Close your eyes for a moment and cast your mind back to the dominant news stories of early 2010. The economy in tatters? Certainly. Global stalemate on climate negotiations and unbreakable gridlock in Congress? Of course. And don't forget the terror — on Christmas Eve, 2009, a lone Nigerian man boards an airplane in Lagos and travels some 18 hours toward Detroit in what can only have been a dizzying combination of anxiety, fear and elation, and a grandiose sense of his own destiny. It all ends with a little ineffectual fumbling in the underpants, cut short by the heroism of Umar Farouk Abdulmutallab's fellow passengers.
The official response to the underwear bomber reveals the usual inability of large bureaucracies to connect the dots or take meaningful action on real threats. Instead of understanding and reassessment, we get yet another late, inappropriate and costly escalation in airport security and political infighting about the treatment of Abdulmutallab — all of it embedded in an unacknowledged but resolute refusal to see the bigger picture.
Meanwhile, the real killing continues to elude the headlines. It is on the Afghanistan-Pakistan border, in Afghanistan's Helmand Province where allied Western soldiers struggle with the almost impossible task of attacking the Taliban without killing civilians. It is in Darfur and the Congo, where death tolls are in the millions, not the thousands, and it is in Nigeria, where Christians and Muslims meet. Here is primeval warfare in full abundance, where bands of men are knit together by ancient bonds of shared violence. They are motivated to kill their neighbors systematically and deliberately, not just by lust for land and resources but also by hatred of the "other" and a too-seldom acknowledged love of war and warring ways. It is in these places, and scores of others where the violence simmers just below the surface, that people live close to one of the darkest realities of human nature.
Humans — human males, really — are not peaceful animals. They are in fact a spectacularly violent species, and very nearly uniquely so. Despite high-minded modern wishes and the received wisdom of three generations of anthropologists and sociologists, warfare is not an aberration in human development, nor is it a learned, culturally determined behavior. War and its ancillary behaviors — including racism, slavery, mass rape and the subjugation of women — are not cultural problems and thus do not have neat, sociological solutions. Along with terrorism, these most destructive of human behaviors derive clearly and directly from our biology, bequeathed to us by an evolutionary pathway that we share with just one other extant species, the chimpanzees.
War, simply put, is in our genes. It is a complex behavior built up out of a series of emotions and impulses that are, in general, expressed more in men than in women, and more in young men than in old. It arose early in our evolutionary history because the most violent of our pre-human male ancestors had more offspring than their more peaceful or timid competitors; it has been with us as long as we have been a species and in all probability will be with us as long as we remain one. Our warlike impulses cannot be stopped with enhanced airport imaging, extrajudicial treatment of terrorism suspects or any attempt at a literal "war on terror."
From biology, medicine, history, literature, political theory, sociology and evolutionary psychology, a clear picture emerges: War is a biological behavior. As robust science demonstrates — and common sense and the experience of warriors around the world and throughout history attest — war is part of the human condition. But does this mean that war is inevitable and peace an unattainable dream?
Emphatically, and demonstrably, no. Most of the world, despite economic challenges, is remarkably peaceful, and as improbable as it seems, the past century has actually been the most peaceful in known human history. The last soldiers who experienced the horrors of trench warfare in France have died, the guns are silent in Bosnia and Herzegovina, and the leaders of Pakistan and India are trying to talk to one another. The Vikings, who once personified the merciless terror of war for an entire continent, have become the Scandinavians, as resolute as anyone in the quest for tolerance at home, and peace and openness around the world.
Crucially, war's deep roots in our evolutionary past do not condemn us to a future as filled with warfare as our history has been. On the contrary, recognizing and accepting the centrality of war in human nature sheds new light on real, practicable policy prescriptions that can help make war less common in the future and less brutal when it does occur. Humans are complex, adaptable animals. And all genes, behavioral or not, are influenced by their environments. If humans truly want peace, they must seek to understand the biology of war and use that understanding to devise policies — chief among them, improved access to family planning services that can control some demographic drivers of war — so as to help the biology of peace win out.
