispositions and environmental factors in the development of personality factors such as aggressiveness.

Of course, aggression has survival value, and that’s why evolution has ensured that humans have aggressive feelings and the capacity to show them in aggressive behavior. Aggression can supply all kinds of resources, ranging from food and living space to mates, as we saw with Genghis Khan. Genghis Khan and his male relatives left at least 16 million present-day descendants. Milquetoast Khan (a creature of my imagination) left none.

But in evolutionary terms, aggression has more value for males than females. That’s because aggressive behavior is risky. It’s a high-stakes game offering the lure of great rewards and also the threat of catastrophic losses, including premature death. This kind of gambling has always made more sense for males than females.

To see this, think about the different reproductive strategies that males and females are forced into. Females of most mammalian species make a much greater investment in reproduction than males do. To reproduce, a female has to invest all the time and metabolic expense of pregnancy, followed by lactation and infant care, before she can think of producing more offspring. Thus females are quite limited in terms of the number of offspring they can produce over a lifetime, but they generally do get fairly close to that limit.

A male, on the other hand, just has to inseminate a female with a few drops of semen, and then he’s free to head off in search of another mate. So males can have much larger numbers of offspring, but—because of choosiness by females and competition from other males—they may very easily have none. In other words, there is more variability in reproductive success for males than for females. The sex difference in variability is what offers a greater incentive for risk-taking by males than by females. To put it simply, why should females take risks when they can get pregnant and have offspring without doing so?

There are all kinds of complications and caveats surrounding this idea, of course. For one thing, it isn’t always the females who make the larger investment in reproduction. In a few species, males do most of the hard work. There are waterbirds called jacanas, for example, in which the males incubates the eggs and feed the nestlings, leaving the female free to fly off and mate with another male. In such species, however, it’s the females who are the aggressive sex, competing fiercely with each other for access to males. In other words, these “exceptions” validate a more general rule: the sex that gets off lightly in terms of parental investment is the one that’s more aggressively competitive in its reproductive strategy.[xx]

In humans, that’s the males. However, the asymmetry between the sexes in this respect is not as great as it is in some other mammals. Men do typically put some investment into reproduction, such as providing food, resources, and protection for the family. That’s in contrast to many other species in which the male does absolutely nothing aside from inseminating the female. Part of the reason why men put some work into reproduction is that they increase their children’s chances of survival by doing so—human infants require more and longer parental care than the young of most other mammals, after all. In addition, women probably choose partners who somehow or other exhibit a commitment to the family.

So far, I have basically treated male aggression as a single coherent trait whose benefit is to maximize an individual’s reproductive success. Genes promoting Genghis-Kahn-type behavior have evolved because of the positive effect they can have on their owner’s reproduction and hence on the survival of those same genes. This is the “selfish-gene” perspective made famous by Richard Dawkins.[xxi]

Of course, some forms of aggression are more closely tied to reproduction than others. Abducting women to stock your harem has direct reproductive significance. Seizing someone’s land doesn’t—it could be seen as an economic form of aggression that has to do simply to do with surviving, or living better than you would have done otherwise. Some opponents of the selfish-gene perspective, such as evolutionary biologist Niles Eldredge, argue that most behavior is “economic” in motivation rather than reproductive—including a lot of behavior that is overtly sexual. For example, he says, much heterosexual intercourse is motivated, not by the urge to make babies, but by a “deal” between partners in which sexual pleasure is traded for some kind of resource.[xxii]

There are a couple of problems with Eldredge’s point of view. For one thing, acknowledging the economic motivation of much human behavior doesn’t really undermine the selfish gene perspective, since it’s necessary to live in order to have children. Living well (having lots of economic resources and power) allows one to have more children than one would otherwise. So, from a selfish-gene perspective, economic strategies can be viewed as reproductive strategies, just less direct ones.

It’s true, as Eldredge stresses, that rich people today don’t have more children than poor people, they have fewer children, which on the face of it indicates that they are not engaged in fundamentally reproductive behavior, they’re simply living as well as they can. But Eldredge’s mistake is to assume that evolution must give people the conscious urge to have children in order to get them to engage in reproductive behavior. Of course, that’s not true. Evolution didn’t even bother to tell people that sex causes pregnancy, a prerequisite for that argument to work. Yes, nearly everyone understands that causal connection today, but the farther we go back in human evolution the less likely it is that people grasped it.

What evolution did do is give people the desire and urge to engage in sexual contacts, the most desirable form of which (for most but not all people) is coitus—heterosexual penile-vaginal intercourse—with gloriously fertile partners. If people engage in this behavior the raw biology—ejaculation, sperm migration, fertilization, implantation, and pregnancy—will take over from there, with no further “urges” required. Babies will be born, and there are then other mechanisms, especially parent-child bonding, that take over to help the babies survive. If people happen to realize the causal connection between coitus and pregnancy, and develop the skills (coitus interruptus, rhythm method) or technology (condoms, pills) to engage in the former without triggering the latter, then they can and do happily subvert the original adaptive purpose of sex, getting the reward without paying the price.

Economic considerations are often relevant to sex and reproduction, of course.[xxiii] There is an aspect of quite a few sexual relationships that has the quality of a “deal”—my beauty for your wealth, for example. And when industrialization reduces the economic value of children, couples have fewer of them—that is the “demographic transition” that is reining in population growth worldwide. But, contrary to a purely economic theory of sexuality, they don’t have fewer by engaging in less sex.

What does evolutionary psychology have to say about that ultimate meeting-point of sexuality and aggression—rape? Randy Thornhill (of the University of New Mexico) and Craig Palmer (of the University of Colorado) addressed this issue in a controversial 2000 book.[xxiv] Thornhill’s own prior research was on mating behavior in insects—specifically, scorpionflies. He showed that forced copulation is an adaptive strategy used by male flies—they even have a special organ that is used only to grasp the female while “raping” her. However, males only engage in forced copulation when their preferred strategy—luring the female into voluntary copulation by means of a nuptial gift—is not possible because they can’t find a gift. Provide such a male with a suitable gift and he will immediately switch strategies.

Thornhill and Palmer considered whether something similar might be true for humans. They cite research by UCLA’s Neil Malamuth, which suggests that men who lack resources, are socially disenfranchised, or are unable to gain access to sex partners for a variety of reasons, are more likely than other men to adopt coercive sexual strategies. These conditions are particularly effective triggering factors when they operate early in life.[xxv] (It’s interesting in this context that Genghis Kahn grew up in conditions of considerable deprivation, after the early death of his father.)

Although Thornhill and Palmer see parallels between forced copulation in insects and rape in humans, they do not conclude that rape is necessarily an evolutionary adaptation—even though almost everyone who has reviewed or commented on the book, including Eldredge, state that they do draw that conclusion. Rather, they say that the evidence presently available doesn’t allow one to decide whether rape is an adaptation or whether it’s a mere byproduct of selection for some other trait (in the same way as the redness of blood is a by-product of selection for blood’s oxygen-carrying capacity). What Thornhill and Palmer do conclude is that, whatever the exact mechanism, the human capacity for rape has been shaped by evolutionary processes, and is not simply some cultural phenomenon that can only be understood in terms of the dynamics of contemporary society.

