Why Humans Kill: Uncovering the Truth About War — Part I
The Deep Roots of War Are a Part of Human Nature
Humans have engaged in warfare with disturbing regularity over the course of history and pre-history. But what really explains this? Are wars typically just tragic mistakes—or perhaps unfortunate glitches or foibles in the brain? Or could they be a product of our evolutionary heritage? Researcher Mike Martin’s Why We Fight and Tel Aviv University scholar Azar Gat’s War in Human Civilization develop penetrating explanations of the fundamental causes of war.
But the explanations might strike some as both illuminating and disturbing. What makes the pair of books unique is their focus on the big picture of war, and their deft integration of disparate and relevant bodies of empirical evidence—ranging from evolutionary psychology, anthropology, history, archaeology, cognitive science, neuroscience, and ethnography.
(And notably, both books are mutually reinforcing and consilient with each other—which is an epistemic goal that the natural and social sciences ought to aim towards.)
Martin’s framework for understanding the causes of war is unashamedly Darwinian—the opening chapter’s epigraph is even the well-known quote from the famous evolutionary biologist Theodosius Dobzhansky, in which he pronounced the dictum that “Nothing in biology makes sense except in the light of evolution”. Martin argues that war will remain mysterious or puzzling if we don’t probe its evolutionary roots. In his words:
I wrote Why We Fight because I found that my experiences as a British soldier in Afghanistan, and as a conflict scholar thereafter, were unsatisfyingly discordant. The lived experience is, by definition, an individual experience—whereas conflict scholarship, and the wider social sciences, seek to understand phenomena on larger scales. Why We Fight attempts to integrate these two views using the incredible advances that have been made in the cognitive sciences over the last thirty years. (p. 5)
A central component of Martin’s account is that humans desire status and belonging, subconsciously if not also consciously. Both status and belonging are key proxies for evolutionary fitness—which is to say, things that tend to increase people’s reproductive fitness, or at least did so ancestrally during human evolution. Gaining membership in a group provided our ancestors with crucial resources like security, trading partners, and access to mates; and the fitness benefits of being in a group significantly outweighed whatever costs group membership tended to have.
And throughout history, leaders have often engaged in warfare to increase their status. Warfare boosted a leader’s status within their own group and gave them access to more of the proxies of status, such as wealth and land; it could even give them greater direct sexual access to women, who are a crucial and scarce reproductive resource. As he puts it, “ … leaders who start wars are pursuing status, just the same as people who kill in gangs, or in bar fights—and that these different forms of violence share the same, evolutionarily-shaped drives” (p. 11).

Leaders also rely on followers to successfully wage war; and followers are often incentivized to join war parties in order to remain in a group—or gain membership in one—and or to elevate their status. At the very least, Martin argues that because participating in war provides men with a way of belonging to a group and or acquiring further status, or proxies of status, men often feel strongly motivated to fight wars. This is so even despite the costly risks of warfare, where death or being maimed are real possibilities.
For instance, it’s commonly observed that men in small military squads or platoons become deeply attached to their fellow soldiers—even to the point of thinking of themselves as brothers. One interpretation of this phenomenon is that the close bonds that soldiers form with one another co-opts—or hacks—our evolved psychological adaptation that governs how we think and feel about kin. In other words, these close bonds make the evolved mind of men think and feel as if their fellow soldiers are biological brothers.
Another central component of Martin’s account is that these two desires—for status and belonging—are the main causes of war. His reading of the scientific literature is that subconscious desires for status and belonging are the real drivers of war, and that conscious reasoning cooks up rationalizations designed to persuade others in your group why fighting a particular war is just or worthwhile.
Indeed, there’s a solid scientific grounding for this general picture of human cognition. Humans typically feel a certain way and then justify that feeling after the fact with a verbal (or written) justification. Thus, according to Martin, when invoked in the service of war, religions and ideologies are mere concoctions that people deploy to persuade others about the supposed rightness of a war; but the real movers are the subconscious desires for status and belonging. In Martin’s words, “reasoning thus confers a fitness advantage in our socially competitive milieu, by making our selfish human drives more acceptable to others” (p. 9).
A key reason why Martin argues for this position is his experience as a soldier in the Afghan War. He and his colleagues were told by his superiors and other authorities that the Taliban and other counterinsurgents were motivated by religious extremism—violent jihad in particular. But, as time passed, what he and others on the ground discovered during their direct interactions with Afghan combatants was that the real motives of those combatants revolved around more concrete concerns.
These included things like old feuds with rival families and rival tribes; disputes with rivals over land and resources; and perceived violations of their honor—including perceived humiliation—after having witnessed family members targeted by foreign military strikes but being powerless to protect them. In his view, Afghan combatants would typically rationalize why they fight by appealing to Islam—for example, by claiming that they were protecting the religion and its adherents from foreign interlopers or from locals who they thought were distorting the faith.