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The idea that warlike violence is not innate actually arose just recently. It can be traced back to Rousseau, and found full-throated proponents in Franz Boas,Margaret Mead, Ashley Montagu and other post-World War I anthropologists. Understandably shocked by the horrors of trench warfare and poison gas, these generally clear-minded academics sought evidence to distance humankind from such barbarism, and they found it — or so they thought — in an updated notion of the Noble Savage and the idea that civilization represents a fall from some earlier state of grace. But archaeologists such as Steven LeBlanc of Harvard and Lawrence Keeley of the University of Illinois at Chicago have found supposedly peaceful societies riddled with violence. Careful investigation reveals histories of murder and long-standing, pervasive and brutally lethal warfare in Mead's Pacific Islanders, the Copper River Inuit, the !Kung people of the Kalahari and many other purportedly "peaceful" societies. As LeBlanc writes in his clarifying 2003 book, Constant Battles, "Prehistoric warfare was common and deadly, and no time span or geographic region seems to have been immune."
Remarkably, the idea that violence and warfare are the fault of culture, not biology, remains widespread in academic circles. As recently as 1986, 20 international scholars drafted the Seville Statement on Violence at a UNESCO meeting asserting that, among other things, "It is scientifically incorrect to say that war or any other violent behavior is genetically programmed into our human nature." Several scientific associations, including the American associations of psychology, of anthropology and of sociology, voted to endorse the Seville Statement. But true science proceeds by observation, experiment and debate, and not by endorsing written statements. And the evidence — which we examined fully in our recent book, Sex and War: How Biology Explains Warfare and Terrorism and Offers a Path to a Safer World — reveals a very different story.
In 2009, fossil hunters in Ethiopia found "Ardi," a nearly complete skeleton of a 4.4 million-year-old ape with a brain slightly larger than that of a chimpanzee. She lived in an open savannah and walked upright, even though she still had an opposable toe, as chimps do, to climb trees. Identified as Ardipithecus ramidus, like other fossils such as the famous "Lucy," Ardi is not a "missing link" in the sense of a literal ancestor, but a cousin, a nearby branch on the tree of human evolution. Yet all the apes have a common ancestor, and Ardi is almost certainly descended from the branch of the ape family that gave rise to chimps and humans. We suggest that this chimp-human ancestor lived in small bands of related males who controlled a defined territory — just as do chimpanzees and virtually every hunter-gatherer society ever studied. And we suggest that war began when those ancestral males first banded together and, as present-day chimpanzees and more recent hunter-gatherers still do, left their territory, found a member of another troop and set about killing it in the most vicious way possible.
At its most basic, we define war as a form of organized violence in which groups of males band together and intentionally set out to kill members of their own species. Many species are violent, of course, and may appear to enjoy hunting and killing "for sport," as humans do. Quite a few predators hunt and kill in packs or coordinated teams. But it is exceedingly rare that they should intentionally hunt and kill members of their own species, as opposed to the occasional and largely accidental deaths that result from male mating competitions. Wolves may do so on occasion, and hyenas, and perhaps one or two other species. But when it comes to warring behavior as a regular, integral part of life, no species come close to human beings and chimpanzees. Taken with the reality that war has been a constant feature of human behavior around the world and throughout time, this commonality of humans and the chimps suggests very strongly that war is an inherited behavior that first evolved in a common ancestor we shared more than 7 million years ago.
So how did war first evolve? As Jane Goodall, Richard Wrangham and others have shown, we share with chimps, our closest living biological relatives, the bizarre propensity to attack and kill others of our own species. Chimpanzees live, as humans did for the vast majority of evolutionary time, in male-dominated social groups in which the males are all blood relatives and only females move between troops. The dominant males largely monopolize mating opportunities and take the best food and other resources. Younger males are left either to work their way up the in-group hierarchy or attempt surreptitious matings with females of their own troop or others — high-stakes strategies that often end in a beating or worse. But, in a unique evolutionary innovation, these young males can also band together and launch attacks on isolated members of neighboring out-groups, ultimately eliminating these "enemies" and securing the territory, resources and females they require to survive and pass on their genes.
Today, we see remarkably similar patterns of territorial raiding, brutal attacks and, ultimately, campaigns of extermination in both humans and chimpanzees. Just as the most successfully violent alpha male chimpanzees have more mates and more offspring than the losers, genetic surveys show that the great human warriors of history have left outsized impacts on the human gene pool. One study published in 2003 estimated that Genghis Khan has 16 million living descendants worldwide. It takes little imagination to see the evolutionary benefit of warfare to Khan and his cohorts, and it leads to the uncomfortable realization that we are all, by definition, the descendants of the victors in conflicts over resources, territory and the right to mate.