It’s worth staying on this topic for a moment longer because rape and other forms of coercive sexuality don’t just cause grave harm to individual victims, they also terrorize communities and even nations (as, for example when rape occurs during a military invasion). One attempt to mathematically model the evolutionary costs and benefits of rape (in an American Indian population in Paraguay) came to the conclusion that the reproductive benefits of rape to the rapist are outweighed by the reproductive costs—in other words, he would leave more offspring, on average, if he refrained from rape.[xxvi] The main reason for this is that, in the researchers’ model, the rapists commit their crimes at random times in the victim’s menstrual cycle. Coitus must occur during the six-day period leading up to and including the day of ovulation to have a significant chance of causing pregnancy, so a randomly-timed rape is not likely to do so. This in turn suggests another intriguing possibility: the fact that women, unlike most other mammals, do not announce by any easily recognizable signal when they are in the fertile phase of the cycle may itself be an adaptive response to the threat of rape, because it makes rape less profitable to the rapist.

In other words, evolution doesn’t necessarily cause all members of a species to act nicely to each other. Rather, it can lead to intense conflicts, including conflicts between the interests of men and women. The rejection or even “honor killing” of rape victims by their husbands or families, for example—morally reprehensible though it seems to us—has a horrible logic in evolutionary terms. If Genghis Khan had rejected or killed his wife after her involuntary stay with Chilger Bote—actions that would assuredly have been acceptable by the standards of the time—his Y chromosome might be even more widespread today than it actually is.

The kind of aggression that most concerns me is mass, collective aggression—war between nations, or large-scale civil conflicts. And here we must ask: are these mass conflicts simply the exhibition of individual aggressiveness, scaled up to an enormous degree by virtue of the numbers of people involved and by modern military technology, or are they something entirely different, with causes unrelated to the causes of street-corner brawls, sexual assault, and domestic violence? My answer—with qualifications—will be that they are the same thing.

When individuals join large groups for some aggressive purpose—whether rioting mobs or disciplined armies—crowd psychology takes over. According to Elias Canetti, the act of joining a crowd involves, more than anything else, a casting aside of the fear of being touched.[xxvii] Personal space vanishes, physically and emotionally. Crowds are dense, equal, desirous of growth, and in movement toward some goal—often an aggressive one. According to Sigmund Freud, joining a crowd signifies the loss of the psychological structures that repress basic instincts: the crowd does have any fundamentally new mental characteristics, but liberates and exaggerates the traits already existing in its members.[xxviii]

The aggressiveness of crowds, says Canetti, flows from a deep individual human need. “The threat of death hangs over all men and, however disguised it may be, and even if it is sometimes forgotten, it affects them all the time and creates in them a need to deflect death on to others.”[xxix] Aggressive crowds—from lynch mobs to armies—allow them to do this.

The crowd’s victim or enemy—the “death-conductor”—is judged to be a legitimate target of violence because of his guilty acts or thoughts. “It is always the enemy who started it,” writes Canetti. “Even if he was not the first to speak out, he was certainly planning it; and if he was not actually planning it, he was thinking of it; and, if he was not thinking of it, he would have thought of it.”[xxx]

A survivor of a battle feels feel superior to the dead, according to Canetti, because he has deflected his own death onto them. The experience, especially when repeated, gives him a sense of invulnerability and immortality—the characteristics of “heroes.” Canetti cites an early experience of Genghis Khan—he was waylaid by six bandits but managed to kill them all without sustaining any injuries—as imbuing him with the sense of invulnerability and certain victory that was the prerequisite for his later career.

Of course, not everyone can be an emperor or general—there have to be those whose only role is to be commanded. Why do they participate, if they are merely ordered around and expended as cannon fodder? Canetti attributes the extraordinary discipline of the Mongol armies to the fact that even the lowliest warrior had subordinates who he in turn commanded—his horses. According to Freud, on the other hand, the followers in a group follow because they identify with the leader, who exerts a force over them resembling hypnosis. At a more practical level, the promise of material and human booty has often been a powerful incentive.

While I agree with Canetti that the roots of group violence lie within the individual psyche, there are problems with accepting his specific ideas. For one thing, they are based more on assertion more than actual evidence. Also, they could only apply to humans, the sole species (so far as we know) in which individuals have conscious knowledge of their own mortality, a key ingredient in Canetti’s model. Yet there is increasing evidence that human aggression, including forms of group aggression such as warfare, has evolutionary roots that predate the origin of our own species.

Ethologist Konrad Lorenz, nearly half a century ago, placed the human aggressive instinct firmly in the context of animal aggression.[xxxi] More recently, Harvard primatologist and anthropologist Richard Wrangham picked up the same theme in a book with the provocative title Demonic Males.[xxxii] From his own field observations, Wrangham describes how gangs of male chimpanzees engage in lethal conflict with other males, often over territory, and also rape females. Some critics have suggested that Wrangham’s observations were atypical, but similar behavior has been observed and filmed by Yale primatologist David Watts. Here’s how Watts described it:

[W]e might be following a group of males, and they will switch into what we call patrol mode. They'll go silent, which is unusual for chimps, and just look and listen. When they hear neighboring chimps, they respond in a pretty predictable way. If there are just a few chimps in the group, for instance, they'll quietly move back toward the center of their own territory. If it's a big group, they'll respond vocally and listen to the responses. If they decide they are evenly matched, that can lead to major aggression. They'll chase down, surround, and attack rivals. Sometimes they kill them.[xxxiii]

Wrangham believes that this kind of behavior was already in the repertoire of the common ancestors of chimpanzees and humans, and that it is has persisted through five million years of human evolution to the present day, forming the instinctual basis for much of the inter-group aggression, including warfare, that bedevils modern societies.

We can’t travel back in time to observe the behavior of our human and pre-human ancestors, but less direct evidence supports the idea that human warfare has ancient roots. Archeologist Steven LeBlanc catalogs the evidence for “constant battles” in human prehistory, battles that were often triggered by resource depletion.[xxxiv] Anthropologist Lawrence Keeley describes the prevalence of inter-group violence in surviving non-literate cultures before they had extensive contact with westerners.[xxxv] These include the tribes of the Papua New Guinea highlands, who engaged in frequent wars with neighboring communities. The conflicts among one group of tribes, the Enga, were also described in horrific detail by Mervyn Meggitt.[xxxvi] These wars, which typically lasted a few days or weeks, were sometimes precipitated by disputes over territory or animals, but they could also involve unprovoked raids on neighboring villages. In the Amazon rainforest, the wars of the Yanomami have been described by Napoleon Chagnon[xxxvii] and others. More than one-third of all Yanomami males died violently, often while participating in raiding parties. In Australia, cave paintings left by aborigines document pitched battles fought with spears and boomerangs: these images reach back thousands of years into the past.[xxxviii]

There are anthropologists who take a dissenting view. Douglas Fry, for example, maintains that most hunter-gatherer societies—the only form of human society through most of our prehistory—rarely engaged in intergroup conflicts and solved disputes fairly amicably.[xxxix] War, Fry maintains, only emerged with the appearance of much larger social entities—states. Fry’s view conflicts with most field observations, however: his assertion that war was rare among the New Guinea highlanders and Australian aborigines, for example, is contradicted by other studies as mentioned above. Fry removes war from the province of early human societies simply by defining war as being organized on a scale that was beyond their capacity—he refers to smaller conflicts as “feuds.” Of course, people living in tiny groups fought tiny wars, but the percentage of the population who died in them was often far greater than in our own battle-scarred age.