It certainly appears that Martin thinks that religions and ideologies are not a fundamental cause of war. However, I would argue that it might not be quite as straightforward. Even if the subconscious desires that he points to are typically the main drivers of war, it still is plausible that religions or ideologies can, at least sometimes, interact with these desires such that they make war more likely.
To take two historical case studies, for instance, it might be that while the two underlying drives for status and belonging are powerful forces that push towards war, religious rationales were quite plausibly necessary causes for the global military-led expansion of Islam and, later, the Crusades—necessary causes that interacted with the desire for status and or belonging.
For instance, the direct appeals to religious rationales that leaders made in both cases plausibly were the decisive factors that persuaded followers and soldiers to fight—even though they were already primed to be quite receptive to such warfare, in virtue of the fundamental drives for status and belonging.
Counterfactually, in a world where neither Islam or Christianity existed, it is possible that something analogous to the military-led expansion of Islam and the Crusades could have still happened absent any religious rationales. Namely, there could conceivably have been military conquests originating in Saudi Arabia in the Middle Ages, and at least one military expedition originating in Europe and pushing into the Near East sometime in or around the 11th- to 13th-centuries. But these counterfactual scenarios are not straightforwardly obvious.

If we take the view that ideologies and even religions arose through cultural evolution, then it stands to reason that they, too, are shaped by evolutionary motives. For instance, a prominent theory argues that pro-social religions evolved because they made groups more cohesive and more effective at competition with other groups. Hence, the individuals in these more cohesive and more competitive pro-social groups would on average gain fitness benefits, all else being equal.
In other words, according to this theory, pro-social religions evolved in the first place because they delivered increased reproductive fitness to people within the religious group. This form of “cultural group-selection” is rather different from the two forms of genetic group-selection. (As Martin rightly notes, genetic group-selection is controversial in evolutionary biology; but we don’t need to enter into that thicket here.) In cultural group-selection, groups do not need to produce offspring groups, like how a cell undergoes mitosis and begets duplicates of itself.
Here the process refers to the cultural evolution of religion itself, but as a response to selection pressures posed by other rival groups, who may also have their own religion. (It’s an open question whether religions also in turn affected genetic evolution—via the co-evolution of genes and culture—but that is a separate issue, and also distinct from processes of genetic group-selection.)
Likewise, other ideologies might evolve in a similar manner—their connection to evolutionary motives, however opaque or indirect, may still be there. After all, a big reason why contagious ideas spread, in general, is because they cohere quite well with evolved human nature, which itself was, of course, powerfully influenced by the Darwinian imperatives of survival and gene replication. And this can be true even if the ideas themselves do not increase someone’s reproductive fitness—which can be the case, for instance, in evolutionarily novel environments. A famous example of a maladaptive belief, in this regard, is suicidal martyrdom—which is likely to decrease someone’s fitness, all else being equal.
On the other hand—and just as Martin describes—religions and ideologies can serve as post-facto rationalizations for warfare. That is to say, the status and group-belonging motives that individuals have, which can sometimes motivate them to engage in war, are merely rationalized, after the fact, with recourse to religions and ideologies.
Martin makes several other interesting empirically backed claims. For instance, historically, as group sizes have increased, the rate of violence within groups has declined. And as the total number of groups around the world has decreased, inter-state war has also declined. And as group sizes have increased, both the number of combatants and the rate of casualties in inter-state wars has declined as well.
A sizeable section of the book also explores a main—if not the main—evolutionary function of moral codes, religions, and ideologies, namely to solve the five chief problems of group living: identity formation; the rules governing status hierarchies; how to keep disease away from the group; the rules regulating trade between members of the group; and the conditions in which people can be punished—for rule violations, moral indiscretions, blasphemy, and so on—and how they’re to be punished.
Martin also discusses how the hormone oxytocin, which first evolved to facilitate the bond between mother and child, was later co-opted to also bond people together in groups. The oxytocin system helps to increase trust between people; and Martin argues that it also helps to demarcate the in-group from out-groups.
Martin also makes an argument for why war has persisted over the course of human evolution, despite its mortality risks—and thus why a propensity to fight wars, even without compulsion, remains, at least in some men:
The positive emotions associated with war suggest that, despite loss of life, it must have paid off evolutionarily in the past (and probably continues to do so). Had the increased reproductive benefits attached to going to war diminished to below the existing loss of reproduction from its roughly 25 per cent death rate,37 then warring would have been selected out of the population as a behaviour. It is not enough to say that people fight in wars because they are exciting. War is exciting because our ancestors received an evolutionary benefit from it, and hence we are motivated to prosecute it. (pp. 16-17)