We are all descended, in other words, from particularly successful rapists, murderers and brigands. Human males today bear the marks of this legacy in the behaviors and impulses that still spur us on to lethal conflict — including the widespread and devastating association between war and rape — even when other solutions are both available and preferable.
There is no doubt that other apes, like people, can be empathetic. They will help one another or slow down a march so a sick or wounded animal can catch up. In her important 2009 bookMothers and Others: The Evolutionary Origins of Mutual Understanding, Sarah Blaffer Hrdy of University of California, Davis, underscores the ability of human mothers to assist one another in the long, arduous task of raising children. How can such intensely social, empathic animals also kill other members of their own species? We postulate that the key that unlocked the full fury of war was an evolved psychological mechanism that allows us to dehumanize (or "dechimpanzize") those we would attack. Tragically, human history is replete with episodes of dehumanizing behavior.
A famous Stanford prison experiment shows that nice young students randomly assigned in a psychology experiment to be "prison guards" will adopt and exploit these roles in a couple of days and emotionally abuse people randomly assigned to play the role of "prisoners." The study lasted only a few days, but the behavior was little different from that of American soldiers in Abu Ghraib abusing Iraqi prisoners. Dehumanizing our enemies is not an aberration — it is default human behavior. New York Times columnist Nicholas Kristof, reporting from the Congo, recently described a woman who, while she was being raped by soldiers, screamed to warn her neighbors. In revenge, her assailants cut off one leg with a machete, cooked the meat and ate it while the woman almost bled to death and her children looked on. When they tried to force her child to eat her mother's flesh, he refused, saying "shoot me." They did. Seemingly, the human ability to dehumanize others knows no limits — and most certainly has not disappeared from our shared evolutionary repertoire.
We go into more detail on the potential mechanisms for our evolved ability to dehumanize and kill our fellow humans in our book. But for a flavor of those root causes, let us suggest that the male sex hormone, testosterone, is in some ways the ultimate weapon of mass destruction. Testosterone levels are highest in men aged 19 to 30, a span that tracks closely the age distribution of convictions for violent crimes. Testosterone levels rise not just among men playing team sports but also their fans — and one need look no further than the passionate partisanship of team sports to see the "in-group" versus "out-group" dynamic that underlies both the camaraderie and cruelty of warfare. Women also secrete testosterone, but at about one-tenth the male level. Intriguingly, women's testosterone output does not change in response to competition.
In 2008, the world suffered the biggest economic collapse since the Great Depression. Like war, the global financial crisis had many specific and technical causes. But it was ultimately driven, in the words of one financial adviser in the City of London, by "a lot of alpha males with testosterone streaming out of their ears." This apparently flippant remark was actually an insightful analysis of the global crisis. Evolutionary psychologists Coren Apicella from Harvard and colleagues found that men with high testosterone levels make riskier investments, and others have observed that women make better investment managers over the long term than do men. The multimillion-dollar bonuses Wall Street bankers pay themselves, which for good reason infuriate the rest of society, can be best understood as a (predominantly male) troop displaying intense internal loyalty and total blindness to the outside world.
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Six months after the publication of Sex and War, we were comforted to read two papers in Science. Two separate studies — Samuel Bowles writing one, and Adam Powell, Stephan Shennan and Mark Thomas the other — used different methodologies from ours but came to the conclusions we had.
Bowles, an economist and behavioral scientist who studies altruistic behavior, used a computer model of between-group competition together with a database of archaeological and ethnographic rates of adult mortality from warfare. His study supports the notion of "parochial altruism," in which humans developed the ability to be altruistic within an in-group, however defined, and callously violent to those outside it. In fact, Bowles' work suggests that the human compassion and altruism most people value today was made possible by the existence of warfare — that cooperation for both defense and offense within the group allowed the most successfully violent of our ancestors to flourish.