The evidence—admittedly incomplete—suggests that war or war-like violence, perpetrated almost entirely by males—has been an age-long characteristic of Homo sapiens and its human and non-human forebears, and that it is a channeling of the male propensity for violence to serve the interests, not only of the warriors themselves, but also of the larger communities in which they live.

I’ve cited evidence that women have less of a propensity to physical aggression, and I believe that this is the reason that women have been much less frequent participants in war than males. Not everyone agrees. David Adams, a psychologist who organized UNESCO’s International Year for the Culture of Peace (the year 2000) argued that women were always eager to go a-soldiering, but were prevented by their menfolk, who feared that they would defect to the enemy.[xl] The supposed reason for this fear was that, in so-called “exogamic” cultures, women married outside of their own communities; thus, if their birth community and their marriage community came into conflict they might experience divided loyalty and perhaps even support the enemy. According to this idea, however, female engagement in war would have become commonplace as soon as human communities became states, because then most marriages were within the state and issues of divided loyalty didn’t arise. Yet that didn’t happen.

Nations are not constantly at war. Even if one includes as “war” the periods of recuperation from the last war, preparation for the next, and the provision of support for other nations’ wars, there are still times and places where life proceeds placidly and war seems to be on nobody’s mind. Thus, we need more than a global statement about male psychology to explain war—we need reasons why wars break out.

Many reasons have been proposed, and doubtless most or all of them have operated at one time or another. Conflicts over territory are probably the number one cause, just as they were for our hunter-gatherer ancestors. These conflicts may be exacerbated by population growth (Malthusian catastrophes), degradation of land by human activity, sea-level rise, climate change, and migrations. Conflicts over other economic resources, such as water, minerals, or trading rights, have been common. Conflicting religions or ideologies have precipitated many wars, or have been used to justify wars whose real causes were more practical. Conflicts over women have started their share of wars—most famously the Trojan War, which tradition holds was fought over Helen of Troy, but also lesser episodes such as Genghis Khan’s campaign to recover his wife from the Merkits. Wars may be sparked by the rise to power of especially charismatic or even mentally unbalanced leaders. Wars may be promoted by those who stand to profit from them, such as weapons manufacturers. Or they may be started as a means to divert the attention of a nation from internal problems. Wars may begin almost unintentionally, as when leaders forget the value of allowing their enemies to save face.

Wars begin, more than anything, because those who start them figure that they are going to be victorious. Small communities or nations that are surrounded by larger or more powerful neighbors rarely initiate conflicts, because doing so would be tantamount to suicide. Germany invades Belgium, not vice versa. The “peaceful” hunter-gatherer communities cited by Fry—those that were actually peaceful, at least—fell into this “little-guy” category.

The fact that war can be triggered by so many causes is important, because many of these causes are ones that can be eliminated or mitigated, in theory at least, and I’ll discuss strategies to do so later. At the same time, the wide variety of “triggers” suggests that they are likely to be underlain by a deeper common cause. This, I believe, is the biopsychological male predisposition to physical aggression.

One theory of war that draws a connection between the biology and social circumstances is the so-called “youth bulge” hypothesis, espoused particularly by Gunnar Heinsohn of the University of Bremen in Germany.[xli] According to Heinsohn, the likelihood of war (as well as internal violence and criminality) is high when the proportion of young (15- to 29-year-old) men in the population increases relative to men in older age brackets—especially when this increase continues over more than one generation. It is not just the sheer numbers of young men, with their capacity for aggressive behavior, that is responsible. A more significant factor is the pyramidal age structure of these societies, which deprives young men of the opportunities and wealth that they have come to expect because they were available to their fathers. Several generations of youth-bulge conditions existed in Europe during the recovery from the Black Death and other fourteenth-century disasters. This youth bulge, says Heinsohn, was the trigger for the wars of colonial conquest that ended up subjugating more than a quarter of the world’s population. Today, youth-bulge conditions exist throughout the Islamic world and other violence-prone regions, whereas continental Europe, with its current antipathy to war, has the opposite, age-dominated population structure.

Another factor that can exacerbate the youth-bulge effect is the lack of sexual outlet for young men, which Heinsohn believes acts as a stressor predisposing them to violence (including sexual violence) and militarism. The lack of outlet can exist because the men have too few resources to be able to marry, or (as is true in some Islamic countries) unmarried women are sequestered and largely unavailable for premarital relationships. China, which does not have a pronounced youth bulge, nevertheless has a looming problem with the availability of sexual outlet for young men, due to the widespread abortion of female fetuses that is leaving China with a shortage of young women. (The only good thing that one can say about this situation is that it is likely to increase women’s worth in Chinese society.)

There has long been a sense that a war should be “just” for it to be morally acceptable, but what constitutes a “just war” has changed over time and still provokes a lot of disagreement. Back in 1415, at the height of England’s youth bulge, King Henry V contemplated the invasion of France, and sought the opinion of the country’s leading moral authority, the Archbishop of Canterbury. The cleric spurred him on and even raised funds to support the campaign. As justification he merely cited centuries-old irregularities in the chain of succession to the French throne. (This if we are to believe Shakespeare’s account in Henry V). In 2002, as Tony Blair considered whether to join the U.S. in an invasion of Iraq, the current Archbishop of Canterbury spoke up against military action, citing the likely human suffering and the dehumanizing effects of the war on those who fought it.[xlii]

In the United States, leading Christian Conservatives sent a letter (the so-called Land letter of October 3, 2003[xliii]) to President Bush, urging him to begin the invasion. The writers, who included the President of the Ethics Commission of the Southern Baptist Convention, wrote that invading Iraq would fulfill the generally accepted criteria for a just war: it was a defensive action, it had just intent, it was an action of last resort, it was legitimately authorized, it had limited and attainable goals, non-combatants would suffer minimal harm, and the action was proportionate to its stated goals. “We believe that every day of delay significantly increases the risk of far greater human suffering,” the clerics wrote.

