Late in his career, Richard Overy, an English historian specializing in the Second World War, identified an epistemological flaw at the heart of his profession: Few human activities are more central to historiography than war, and yet historians are poorly equipped to understand its evolutionary and psychological roots: Why War? attempts, with only partial success, to close this gap.
Overy’s narrative begins with the 1932 correspondence between Albert Einstein and Sigmund Freud. Einstein, a committed pacifist aghast at the carnage of the First World War, sought reassurance from Freud that humanity could somehow free itself from the dogs of war that had cursed the new century. He was soon disappointed: humanity, Freud told him, was afflicted with an incurable “death drive.”
Freud’s intuition was supported by subsequent discoveries in primatology and evolutionary psychology, prime among which was the work of Jane Goodall, who documented the murderous habits of our closest genetic relatives, chimpanzees, with whom we share 98 percent of our DNA.
Are humans as irretrievably hardwired for lethal violence as chimpanzees? Maybe, maybe not; bonobos, which were only recently recognized as a species separate from chimps, are famously peaceful (and, perhaps not coincidentally, compulsively promiscuous).
Overy documents the large-scale violence pervasive in human history and leaves little doubt just how murderous, and for how long, our own species has been:
Supporting data
Why War? marshals four different kinds of evidence to support this depressing conclusion. First and foremost is the “oseteoarchaeological” record, geographically pervasive mass gravesites extending back tens of thousands of years.
Typical is a site near Herxheim, Germany dating to 5000 BCE that features 173 skulls and skull parts and the dismembered skeletons of 334 men, women, and children. Even more depressingly, their bones showed features suggestive of cannibalization (marrow extraction, knife marks, and human tooth marks). Less spectacularly, but just as persuasive, about one quarter to one third of prehistoric human skeletons exhibit skull fractures and “parry fractures” of the ulna (the medial forearm bone) characteristic of reflexive face shielding during violent attack.
Documentary data supplies the second body of evidence of mankind’s homicidal nature, from the God of 1 Samuel demanding of the Israelites “the slaughter of the men, women, children, and ox, sheep, camels, and donkeys of the Amalek” to stone age reliefs and murals from around the world depicting the defeat, capture, and ritual massacre of soldiers and civilians. Roman texts as well as Mayan hieroglyphic records describe a stereotyped event sequence in which the victors slaughter defeated populations, raze their sacred sites, and then replace them with their own.
Third is the pervasive fortifications that first appear during the transition from hunter-gathering to settled agriculture, as dense populations of farmers, and especially their food stores, found themselves the targets of neighboring settlements and raids from pastoral tribes; not infrequently, these fortifications are also the sites of mass graves.
Last, anthropological research from today’s aboriginal societies reveals that most have gone to war with their neighbors at one time or another; one study of 298 peoples found only about a dozen that had not experienced war. Another survey found that two thirds of tribes experienced it regularly, and over a quarter fought wars almost continually.
In short, the evidence of our homicidal nature overwhelms; around the world and throughout history, when humans reach a certain level of organization, warfare becomes a way of life: Writes Overy, “Once the genie was out of the bottle, it proved impossible to force it back in.”
Digging into the reasons behind the tendencies
But exactly why are humans so warlike? At this point, Overy throws out the baby—the evolution of tribal in-group/out-group behavior. Inexplicably, he obsesses for the better part of a chapter over the bath water, in this case the comical and untestable hypotheses put forward by Freud and his acolytes that our warlike ways somehow flow from Oedipal complexes, castration fears, and other psychoanalytic piffle. To his credit, Overy eventually discards these by the gallon, but not before wearing out the reader’s patience.
The evolutionary “baby” in the story of our murderous nature is the work of biologists and anthropologists like Joseph Henrich, Dominic Johnson, Christopher Boehm, and Robin Dunbar on human in-group/out-group behavior—more concisely, if clumsily, referred to as “groupishness”—that is so vital to the question of the human proclivity to war and violence.
Dunbar’s research, in particular, provides the key to understanding how neurologically modern humans became so homicidal. His work with primates correlated their neocortical volume with the size of their social groups: Humans, with the largest neocortexes, use that brainpower to maintain stable social networks of up to about 150 members; beyond that number, tribal bands fracture into smaller ones. (Lemurs, with the smallest neocortexes, can maintain groups of only two.)
Between 100,000 years ago and the dawn of settled agriculture around 10,000 years ago, humankind lived in groups with a 150-member maximal size—“Dunbar’s number”—and humans, sadly, have struggled along with the fallout of this long evolutionary history marinated in Dunbar’s number-sized tribes. Tragically, today’s humans live in the nuclear age in populations of hundreds of millions with stone-age brains that evolved in bands of about a hundred.
The importance of iribal affiliation
Not until the book’s final few pages does Overy even glance at the heart of the matter. This is the groupishness selected for at the tribal level, particularly pertaining to the sharing of meat from hunts and sacrifice in battle. This tribal-level altruism conferred an advantage over competing tribes, the problem being that within the tribe, the genes of cheaters benefit at the individual level from a survival advantage that is even more powerful than the tribal-level advantage.