Powell and his co-authors focus on the evolution of technological and cultural complexity, as evidenced by the appearance of art, sophisticated tools — including such potential weapons as bows, boomerangs and spear-throwers — and long-distance trade. Their main purpose is to explain patterns of "modern" behavior in Africa and Eurasia. But in the process, they show convincingly that population growth and other demographic features may hold the key to some of our most complex behaviors.
Each particular war and battle has its own history and specific sets of grievances, turning points and precipitating triggers and personalities. But there is also a set of factors — social, political and environmental — that many wars and violent conflicts have in common. In fact, these shared characteristics are so common that statistical modeling of social, economic and environmental conditions can result in stunningly accurate predictions of armed conflict and unrest.
Briefly, the factors that seem most likely to increase the probability of open war or armed conflict include:
• Environmental stress and/or resource limitation.
• Extreme economic disparity within or between groups and lack of opportunities, especially for young men.
• Subjugation of women and a culture of male dominance.
• A high proportion of young males relative to older males.
All of these factors interact in one way or another with the warlike biology of the human male, and each is influenced quite directly by population growth rate, and as a result, population age structure or the relative ratios of young to old in a society.
We argue in Sex and War that our warring behaviors are essentially a hangover from our evolutionary past. It also seems clear that these behaviors have been rendered wildly maladaptive in the dual modern contexts of stable societies with social norms that condemn wild warring on the one hand and allow weapons of mass destruction on the other. (With simple technology, the impulses of war can kill hundreds or thousands; with nuclear and biological weapons, they can potentially kill us all.)
But the problem with evolved traits is that they neither know nor care when they're no longer wanted. And as catastrophically troublesome as our warrior genes often are today, the conditions in which they rose to prominence also persist. Our prescription for a more peaceful future follows directly from that observation: To limit the damage from unchecked warfare, humanity must understand and limit the physical, cultural and demographic conditions that make it most likely to occur.
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War is the ultimate zero-sum game. Whether one side or another wins, and no matter what short-term economic stimulation comes to pass, war represents a squandering of resources and the greatest imaginable waste of human effort, ingenuity and life. We cannot argue that wars are never justifiable or necessary. But whatever utility this unique set of behaviors once had, human culture and morality have moved beyond the point where wars of extermination are acceptable, and killing technologies have become far too deadly, and indiscriminately so, for our warring impulses to be given free rein.
But can humans change? The answer, thank goodness, is that we can — and in fact we already have. As mentioned earlier, the 20th century was more peaceful than any other, both in terms of the number of people directly involved in warfare and in the percentage of adult males dying in armed conflict. As just one example, the Soviet Union suffered the greatest casualties of World War II — perhaps 15 million died, representing 8 percent of the population. As unimaginably horrific as those losses were, they are small compared to violent death rates in hunter-gatherer societies. Among the hunter-gatherers of New Guinea, studies show, from 5 to 30 percent of adults typically die from raids and wars; in the Yanomamo of the northern Amazon basin, a staggering four out of 10 adults have participated in killing another person and 20 percent of people over 40 have lost a parent, child or sibling to violence.
Just as there are many factors that initiate wars, there are also many factors that can in theory be tweaked or refined to make war less likely. The clarifying lens of biology helps show that one factor influences them all: The population growth rate turns out to be a crucial component in the biology of war, for reasons both direct and indirect.
Foremost, growing populations are young populations, and young men are the true engines of war. They provide the recklessness and bravery, the intense inward loyalty and outward hostility, and the other raw behavioral "material" that can be shaped easily into small, tightly bonded fighting units, which in turn can be built up into armies of millions with each soldier still fighting, ultimately, for the men at his side. At the same time, women in rapidly growing populations are women spending a great deal of time having and raising children — and not, usually, taking an equal role in politics at any level outside the home. Careful statistical studies show that the probability of violent conflict increases as the ratio of young men in a society rises above that of older men, and that the probability of war falls as the percentage of women involved in local politics rises.
More tangentially, growing populations stress their environments and lead to competition for increasingly scarce natural resources. The link between environmental instability and violent conflict is made frighteningly clear in a 2009 study by researchers at Stanford University, New York University, Harvard University and the University of California, Berkeley. Extrapolating from historical correlations of temperature rise and increased armed conflict in sub-Saharan Africa, the researchers project that expected climate change alone could spur a 60 percent increase in armed conflicts by 2030. That projection, if it came true, would translate to an additional 459,000 deaths from war in just two decades, and that is without taking population growth into account — in the fastest-growing region of the world.