At the same time, the President of the Catholic U.S. bishops’ conference also wrote to President Bush, urging him not to order the invasion. The bishops discussed the very same moral criteria raised by the authors of the Land letter, but argued that an invasion of Iraq would not satisfy any of them.[xliv] I am left wondering how much the conclusions of these two ecclesiastical groups resulted from unbiased consideration of the moral issues involved, and how much they represented attempts to justify points of view whose roots lay elsewhere.

Earlier, I mentioned the idea that there are profound differences between the moral instincts of men and women. To the extent that these differences exist, one would expect that women would be more opposed to war than men, and would use whatever influence they have to prevent the outbreak of war or to bring war to a halt. A famous fictional example of such an effort was the sex strike imposed by the women of ancient Athens, and by those of its enemy cities, in an effort to halt the Peloponnesian war—the story line of Aristophanes’s comedy, Lysistrata. A more recent and real-life effort to bring warring menfolk to their senses was the grass-roots campaign by housewives Betty Williams and Mairead Corrigan to end the sectarian violence in Northern Ireland, a campaign that earned them the 1976 Nobel Peace Prize.

Nowadays, women’s influence has spread far beyond the bedroom, but this process of liberation has varied greatly from country to country. Some countries are approaching gender equality, while in others women are as heavily repressed as they ever were. If women want peace more than men do, one would predict that countries that have attained a high degree of gender equality would have less aggressive foreign policies than those in which women are still dominated by men. One study, by Mary Caprioli of the University of Tennessee, found exactly that result: in the period from 1978 to 1992, 141 states were involved in international conflicts, but states with gender equality were less likely to have initiated such conflicts than states where women were markedly less powerful than men.[xlv]

Although I like Caprioli’s findings and hope they are correct, there is some reason for caution. For one thing, Caprioli used as one of her two measures of a state’s gender equality the average number of children women have (on the assumption that there is less equality when women have many children.) But countries where women have many children are also often the countries with pronounced “youth bulges.” Thus the militarism of those countries could result from their youth bulges, as discussed earlier, rather from any benign influence of women’s values on a state’s international policies.

Also, the well-established gender difference in physical aggression relates mainly to aggression by the very persons who are studied or interviewed. But women may be far more permissive, even admiring, of aggression in men than they are in themselves or other women. Women tend to like “strong” political leaders in the much same way as men do. Over the last seven U.S. presidential elections, women have consistently voted to the left of men, but the differences have been fairly small—in the range of five to ten percentage points.[xlvi] The twentieth century saw women’s suffrage sweep the globe, but that dramatic change hasn’t rung in an era of international peace, so far at least. A more obvious effect has been on domestic policies relating to rape, domestic violence, and other forms of physical and economic victimization of women, where the last century saw positive changes in many countries.

What is the impact of human aggression, especially in its large-scale, organized form? I don’t need to review the direct human costs of war, of course—the bloody slaughter at Merv, mentioned earlier, can stand in for all of that. But we should consider the impact of war on the world we inhabit, for these environmental effects are not so widely appreciated.

Deliberate environmental destruction has long been a tool of warfare. The Assyrians, after quelling a rebellion by the Hurrites, sowed salt over the ruins of a Hurrite city, thus ensuring that crops would not grow there. Fast forward three thousand years, and we witness the application of modern technology to the same end. During the Vietnam War for example, it was U.S. policy to render much of rural South Vietnam uninhabitable, thus depriving the Viet Cong of its support among the peasantry. To achieve that purpose, U.S. forces dropped millions of tons of bombs and hundreds of thousands of tons of herbicides and defoliants. To complete the job on the ground, lines of bulldozers connected by heavy chains destroyed every tree or plant in their paths, even ripping up the soil itself. Since that time, there have been considerable efforts at remediation, but the massive erosion triggered by the loss of plant cover has left great swathes of southern Vietnam as barren wastelands—zones that the Vietnamese call “Agent Orange Museums.”

Military action is not the whole story of the environmental degradation caused by war: tremendous damage is also done by the preparation for war. The construction of HMS Victory, Lord Nelson’s flagship at the battle of Trafalgar, required the felling of 2,000 oak trees—and the Victory was but one of thousands of Royal Navy ships that consumed timber on that scale.[xlvii] Warship construction stripped Britain of immense tracts of mature trees, as it has done in many other parts of the world. And in the aftermath of wars, the demands of reconstruction and the needs of refugees lead to further loss of trees. In Vietnam, for example, reconstruction essentially finished off the job of deforestation that had been begun by the Americans. When we add in land rendered unusable by the testing or storage of munitions (including nuclear weapons), by landmines and unexploded ordnance, by contamination with rocket fuels and depleted uranium, by deliberate alteration of the ecology (such as Saddam Hussein’s draining of the marshes of southern Iraq in his campaign against the Marsh Arabs), and by the firing of oil wells (another of Hussein’s strategies), we begin to appreciate the magnitude of war’s ecological cost to humans. The damage is even spreading beyond Earth: both the U.S. and China have tested anti-satellite technology by knocking out their own satellites; the resulting debris clouds, containing hundreds of thousands of particles and larger objects, pose a long term threat to other satellites.[xlviii]

For the non-human inhabitants of Earth the costs of war are sometimes even graver, for war involves the direct destruction of unimaginable numbers of animals and plants (including those domesticated species, horses especially, that have participated in battles), the loss of their habitats and the disruption of critical conservation efforts. Among the iconic species whose survival is threatened by wars now underway are the snow leopard and the Siberian crane—both as a result of the war in Afghanistan. And finally, war destroys the spirit of trust and cooperation between nations that is essential for meeting the global environmental challenges that we face.

So, war is a bad thing—or is it? Not that I feel compelled to mull over outmoded militaristic doctrines, such as the idea that war has a purifying or rejuvenating effect on nations. More realistically, however, it is doubtless true that there have been long-term benefits of some wars, particularly the successful wars of conquest that melded tribes into city-states and city-states into nations. Civilization as we know it—the arts and sciences, engineering, trade, discovery, organized religion, medicine, and most of the forms of social intercourse that we value—this cultural flowering owes its existence in large measure to the forced merging of smaller social units into larger ones.

Also, there have been plenty of wars that have been initiated with the purpose of correcting some egregious wrong. The wars of colonial liberation, including the one that freed Britain’s American colonies, surely fall into this category. And the vagaries of history have often left us with national boundaries that make no sense and deprive certain peoples of unity or self-government. The Kurds, for example, form an ethnically and culturally coherent people who occupy about 390,000 square kilometers of the Middle East. But this territory (“Kurdistan”) is divided up among six different nations (Turkey, Armenia, Azerbaijan, Iraq, Iran, and Syria), few of whom treat their Kurds as first-class citizens. In these and other ways, there is the possibility that abolishing war—if such a feat be possible—would perpetuate wrongs by freezing them in place.

