Michael L. Wilson and Richard W. Wrangham

In a recent opinion piece for Scientific American, John Horgan writes “Ending war won’t be easy, but it should be a moral imperative, as much so as ending slavery and the subjugation of women. The first step toward ending war is believing it is possible.” As part of his argument he declared that “far from having deep evolutionary roots, [war] is a relatively recent cultural invention.” We cherish the goal of ending war, but regard the claim of war being independent of our evolutionary past as unjustified, irresponsible and improbable. It is possibly based on a misunderstanding of what it means for a behavior to have evolutionary roots, but regardless of its derivation, we see it as dangerous. The problem is that if Horgan’s view prevails, we close our minds to potentially important contributions towards understanding the causes of wars.

Like any other sane individual aware of the power of nuclear weapons, we of course agree that is desirable to reduce major wars to zero. We consider this goal to be part of the “possibilist agenda” —  an approach that begins by ruling out what is impossible, given the laws of the universe, rather than pessimistically focusing on what results are most probable to occur. ¹  Is ending war possible? Well, we don’t know of any laws of physics, or even principles of evolutionary biology, that would make it impossible to end war. The task before us remains finding the best means to achieve this goal. This includes gaining a clear, scientific understanding of what war is, what its causes are, and what factors can prevent it.

There are reasons for optimism. The examples Horgan cites shows that extended peace is a reasonable possibility, given that wars have been absent for decades in parts of the world. As Stephen Pinker described in exhaustive detail in his 2011 book, The Better Angels of Our Nature, deaths from international warfare declined steeply after 1945. Many of the richer countries that formerly engaged in long and bitter wars with one another — the United States, Japan, Germany, France, England — are currently closely allied in long-term friendships. The United States and Canada, for example, haven’t fought each other in earnest since the War of 1812, and as each other’s largest trading partners, benefit greatly from trading rather than invading. At the same time, the rise of authoritarian and nationalistic leaders worldwide, and events such as Russia’s invasion of Ukraine, provide reasons for concern. 

We also agree that much about war involves “relatively recent cultural inventions.” Certainly contemporary weapons are entirely different from those used by the hunter-gatherers of Nataruk, Kenya, 10,000 years ago to execute their enemies, or by the several hunter-gatherer societies of India’s remote Andaman Islands who lived in permanent war with each other. Equally novel are the hierarchies that give leaders power to order warriors into battle, and the military training that promotes efficient tactics, and ideologies that encourage self-sacrifice even to the point of suicide, and the assembly of huge armies. There are many such novelties. But so what? They cast no direct light on the question of the antiquity of war, or more importantly on the question of whether the possible practice of lethal intergroup aggression by our remote ancestors has left a legacy in the human psyche.

Whether war has deep evolutionary roots remains an unsettled topic, contrary to Horgan’s assertions. For example, Horgan links to his 2016 blog post, in which he describes a study led by Hisashi Nakao of skeletal trauma in hunter-gatherers living in Japan 12,000 to 2,800 years ago, which reports that only a small proportion of skeletons showed evidence of violent death. However, as noted in a 2020 review by Mark Hudson and colleagues, “[i]ndividual finds of arrowheads embedded in human bones and other similar, clear-cut traces of violence have long been known in Japan as elsewhere.”  Although Nakao and colleagues consider their findings to represent a low rate of violence,  their sample includes cases of “blunt force injuries to the cranium and embedded stone and bone projectile points in the post-crania with no signs of healing – providing a prevalence of 1.8 per cent of adults dying violently.”   

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Do these data indicate a high or low rate of death from violence? The actual rate of deaths from violence is difficult to determine from archaeological evidence alone: not all deadly violence damages bones, and only a fraction of deaths end up in the archeological record. But if we take the rate of 1.8% adult deaths from violence at face value, this is about 2.5 times the rate of death from homicide in the United States in 2020², a year marked by a dramatic increase in violence. What does this tell us about the evolutionary roots of war? It provides additional evidence that people in prehistory faced a threat of death from violence, but also provides additional evidence that rates of violence vary across space and time, for reasons that are worth exploring further. 

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For anyone seriously interested in understanding the causes of war many other kinds of evidence are thought-provoking. Warfare is documented among hunter-gatherers in every inhabited continent, and was especially prominent where the neighbors were also hunter-gatherers rather than dominant farmers. Chimpanzees, our evolutionary cousins, practice a form of intergroup aggression that is strikingly similar to the hit-and-run aggression characteristic of human small-scale societies. In chimpanzees and in humans the initiators and the victims of lethal intergroup aggression are overwhelmingly males. In both species, males are more aggressive than females, a difference that starts long before maturity.