Growing populations, especially in poorer areas, also tend to overburden existing infrastructure and outstrip the available employment. This leads to high operating costs for businesses and lost opportunities for individuals. Educational opportunities are lost for the same reasons, and in many cultures, it is girls who lose out first when education is rationed.
There is no one solution to the problem of war. But biology suggests — and quantitative studies support — the notion that if testosterone is the ultimate weapon of mass destruction, then the birth control pill may be the ultimate prescription for peace.
It is sobering to think of how many millions of our forebears died of sword and siege and famine, but it is also heartening to realize that humans have already found many ways to rein in our most violent impulses. In light of the true, root, biological causes of warfare, it becomes obvious that there is much more humans can and should be doing now.
Let's look again at some of the main predictors of war in the light of one particularly well-known conflict, the alternating battles and standoffs between Israel and Hamas in the Gaza Strip. Young men, in general, are motivated to fight for resources, or in revenge for those killed, or because they feel a sense of injustice, and those emotional cues surely help motivate young Palestinians to risk their lives firing rockets at Israel, or to become suicide bombers on Israeli streets. Israel's desire to stop such attacks is understandable.
Interestingly, the Palestine Liberation Organization under Yasser Arafat showed very clearly what it takes to truly stop terrorism. In 1972, the terrorist group Black September, based in Beirut, gained world attention by killing Israeli athletes at the Munich Olympic Games. But Arafat had his eye on the possibility of gaining observer status at the United Nations. Afraid Black September might launch more high-profile raids and undermine his U.N. ambitions, the PLO flew eligible young female volunteers to Beirut and offered militant members of Black September $3,000, an apartment with a TV, long-term employment and $5,000 if they married and had a child. The offer was overwhelmingly accepted, and Black September as a terrorist movement collapsed almost overnight.
Since the Six-Day War in 1967, the population of the Occupied Palestinian Territory has grown from just over 1 million to 3.9 million. The average woman there has about five children, and the U.N. estimates that by 2050 the Palestinian population could reach a mind-boggling 8.8 million to 11.8 million. Two-thirds of the current population is under age 25, giving rise to an unemployed, volatile, testosterone-fueled group of young men — an endless source of terrorists. Adding to their frustrations, and their motivation to lash out, Arab society discourages premarital sex, and unemployed men don't have the resources needed to marry.
The world is not going to pay young Palestinians to marry. But we do have options. Gaza has few jobs and virtually no natural resources. Already, Palestinians pull more water out of the ground than falls from the sky, and their drinking water is increasingly saline. Palestinians also have a strong entrepreneurial tradition, however, and, like Singapore in the 1960s and 1970s, could build a future in their overcrowded space by living by their brains. Israel may understandably want to build a wall around Gaza, but without the free flow of goods, capital and ideas — and education at all levels for both sexes — the problem of Palestinian terrorism will get worse, not better.
In the early 1990s, one of us spent time in Gaza working with an Arab colleague to develop family planning services. Palestinian women wanted help from the international community, but no government or donor would provide the modest support needed to improve access to family planning. That opportunity to improve the lot of Gazans was lost, and the situation has deteriorated to the point where today's radical Palestinians claim, "the Palestinian womb is the one weapon that Palestinians have." In a tragic sense, they are right. But as a strategy for building a better future, rapid population growth could not be more wrong.
There is a popular notion that education and rising economic fortunes lead to decreasing family size. We argue elsewhere that, conversely, decreased family size is actually a prerequisite for economic growth and social stability. We suggest that the cases of China, South Korea and Thailand, and even post-revolution Iran, present particularly powerful examples. In each of these cases, governments realized early on that rapid population growth threatened their continued peace, stability and prosperity. In China by coercive means, and more by top-down social consensus building in South Korea, Thailand and Iran, these countries were able to slow population growth rates dramatically, and each has had a more prosperous-than-expected outcome as a result.
Other problems persist of course. But even in Iran, with its incongruously antagonistic government and truncated economy, the benefits of slowing population growth are plain to see. There are now more women in the University of Tehran than men, and while Iran's chaotic president may support terrorist groups, young Iranians are not strapping on explosive vests and killing people — they are marching peacefully in the streets demanding more openness, democracy and peace.