At a deeper level, one can ask whether the physically and sexually aggressive impulses of men, which I maintain are key causal factors underlying war, are wholly bad things. To Freud, the sexual instinct (“libido”) provided the fuel for every kind of cultural achievement, by a process of energy diversion that he called “sublimation.”[xlix] More recently, Camille Paglia asserted that men’s difficult-to-control sexual urges are an indivisible part of the masculine projective quality that has created civilization as we know it, an idea that she encapsulated in two infamous aphorisms: “There is no female Mozart because there is no female Jack the Ripper,” and “If civilization had been left in female hands, we would still be living in grass huts.”[l] If there is truth to these ideas, the harm caused by men’s sexually charged aggressiveness might simply be the price we have to pay for everything we value about human culture.

Thus strategies to limit or prevent war must not merely work—an awesome challenge it itself—but they must do so without perpetuating injustice and without destroying the positive benefits that the deeper causes of war may bring with them. How could we hope to achieve these goals? Are they achievable at all? Given the terrible costs even of conventional war, and the danger that nuclear war poses to civilization itself, humanity has the deepest obligation to search for answers to these questions.

Efforts to prevent war can be undertaken at many different levels. At the top level, there is the obvious fact that international relations exist in a largely anarchic environment. Every effort to strengthen the United Nations, the force of treaties, and respect for international law is an effort to strengthen peace.

Democracies rarely wage war against each other, perhaps because democracy inculcates a culture of dialogue and compromise. Thus, as argued by R.J. Rummel (a professor of political science retired from the University of Hawaii), encouraging the spread of democracy may lessen the likelihood of war.[li] (Nevertheless, democracies are fairly willing to go to war with non-democratic states.) Encouraging women’s rights worldwide may also help prevent war, as discussed earlier.

The details of military arrangements can make war less (or more) likely.[lii] For example, nations may be tempted into a first strike if they are uncertain of their enemy’s military capacity or if they believe that a first strike will be quickly victorious. Thus military arrangements that make information about each side’s forces available to the other, or that ensure the capacity for a second strike, tend to lessen the likelihood that war will break out.

Encouraging an improvement in economic conditions in underdeveloped countries may lessen the likelihood of war, both by directly lessening social discontent and also by speeding the demographic transition, which eventually terminates youth bulges. Encouraging population control measures may have the same effect.

Encouraging international trade may knit nations together in ways that make war unprofitable. This can work in the opposite direction, however, because nations that import key commodities, such as oil, may feel and actually become less secure as a result. Japan’s entry into World War II in 1941, for example, was motivated in large part by the fact that its oil imports had been cut off by an international boycott.

Cultural exchange is often touted as a force for peace, and it may often be so. Still, the spread of western culture has triggered violent antipathy among some more traditionally-minded populations, thus making conflict more likely, not less. And sometimes partition may be a more effective solution to ethnic strife than integration.

Efforts to reduce the power of the military-industrial complex could lessen the pressure for war. This is particularly true in the United States, whose annual military budget of $550 billion (in 2006) nearly matches those of all other nations combined.[liii] Unfortunately, the efforts of some U.S. presidents, such as Jimmy Carter, to rein in the military-industrial complex have been negated by others, such Carter’s successor Ronald Reagan, who have fed its growth.

Worthwhile though all these approaches may be, I believe that their effectiveness will be limited if attention is not paid to the underlying engine that drives nations to war—men’s biological propensity for physical aggression.

There is a widespread misperception that acknowledging the role of biological predispositions to violence in the causation of war is tantamount to saying that war can never be prevented, because biological attributes are immutable. This was the perspective of David Adams, for example—the psychologist whom I mentioned earlier as not believing in a difference in men and women’s enthusiasm for warfare. In an article titled “There is no instinct for war,” Adam summarized his views as follows: “[T]here is considerable evidence that warfare is a cultural institution with an ancient history and close relations to other cultural institutions, e.g., marriage systems. Therefore, we can encourage the millions of people around the world who are engaged in the struggle to abolish war and militarism. We can say to them that there is nothing that we know in biology that stands in the way of the abolition of war.”[liv]

The fact that war has been with us for so long suggests that it is not going to be easy to abolish, whatever its causes. But I disagree that biological causes are inherently more resistant to mitigation than cultural ones. What matters is that strategies for peace address the real causes, whatever they may be.

First, I emphasize what I’ve already stated: what biology provides is a predisposition for physically aggressive behavior by men, a predisposition whose actual behavioral expression is promoted or restrained by a variety of circumstances and cultural factors. I am not preaching the biological inevitability of war, but I am saying that the biology creates a key underlying causal factor.

One much discussed approach to the problem of male violence involves diverting it into less harmful channels. This strategy has ancient roots, to judge by the behavior of some preliterate societies. The Enga of the New Guinea highlands, already mentioned on account of their fondness for lethal raiding, also often engage in ritualized “battles” in which the males of opposing clans form two lines and charge at each other, usually without causing injuries. Only occasionally—when one side is much stronger than the other—do these events proceed to lethal violence.

Many contact sports have the flavor of ritualized physical aggression. Thus Konrad Lorenz, among many others, suggested that engaging in or watching such sports could have a cathartic effect, allowing aggressive urges to be vented and thus reducing the likelihood that they would erupt later in some more harmful fashion. This effect might be similar to the satiating effect of eating on hunger or of sexual intercourse on “horniness.”

But this widespread and seemingly logical belief is not supported by research: most such studies have reported that participation in violent sporting activities does nothing to lessen the participants’ aggressiveness, and that watching violent sports actually increases it.[lv] Similarly, playing violent video games increases aggressiveness, and watching violent pornography increases men’s willingness to harm women.[lvi]

Why there is no cathartic effect of involvement in sports or playing violent video games is not entirely clear. However, if the aggressive drive seeks the actual killing of others as its endpoint, as much of the anthropological research suggests, then anything less than that might have little cathartic effect, just as sexual activity that stops short of orgasm is unlikely to reduce a man’s horniness. In addition, there is probably a desensitizing effect of violence, making those who engage in it less likely to experience revulsion when exposed to it in the future, and therefore more willing to engage in it.[lvii] This effect is likely to overwhelm any “satiating” or “diversionary” effect. Whatever the exact reason, it seems that providing a non-lethal outlet for men’s aggressive feelings is an ineffective strategy for reducing physical aggression of any kind, including war.

Maybe catharsis would work better if it diverted aggressive energy at a deeper level or earlier in life? Lee Ellis, of Minot State University, has suggested that all males have a biological predisposition to “competitive/victimizing” behavior, but that at adolescence more intelligent boys learn that physical victimization is unrewarding (because of the risk of punishment, mainly), and they therefore divert this energy into other forms of competitive/victimizing behavior that are socially approved, such as making a career in business.[lviii]

Consistent with this model are the findings of a prospective study done in England: boys’ intelligence at age 8-10 was a good predictor of delinquency and criminality, including violent criminality, later in life (low intelligence predicted high criminality).[lix] Interestingly, marriage tended to terminate criminal behavior even in the less intelligent subjects. This may in part be because marriage, and other stable sexual relationships, dramatically lower a man’s testosterone levels.[lx]

There are doubtless many ways to encourage the development of life paths that allow male aggressive energy to be expressed in more positive forms than physical violence against others. These could include the provision of better prenatal care and nutrition, especially in high-risk populations; the protection of children from physical and sexual abuse; improvements in economic conditions, educational quality, and policing in deprived areas; alternatives to gang membership; better role modeling; the provision of rewarding career opportunities; the reduction of violence on television, in video games, at sports arenas, and in popular music; and so on. I support all such efforts—but they don’t cut to the heart of the matter.

