Why war? Darwinian explanations, such as the popular "demonic males" theory of Harvard anthropologist Richard Wrangham, are clearly insufficient. They can't explain why war emerged relatively recently in human prehistory—less than 15,000 years ago, according to the archaeological record—or why since then it has erupted only in certain times and places.

Many scholars solve this problem by combining Darwin with gloomy old Thomas Malthus. "No matter where we happen to live on Earth, we eventually outstrip the environment," the Harvard archaeologist Steven LeBlanc asserts in Constant Battles: Why We Fight (Saint Martin's Griffin, 2004). "This has always led to competition as a means of survival, and warfare has been the inevitable consequence of our ecological-demographic propensities." Note the words "always" and "inevitable."

LeBlanc is as wrong as Wrangham. Analyses of more than 300 societies in the Human Relations Area Files, an ethnographic database at Yale University, have turned up no clear-cut correlations between warfare and chronic resource scarcity. Similarly, the anthropologist Lawrence Keeley notes in War before Civilization: The Myth of the Peaceful Savage (Oxford University Press, 1997) that the correlation between population pressure and warfare "is either very complex or very weak or both."

Two tribal societies—the Semai of Malaysia and the Waorani of the Ecuadorian Amazon—represent especially striking exceptions to the Malthusian model. According to the anthropologists Clayton and Carole Robarchek (pdf), who lived among both societies, the Semai population is 60 times denser than the Waorani, and they have much less food, because their soil less fertile and game less plentiful. And yet the Semai, the Robarcheks pointed out, "are among the most peaceful people" known to anthropology (even though some Semai helped British colonialists fight communist insurgents in the 1950s). The Waorani, however, are one of the most violent known societies, with casualties from warfare claiming as much as 60 percent of the population.

War is both underdetermined and overdetermined. That is, many conditions are sufficient for war to occur, but none are necessary. Some societies remain peaceful even when significant risk factors are present, such as high population density, resource scarcity, and economic and ethnic divisions between people. Conversely, other societies fight in the absence of these conditions. What theory can account for this complex pattern of social behavior?

The best answer I've found comes from Margaret Mead, who as I mentioned in a recent post is often disparaged by genophilic researchers such as Wrangham. Mead proposed her theory of war in her 1940 essay "Warfare Is Only an Invention—Not a Biological Necessity." She dismissed the notion that war is the inevitable consequence of our "basic, competitive, aggressive, warring human nature." This theory is contradicted, she noted, by the simple fact that not all societies wage war. War has never been observed among a Himalayan people called the Lepchas or among the Eskimos. In fact, neither of these groups, when questioned by early ethnographers, was even aware of the concept of war.

In discussing the Eskimos Mead distinguished between individual and group violence. Eskimos were "not a mild and meek people," she noted. They engaged in "fights, theft of wives, murder, cannibalism," often provoked by fear of starvation. "The personality necessary for war, the circumstances necessary to goad men to desperation are present, but there is no war."

Mead next addressed the claim that war springs from "the development of the state, the struggle for land and natural resources of class societies springing, not from the nature of man, but from the nature of history." Here Mead seems to invoke Marx as well as Malthus. Just as the biological theory is contradicted by simple societies that don't fight, Mead wrote, so the theory of "sociological inevitability" is contradicted by simple societies that do fight. Hunter–gatherers on the Andaman Islands "represent an exceedingly low level of society," but they have been observed waging wars, in which "tiny army met tiny army in open battle."

Australian aborigines, similarly, occasionally interrupted their wanderings "from water hole to water hole over their almost desert country" to battle each other. They seemed to fight not for any of the usual reasons—the "the struggle for lands, struggle for power of one group over another, expansion of population"—but because war was part of their tradition.

Warfare is "an invention," Mead concluded, like cooking, marriage, writing, burial of the dead or trial by jury. Once a society becomes exposed to the "idea" of war, it "will sometimes go to war" under certain circumstances. Some people, Mead stated, such as the Pueblo Indians, fight reluctantly to defend themselves against aggressors; others, such as the Plains Indians, sally forth with enthusiasm, because they have elevated martial skills to the highest of manly virtues; fighting bravely is the best way for a young man to achieve prestige and "win his sweetheart's smile of approval."

The original motivations for war's invention may have been those mentioned by Mead: conflicts between different groups over food, fertile land, women and status, perhaps driven by overpopulation. But the question remains why war spread so rapidly around the world after its initial invention. After all, unlike inventions such as cooking, agriculture and writing, which have obvious benefits, war is an extremely risky enterprise.

Mead did not directly address this question, but her successors have. The Robarcheks pointed out that war is in a sense "contagious," because when one group in a region resorts to war, "others must either take it up or be destroyed." Keeley, similarly, noted that war among North American Indians often stemmed from the aggression of just a few extremely warlike tribes, "rotten apples that spoiled their regional barrels." He added, "Less aggressive societies, stimulated by more warlike groups in their vicinity, become more bellicose themselves."

Societies in a violent region, the political scientist Azar Gat emphasized in War in Human Civilization (Oxford University Press, 2006[HARDCOVER]), have a strong incentive to carry out preemptive attacks. Societies may "attack the other side in order to eliminate or severely weaken them as a potential enemy. Indeed, this option only makes the other side more insecure, rendering the security dilemma more acute. War can thus become a self-fulfilling prophecy. The fear of war breeds war."

War, in other words, is a self-perpetuating meme. So how can we end it? Contrary to the claims of her critics, Mead was far from a naive optimist. In "Warfare Is Only an Invention" she asked, "If we know that it is not inevitable, that it is due to historical accident that warfare is one of the ways in which we think of behaving, are we given any hope by that?" Not necessarily, because "once an invention is known and accepted, men do not easily relinquish it." Writing at the dawn of World War II, Mead had good reason to fear that militarism had become too deeply embedded in modern culture to eradicate. "The deeds of our warriors are immortalized in the words of our poets; the toys of our children are modeled upon the weapons of war," she wrote.

For an invention to become obsolete, Mead argued, "people must recognize the defects of the old invention, and someone must make a new one." In this way trial by jury supplanted trial by ordeal or combat, which had come to seem "unfair, capricious, alien." She added that "to invent new forms of behavior which will make war obsolete, it is a first requirement to believe that such an invention is possible."

Only on this point do I disagree with Mead. We already have inventions—notably the United Nations—for resolving conflicts peacefully. We just need to use them instead of resorting to the worst invention of all time: war.

Photo of Mead courtesy of Wiki Commons