How, then, to suppress cheating and free riding on the sacrifices of others? The genetic bonds that power natural selection at the individual level are relatively weak in tribal bands, so humans evolved cultural norms that fostered intra-tribal altruism and cohesion, namely a Manichean groupish mindset that glorifies and mythologizes one’s own tribe and demonizes other tribes as subhuman, “vermin” being the most commonly used metaphor, and thus worthy of extermination.
Numerous field and experimental studies demonstrate just how easy it is to set this human groupish dynamic in motion, most famously in high school class exercises in which students are arbitrarily divided into groups by shirt or hair color, which almost immediately produces both intra-group solidarity and inter-group demonization.
The most powerful cheating suppressant is, of course, religion; it’s no accident that there are no atheist tribes, societies, or nations, and that, at least in the West, religious belief has inexorably moved towards monotheism: What better way to keep cheating in check than an ever present, all seeing, and at least intermittently vengeful deity—in the words of evolutionary biologist Dominic Johnson, a “moralizing high god”?
In summary, humans evolved to behave altruistically towards members of our own tribe or nation and with homicidal ruthlessness towards everyone else. This evolutionarily driven groupishness makes war a constant of human history that inevitably plays out in conflicts revolving around resources, belief, power, and security.
Margaret Atwood succinctly described this demon at the heart of human history in an interview with Ezra Klein about how demagogues and military adventurists exploit the human desire to think well of ourselves in the service of murderous ends:
You might like a story about what a good person you are. You’re a good person, Ezra. Do you want to do the right thing? Sure you do, I can tell. Well, you can really help out humankind. So all you have to do is sacrifice 17 children at the full of the moon, and you’re going to do that, aren’t you, Ezra? Because you’re a good person and you want to help.
The causes underlying war
Overy’s omission of this critical evolutionary psychological story is a serious but not fatal stumble, which he recovers from by devoting a chapter each to the four historical causes of war: resources, belief, power, and security.
First, resources. The book’s most fascinating chapter unpacks the connection between climate and warfare. A period of dramatic global cooling and lower crop yields that occurred around 4000 BCE saw a significant increase in fortress construction in Eurasia; over the past millennium of Chinese history, warfare was twice as frequent during cold periods as during warm periods. A dearth of cropland drove Adolph Hitler’s blitzkriegs towards a swath of Lebensraum stretching to the Urals, a Greater Reich big enough to feed 600 million Germans. Over the past few decades, heat and drought have driven thousands from their homes in Central America’s northern triangle towards the Rio Grande; in 2007 16 retired three- and four-star generals penned an open warning about the serious geostrategic risks of climate change, which they labeled a “catalyst for conflict.”
Belief, not only of the religious sort but also, in the twentieth century, of the ideological variety, features prominently in the long history of human conflict. This holds especially true when it involves the end times, as does the ideology of ISIS. This belief system centers on a prophesized battle with the modern West—a reincarnation of Christian Rome—near Dabiq, a dusty town in northwestern Syria.
The book’s chapter on the pursuit of power, recounts the malignant hubris of an Alexander, Napoleon, or Hitler that aims at nothing less than world conquest, stopped only by defeat, or in Alexander’s case, by illness. The modern reader will have little trouble connecting their spellbinding stories to those of deluded modern autocrats unwilling to let go of power.
The section’s final chapter demonstrates how the quest for strategic security, because its pursuit by one tribe or nation invariably decreases the security of its neighbors’, proves invariably futile.
Overy maintains his dim view of our future right until the book’s very last sentence, which references the first large-scale European land war in three-quarters of a century: “No one [observing the Ukraine war] could be persuaded that warfare in all its many iterations will become a thing of the past. If war has a very long human history, it also has a future.”
Why War? is an insightful and well-written book that should be mandatory reading for anyone with even a passing interest in the past, and future, of the human race. However, those wanting a deeper understanding of evolutionary roots of mankind’s addiction to war would do well to supplement it with the relevant works of Robin Dunbar or Joseph Henrich. I particularly recommend the latter’s The Secret of our Success, which provides the evolutionary keys necessary to unlock Overy’s fine volume.
William J. Bernstein is a neurologist, the co-founder of Efficient Frontier Advisors, an investment management firm, and a writer with several titles on finance and economic history. He has contributed to the peer-reviewed finance literature and has written for several national publications, including Money Magazine and The Wall Street Journal. He has produced several finance titles, and four volumes of history, The Birth of Plenty, A Splendid Exchange, Masters of the Word, and The Delusions of Crowds about, respectively, the economic growth inflection of the early 19th century, the history of world trade, the effects of access to technology on human relations and politics, and financial and religious mass manias. He was also the 2017 winner of the James R. Vertin Award from the CFA Institute.
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Late in his career, Richard Overy, an English historian specializing in the Second World War, identified an epistemological flaw at the heart of his profession: Few human activities are more central to historiography than war, and yet historians are poorly equipped to understand its evolutionary and psychological roots: Why War? attempts, with only partial success, to close this gap.