The heart of the matter is the need to change male biology in a way that lessens the male proclivity for violence without lessening the capacity to lead energetic, productive, and happy lives. A challenging experiment, certainly, but one that has already been completed successfully.

That experiment took place in Africa, within the giant northward-arcing loop of the Congo River, starting 2 or 3 million years ago. At that time the ancestors of chimpanzees—probably very similar to today’s chimpanzees in appearance and behavior—inhabited the rainforest that lay in a vast swathe across equatorial Africa. These proto-chimpanzees shared their range with gorillas, but within the loop of the Congo the gorillas eventually disappeared, probably as a result of a drying episode that deprived them of the rain-forest herbage that is their main diet. Once gone, they could not return when the climate became wet again, because the river presented too wide a barrier. Freed of competition, the proto-chimpanzees remaining in this zone expanded their diet to include the types of food that the gorillas had formerly consumed. This new abundance allowed the proto-chimpanzees to associate in larger groups than formerly—groups in which females could form alliances to successfully resist the dominance of males. Over the millennia, this new social reality led to genetic changes that affected the animals’ psychology much more dramatically than their anatomy. They became a new species of “peaceful chimpanzees”—the bonobos, who now form a separate species from the regular chimpanzees found elsewhere in Africa.

This is Richard Wrangham’s fairly speculative reconstruction of how bonobos evolved from their chimpanzee-like ancestors. Whether the story is correct in its details or not, one main point seems clear: a sub-population of apes with the aggressive male traits characteristic of chimpanzees and humans went through a rapid process of evolutionary change, in which this trait was largely lost. Bonobos don’t engage in the raiding parties that cause so many deaths among chimpanzees. In fact, bonobos rarely resolve conflicts through physical aggression. That’s both because the males themselves are relatively unaggressive, and because any aggression they do show is generally restrained by alliances among females. Interestingly, the bonobos’ unbelligerent nature also affects their relationship with other primates: unlike chimpanzees, bonobos don’t hunt monkeys, even though they are physically capable of doing so.

Instead of force, bonobos use sex to cement relationships and to settle disputes.[lxi] As many observers have commented, their slogan seems to be “make love, not war.” It may happen, for example, that one bonobo finds another in possession of a prime feeding site—a classic trigger for aggression among chimpanzees. But in this kind of situation the two animals often engage in sexual contact and then share the food resource amicably. Such conflict-resolving sexual contacts occur between males, between females, between males and females, and between adults and juveniles.

This extraordinary change in the psychological makeup of bonobos is not a cultural phenomenon; it is genetic. Something happened in the bonobo genome over the last 2 or 3 million years that greatly weakened the predisposition of males to engage in physical aggression against other bonobos (and monkeys).

We don’t know what that change was, but we can certainly find out. The chimpanzee genome has already been decoded. Decoding the bonobo genome will probably be undertaken in the next decade or two, because the value of being able to identify the genetic differences between the two species (and between those two species and humans) is well appreciated.[lxii]

Until not long ago, most scientists would have considered it impossible to identify the DNA changes responsible for the peaceable nature of the bonobos, even with their genomes in the bank. That was because behavioral traits such as aggressiveness were thought to be regulated by vast networks of genes, any one of which had effects too subtle and too interactive to be analyzed. But now the pendulum is swinging the other way. Earlier, I mentioned the evidence that variations in just one gene, MAOA, can profoundly affect the likelihood that a man will exhibit violent behavior, including the likelihood that environmental factors such as abuse during childhood will drive him to antisocial or aggressive behavior in adult life. It would be a fairly simple matter to compare this gene, and the way that it is regulated, in the two species. Other genes will doubtless also be identified that could be important in creating the difference in the two species’ aggressiveness. Sooner or later, the difference will be fully understood in biological terms.

At the same time, progress will be made in understanding the genetic basis of aggressiveness in humans, as well as the neural circuitry and hormonal environment that also affect this trait. Putting what we learn about our own biology together with what we learn about the transformation from proto-chimpanzees to bonobo will tell us how to transform humans—and especially human males—into more peace-loving creatures.

How could we put such knowledge into practice? Let’s take the MAOA gene as an example. (And it is only an example, chosen because it’s a gene whose link to aggression happens to be already known.) In the Dutch family that I mentioned earlier, the defective MAOA gene has caused absolute havoc, not only on account of the aggressive acts perpetrated by the affected males, but also because the gene has another effect: it reduces their intelligence to an IQ of about 85—low enough to markedly affect their educability. On both accounts, then, the genetic defect is something that family members would like to be rid of.

What’s more, the MAOA gene lies on the X chromosome, which means that some female family members, though unaffected themselves, are carriers who can pass the trait on to their male children. This, according to news reports, has led to understandable anxiety among the women who fear that they may give birth to an affected child.[lxiii] (The confidentiality of the family’s identity has been maintained, so it’s not known whether these women have been offered prenatal diagnosis, now that the gene has been identified.)

In this atmosphere of distress and anxiety, a medical intervention to replace the non-functional MAOA gene with its normal counterpart would surely find acceptance. To perform the replacement the normal gene would be carried into the cells of the affected men (or children, or embryos) by a genetically engineered virus or some other vector. If this led to the successful correction of the individuals’ psychological problems, or to the birth of a child who was spared ever having to experience them, this would be seen as a major medical success.

The next step might be to offer the treatment to persons with the milder mutation of the MAOA gene—the one that increases a man’s likelihood of engaging in antisocial or violent behavior only if he has also experienced childhood abuse. Perhaps at first it would be offered only to those affected individuals who have actually experienced abuse. Then, if the results were good and no negative side effects became apparent, it might be offered prophylactically to all boys with this mutation.

The question might then arise, as to whether it would be possible to engineer a novel version of the MAOA gene that had the opposite effect from the defective gene, increasing the metabolism of serotonin, norepinephrine, and dopamine to levels higher than seen normally. Would this also have the opposite psychological effect, decreasing the person’s predisposition toward violence? At present, of course, we don’t know whether this would happen or not, but at some point in the future the genomic research on chimpanzees and bonobos may already have hinted at the answer. Even if this particular gene could not be manipulated in the direction of lowering the “normal” human propensity for aggression, other genes will be found that can be engineered to have this effect.