In his 2009 speech accepting the Nobel Peace Prize, Barack Obama noted, quite correctly, that "security does not exist where human beings do not have access to enough food, or clean water, or the medicine they need to survive. It does not exist where children cannot aspire to a decent education or a job that supports a family." He did not, however, mention his single most significant contribution to world peace to date: reversing the Bush-era policy of refusing U.S. development funds to any agency supporting the availability of family planning. And it doesn't matter whether the goal is specifically to build peace — that result will come if the policies succeed and women in the most impoverished areas of the world are simply able to determine their own family size.
We have argued that offering women a range of family planning is always associated with falling family size. This assertion has recently been validated by the analysis of what might be called a natural experiment. In Kenya between 1970 and the early 1990s, considerable emphasis was put on improving access to family planning, and average family size fell from 8 to 5. But then the focus was taken off of family planning, and health professionals migrated to work on the AIDS epidemic.
Kenya's fertility decline stalled as contraceptive use fell, and unwanted pregnancies and unsafe abortions increased dramatically. In 1990, it was estimated that the population of Kenya would grow from 23.5 million then to 54 million in 2050. As a result of the stalled fertility decline, today's estimate for 2050 population has been raised to 83 million. Already, the ethnic violence following the disputed presidential election of December 2007 has undermined generations of peaceful coexistence and friendship within Kenya; population increases of this magnitude could well turn Kenya into a failed state like its neighbor, Somalia. If population growth is not slowed again in Kenya, the results will be as horrifying as they are avoidable: Ethnic violence related to diminished access to resources will increase, and a once shining light for stability and prosperity in Africa will have been snuffed out for generations because of the lack of attention to family planning over the past 15 years.
In fact, we have just come through a "lost decade" of family planning — and by extension, a lost decade for building peace in the world. Especially in sub-Saharan Africa and parts of the Middle East, this lost emphasis on both domestic and international family planning programs is already having tragic consequences. The latest research shows that a "contraception gap" between rich and poor — already a common phenomenon around the world — is widening in those countries where the poor are already most vulnerable. (The wealthy have always been more able to find ways to separate sex from reproduction.) As the unmet demand for family planning continues to rise throughout the most impoverished nations, so too will the disparities in family size between rich and poor. Inevitably and as a direct consequence, the inequalities in education, health, employment and income will also continue to widen, infrastructure will continue to crumble, and the risk of food insecurity, environmental catastrophe and devastating warfare will continue to rise.
Ultimately, the decision to support family planning efforts comes down to making a moral choice. The profound success of humanitarian aid efforts and improved nutrition and health care for many in the developing world is greatly to be admired and celebrated. But in decreasing infant mortality, we have engendered a grim unintended consequence — millions of women throughout the developing world are now able to bear healthy children safely, but have no access to safe and effective contraception.
A prominent evolutionary biologist recently shared with one of us his recent realization that when it comes to vaccination and other means of preventing tropical illness, "to provide these measures without providing family planning assistance is tantamount to homicide/genocide." As important as it obviously is to work for greater health and longer life, we could not agree more emphatically that to do so without also giving people the ability to determine family size is to condemn them to an increased likelihood of overpopulation, poverty and environmental degradation, as well as a dramatic and quantifiable increase in the likelihood of bloody conflict.
It is not just perverse and foolhardy from a national security standpoint to pursue policies that increase the likelihood of famine, unemployment and war; it is morally wicked, on a historically vast scale, to condemn untold hundreds of millions of fellow humans to longer lives of decreasing opportunity and increasing misery. This is, of course, not an argument against health care and hygiene throughout the developing world, and it is emphatically no brief for eugenics or forced or coercive abortion, sterilization or contraception.
It is, rather, the strongest possible argument for the immediate, universal provision of the means of family planning and maternal health care, so that women throughout the world can have the freedom to choose the family size that's best for them. Those individual choices, made freely and without coercion, will inevitably lead to more stability, peace and prosperity. If those millions of women are denied the means to choose for themselves, then choice will diminish for the rest of us. We will all have continued population increase, a devastated environment and the looming prospect of a future just as bloody and war-filled as our past.