Why are chimpanzees and humans two of the few species of mammals that regularly take advantage of opportunities to kill members of neighboring groups? Are the reasons wholly different in the two species, or do they share some aspects of motivation and explanation, in which case, what are they? To rule out such questions is unworthy of an essay in a scientific journal. They concern topics being actively explored in behavioral ecology, evolutionary psychology and the neurosciences, among other areas.

There is much to learn, but we believe that some current hypotheses are helpful. One is that warfare has evolved from the human ability for proactive aggression. Proactive aggression involves making plans to attack vulnerable victims using overwhelming power, a practice followed by chimpanzees and humans. It benefits the aggressors by leading to increased territory size, or greater safety. Importantly, however, proactive aggression is very predictably inhibited when potential victims turn out to be sufficiently well defended that the aggressors risk getting hurt. Thus, even though proactive aggression is reined in for most of the time, it pays dividends under some circumstances; and human brains are likely very sensitive to such assessments. Would Russia have dared to invade Ukraine in 2014, much less escalated its attack in 2022, if NATO had accepted Ukraine’s application to join in 2008?

The challenge of preventing major wars is mainly undertaken by politicians and lawyers, but we think that every contribution might help. If a deep legacy of using coalitionary proactive aggression has left its mark on the human psyche, what does that mean for differences in war motivation between men and women? Or for how war leaders perceive the costs of wars in which they are directors rather than participants? Or for the point at which leaders will perceive the benefits of peace as outweighing the costs of war? Or for how individuals categorize others as friend or foe? Is it possible that the psychological mechanisms for evolutionarily-based risk assessments are predictably affected by certain medicines that could make leaders more or less risk-prone? If so, we should all be especially alert to a leader’s ailments. To ignore such questions is to bury one’s head in the sand.

Retaining an open mind about the etiology of war is surely the wisest course in a world still threatened by major conflict, despite the many institutional efforts, from the UN down, to control it.

Additionally, whether war has deep roots in our psyches, or is a recent invention like agriculture and empires, evolutionary game theory suggests that in looking to the future, we need to consider, not just our hopes and dreams, but also the strategies available to all players in the game. Players don’t act in a vacuum: they act in a world of competing strategists. We may prefer to plan for peace rather than war, but if other players pursue war as a strategy, we must either prepare to oppose them, or be defeated by them.

Russia might one day fulfill its promise of becoming a Canada to Europe and Asia: a peaceful, prosperous neighbor to the north. In the meantime, however, Russia’s leaders remain trapped in a zero-sum view of the world, seeing neighboring states more as opportunities for conquest than commerce. 

As an example of a fatalism that will lead to more war, Horgan offers this quotation from Kaja Kallas, the prime minister of Estonia: “Sometimes the best way to achieve peace is to be willing to use military strength.” People living in Estonia, however, have bitter historical experience with being invaded and occupied. If you live next door to Putin’s Russia, rather than Trudeau’s Canada, the best way to prepare for peace may indeed be to prepare for war. International peace will not be achieved by simply giving in. Peace among nations surely requires making the costs of war so high that when appeals to law or morality fail, potential aggressors recognize that it is in their own best interests to find a constructive solution.

References

Horgan, J. (2016, April 4). Japanese study deals another blow to deep-roots theory of war. Scientific American Blog Network. Retrieved May 12, 2022, from https://blogs.scientificamerican.com/cross-check/japanese-study-deals-another-blow-to-deep-roots-theory-of-war/ 

Horgan, J. (2022, April 27). Will war ever end? Scientific American. Retrieved May 12, 2022, from https://www.scientificamerican.com/article/will-war-ever-end/ 

Hudson, M., Schulting, R., & Gilaizeau, L. (2020). The Origins of Violence and Warfare in the Japanese Islands. In G. Fagan, L. Fibiger, M. Hudson, & M. Trundle (Eds.), The Cambridge World History of Violence (The Cambridge World History of Violence, pp. 160-178). Cambridge: Cambridge University Press. doi:10.1017/9781316341247.009

Nakao, H., Tamura, K., Arimatsu, Y., Nakagawa, T., Matsumoto, N., & Matsugi, T. (2016). Violence in the prehistoric period of Japan: The spatio-temporal pattern of skeletal evidence for violence in the Jomon period. Biology Letters (2005), 12(3), 20160028.

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1.  “Following the possibilist agenda means working tirelessly to imagine both possibilities and impossibilities and then laying plans to arrange events so that the desirable can be realized and the undesirable avoided—working to avoid unintended consequences. In this way, by superposing such thoughts onto recognized physical–biological–social problems of the world . . . seemingly intractable problems may have possibilist solutions.”  PNAS [↩]
2. Proportion of deaths in the United States from homicide = 0.73% in 2020, based on 24,576 homicide deaths and 3,358,814 total deaths [↩]