So, at some point down the road, I believe that it will be possible to prevent violence by medical means—and by means that do not turn men into eunuchs or impair their initiative, productivity, or social skills. But when this technology becomes available, there would still remain the political and social challenge of enacting a program to genetically alter the entire human race.

At first blush it seems inconceivable that such a program could ever be enacted. Not only because of the sheer scale of the operation, but also because people might well worry about the effects of the treatment. In particular, they might be concerned that any humans who remained untreated would be in a position to take over the world from the non-belligerent majority.

With regard to the scale, that’s not really an issue. The global campaign to eradicate polio—now tantalizingly close to completion—has involved the immunization of 2 billion children worldwide in a period of twenty years. The campaign has cost only $3 billion—an amount equivalent to a mere 23 hours of global military expenditure.[lxiv] Similar international immunization campaigns have been undertaken to combat smallpox and many other diseases. Of course I am ignoring the possible extra cost of a genetically engineered vector as compared with a vaccine, but I anticipate that most of that extra cost would lie in development of the vector, so that the marginal cost of administration would be low.

With regard to acceptance, that could be a more challenging problem. However, it will be decades before the technology is ready. By that time, I expect that genetic engineering and gene therapy will be far more widely practiced and widely accepted than they are now. The idea will not be as shocking as it is to current generations.

In addition, the treatment will in all likelihood be made available to young children rather than adults. It may be included as part of a package of prophylactic treatments that will be routinely administered to newborns. Parents will be far less concerned about preventing their children from become aggressive adults than they would about undergoing similar changes themselves.

The idea that a small population of untreated individuals would dominate the world is unreasonable. The treatment would not render people passive or lacking in drive or the ability to face up to threats. Not even a Genghis Kahn could do his evil work without the support of the people. A culture would emerge in which violent behaviors would simply not be accepted.

All this is a lot more sketchy and speculative than I am comfortable with. I cannot predict exactly what technology would emerge or how people would choose to use it. What I do know is that there is little future for humankind if we do not face up to and deal with our biological predisposition for violence. This trait evolved because it helped men reproduce, but now it can only help us destroy ourselves. The question is whether we choose to do something about it before war brings civilization itself to an end.

[i] Zerjal, T., Xue, Y., Bertorelle, G., Wells, R. S., Bao, W., Zhu, S., Qamar, R., Ayub, Q., Mohyuddin, A., Fu, S., Li, P., Yuldasheva, N., Ruzibakiev, R., Xu, J., Shu, Q., Du, R., Yang, H., Hurles, M. E., Robinson, E., Gerelsaikhan, T., Dashnyam, B., Mehdi, S. Q. & Tyler-Smith, C. (2003). The genetic legacy of the Mongols. Am J Hum Genet. 72, 717-721.

[ii] John Man (2004). Genghis Khan: Life, Death, and Resurrection. Thomas Dunne Books.

[iii] Derenko, M. V., Malyarchuk, B. A., Wozniak, M., Denisova, G. A., Dambueva, I. K., Dorzhu, C. M., Grzybowski, T. & Zakharov, I. A. (2007). Distribution of the male lineages of Genghis Khan's descendants in northern Eurasian populations. Russian Journal of Genetics, 43, 334-337.

[iv] Xue, Y., Zerjal, T., Bao, W., Zhu, S., Lim, S. K., Shu, Q., Xu, J., Du, R., Fu, S., Li, P., Yang, H. & Tyler-Smith, C. (2005). Recent spread of a Y-chromosomal lineage in northern China and Mongolia. Am J Hum Genet. 77, 1112-1116.

[v] Roewer, L., Croucher, P. J., Willuweit, S., Lu, T. T., Kayser, M., Lessig, R., de Knijff, P., Jobling, M. A., Tyler-Smith, C. & Krawczak, M. (2005). Signature of recent historical events in the European Y-chromosomal STR haplotype distribution. Hum Genet. 116, 279-291.

[vi] Gatewood, J. D., Wills, A., Shetty, S., Xu, J., Arnold, A. P., Burgoyne, P. S. & Rissman, E. F. (2006). Sex chromosome complement and gonadal sex influence aggressive and parental behaviors in mice. J Neurosci. 26, 2335-2342. Roubertoux, P. L., Guillot, P. V., Mortaud, S., Pratte, M., Jamon, M., Cohen-Salmon, C. & Tordjman, S. (2005). Attack behaviors in mice: from factorial structure to quantitative trait loci mapping. Eur J Pharmacol. 526, 172-185.

[vii] Buss, A. H. & Perry, M. (1992). The aggression questionnaire. J Pers Soc Psychol. 63, 452-459. Oliver, M.B. & Hyde, J.S. (1993). Gender differences in sexuality: A meta-analysis. Psychological Bulletin 114, 29-51. U.S. Department of Justice, Bureau of Justice Statistics (2006). Homicide trends in the U.S.: Trends by gender. Available at:

[viii] Schmitt, D. P., Alcalay, L., Allik, J., et al. (2003). Universal sex differences in the desire for sexual variety: tests from 52 nations, 6 continents, and 13 islands. J Pers Soc Psychol. 85, 85-104. Baillargeon, R. H., Zoccolillo, M., Keenan, et al. (2007). Gender differences in physical aggression: A prospective population-based survey of children before and after 2 years of age. Dev Psychol. 43, 13-26.

[ix] Russell, D.E.H. (1984). Sexual exploitation: Rape, child sexual abuse, and workplace harassment. Sage.

[x] Malamuth, N.M. (1981). Rape proclivity among males. Journal of Social Issues 37, 138-157

[xi] Clark, G. (1969). What happens when the police go on strike? New York Times Magazine, November 16. Kamber, M. (2004). Five months after Aristide, mayhem rules the streets. New York Times, August 2.

[xii] Wilson, J.Q. (1993). The moral sense. Free Press.

[xiii] Ellis, L. (2000). Criminology: A global perspective, supplemental tables, and references. Pyramid Press.

[xiv] Bailey, A. A. & Hurd, P. L. (2005). Finger length ratio (2D:4D) correlates with physical aggression in men but not in women. Biol Psychol. 68, 215-222.

[xv] Christine Knickmeyer, R. & Baron-Cohen, S. (2006). Fetal testosterone and sex differences. Early Hum Dev. 82, 755-760.

[xvi] Olivier, B. (2004). Serotonin and aggression. Ann N Y Acad Sci. 1036, 382-392.

[xvii] Coccaro, E. F., Bergeman, C. S., Kavoussi, R. J. & Seroczynski, A. D. (1997). Heritability of aggression and irritability: a twin study of the Buss-Durkee aggression scales in adult male subjects. Biol Psychiatry. 41, 273-284.

[xviii] Brunner, H. G., Nelen, M., Breakefield, X. O., Ropers, H. H. & van Oost, B. A. (1993). Abnormal behavior associated with a point mutation in the structural gene for monoamine oxidase A. Science. 262, 578-580. Cases, O., Seif, I., Grimsby, J., Gaspar, P., Chen, K., Pournin, S., Muller, U., Aguet, M., Babinet, C., Shih, J. C. & et al. (1995). Aggressive behavior and altered amounts of brain serotonin and norepinephrine in mice lacking MAOA. Science. 268, 1763-1766.

[xix] Caspi, A., McClay, J., Moffitt, T. E., Mill, J., Martin, J., Craig, I. W., Taylor, A. & Poulton, R. (2002). Role of genotype in the cycle of violence in maltreated children. Science. 297, 851-854.

[xx] Trivers, R. (1972). Parental investment and sexual selection. In B. Campbell (ed.), Sexual selection and The Descent of Man. Aldine (pp. 136-179). Emlen, S. T. & Oring, L.W. (1977). Ecology, sexual selection, and the evolution of mating systems. Science 197:215-223.

[xxi] Richard Dawkins (1976/2006). The selfish gene. 3rd edition. Oxford University Press.

[xxii] Niles Eldredge (2004). Why we do it: Rethinking sex and the selfish gene. W.W. Norton.

[xxiii] Richard A. Posner (1992). Sex and reason. Harvard.

[xxiv] Randy Thornhill and Craig T. Palmer (2000). A natural history of rape: Biological bases of sexual coercion. MIT Press.

[xxv] Malamuth, N. (1998). An evolutionary-based model integrating research on the characteristics of sexually coercive men. In: Advances in Psychological Science, volume 2: Personal, social, and developmental aspects. (ed. Adair, J., et al.). Psychology Press.

[xxvi] Smith, E. A., Mulder, M. B. & Hill, K. (2001). Controversies in the evolutionary social sciences: a guide for the perplexed. Trends Ecol Evol. 16, 128-135.

[xxvii] Elias Canetti (1960/1978). Crowds and power (trans. Carol Stewart). Seabury Press.

[xxviii] Sigmund Freud (1921/1959). Group psychology and the analysis of the ego (trans. James Strachey). W.W. Norton.

[xxix] Canetti, p. 49.

[xxx] Canetti, p. 73.

[xxxi] Konrad Lorenz (1963/1974). On Aggression. Harvest Books.

[xxxii] Richard Wrangham and Dale Peterson (1996) Demonic males: Apes and the origins of human violence. Houghton Mifflin.

[xxxiii] PBS. Deep jungle: The beast within. Available at

[xxxiv] Steven LeBlanc with Katherine Register (2003). Constant battles: Why we fight. St. Martin’s Press.

[xxxv] Lawrence Keeley (1996): War Before civilization: The myth of the peaceful savage. Oxford University Press.

[xxxvi] Mervyn Meggitt (1977). Blood is their argument: Warfare among the Mae Enga tribesmen. Mayfield.

[xxxvii] Napoleon Chagnon (1968). Yanomamö: The fierce people. Holt, Rinehart and Winston.

[xxxviii] Tacon, P., & Chippendale, C. (1994). Australia’s ancient warriors: Changing depictions of fighting in the rock art of Arnhem Land, N.T. Cambridge Archaeological Journal 42, 211-248.

[xxxix] Douglas Fry (2007). Beyond war: The human potential for peace. Oxford University Press.

[xl] Adams, A. (1983). Why there are so few female warriors. Behavioral Science Research, 18, 196-212.

[xli] Gunnar Heinsohn (2003). Söhne und Weltmacht: Terror im Aufstieg und Fall der Nationen. Orell Füssli Verlag. Available at

[xlii] BBC News (2002). Archbishop’s anti-war message. December 26. Available at

[xliii] The Ethics and Religious Liberty Commission of the Southern Baptist Convention: The so-called “Land Letter.” Available at

[xliv] Willy Thorn (2002). U.S. bishops oppose preemptive strike against Iraq. Catholic Herald, September 19. Available at

[xlv] Caprioli, M. (2003). Gender equality and state aggression: The impact of domestic gender equality on state first use of force. International Interactions, 29, 195-214.

[xlvi] Center for the American Woman and Politics (2005). The gender gap: Voting patterns in presidential elections. Available at

[xlvii] National Maritime Museum (undated). An A to Z of Nelson: V: HMS Victory. Available at

[xlviii] Marc Kaufman and Dafna Linzer (2007). China criticized for anti-satellite missile test. Washington Post, January 19. Available at

[xlix] Sigmund Freud (1905/1962). Three essays on the theory of sexuality (trans. James Strachey). Basic Books.

[l] Camille Paglia (1990). Sexual personae: Art and decadence from Nefertiti to Emily Dickinson. Yale University Press, pp. 247 and 38.

[li] R.J. Rummel (1997). Power kills: Democracy as a method of nonviolence. Transaction Publishers.

[lii] Geoffrey Blainey (1988). The causes of war. (3rd. edition). The Free Press.

[liii] Stockholm International Peace Research Institute (2007). Recent trends in military expenditure. Available at

[liv] Adams, D. (1984). There is no instinct for war. Psychological Journal (Moscow), 5, 140-144. Available in translation at

[lv] Wann, D.L., Carlson, J.D., Holland, L.C., Jacob, B.E., Owens, D.E., & Wells, D.D. (1999). Beliefs in symbolic catharsis: The importance of involvement with aggressive sports. Social Behavior and Personality, 27, 155-163.

[lvi] Anderson, C.A., & Bushman, B.J. (2001). Effects of violent video games on aggressive behavior, aggressive cognition, aggressive affect, physiological arousal, and prosocial behavior: A meta-analytic review of the scientific literature. Psychological Science, 12, 353-359. Donnerstein, E., & Berkowitz, L. (1981). Victim reactions in aggressive erotic films as a factor in violence against women. Journal of Personality and Social Psychology, 41, 710-724.

[lvii] Carnagey, N.L., Anderson, C.A., & Bushman, B.J. (2007). The effect of video game violence on physiological desensitization to real-life violence. Journal of Experimental Social Psychology, 43, 489-496.

[lviii] Ellis, L. (2005). A theory explaining biological correlates of criminality. European Journal of Criminology, 2, 287-315.

[lix] Farrington, D.P. (1994). The development of offending and antisocial behaviour from childhood: Key findings from the Cambridge Study in Delinquent Development. Journal of Child Psychology and Psychiatry, 36, 924-964.

[lx] Burnham, T. C., Chapman, J. F., Gray, P. B., McIntyre, M. H., Lipson, S. F. & Ellison, P. T. (2003). Men in committed, romantic relationships have lower testosterone. Hormones and Behavior,. 44, 119-122.

[lxi] Frans B.M. de Waal and Frans Lanting (1997). Bonobo: The forgotten ape. University of California Press.

[lxii] Hauser, M.D. (2005). Beyond the chimpanzee genome: The threat of extinction. Science, 309, 1498-1499.

[lxiii] Richardson, S. (1993). A violence in the blood. Discover, October 1. Available at

[lxiv] Stockholm International Peace Research Institute, SIPRI Yearbook 2006. Available at