Why Humans Go To War

Humans, chimpanzees, and wolves are the only mammals that form coalitions with other members of their species to inflict deadly violence on yet other members of their species. Why do we do it?

We may have evolved an instinct for war. Support for this idea comes from what anthropologist Richard Wrangham calls the Chimpanzee Violence Hypothesis. The Chimpanzee Violence Hypothesis has its origins in Jane Goodall’s observation that adult male chimpanzees often collaborate to kill or severely wound chimpanzees of neighboring communities in a manner reminiscent of human war raids. This has led some anthropologists to suggest that there could be an evolutionary link between chimpanzee violence and human warfare. If those anthropologists are right, then maybe we can understand human warfare better if we can figure out the nature of chimpanzee violence.

What drives chimpanzees to form groups and go out and kill chimpanzees in neighboring territories? Plenty of primates form coalitionary alliances but don’t carry out lethal raids against neighboring groups.

What is unique to chimpanzees is the relative scarcity of resources in their natural habitat and the imbalance of power between groups in the wild. Because of their diet, which is specialized for eating ripe fruit, chimpanzees are vulnerable to what’s known as “scramble competition.” Scramble competition is when there aren’t enough resources for every member of a group, but when every member of the group has – theoretically – equal access to those resources. In the case of chimpanzees, any of them can wander around looking for fruit, but often there aren’t enough fruit for everyone. And one territory can have an abundant supply of fruit whereas in another territory food is scarce.

Chimpanzees display violent behavior.
Chimpanzees display violent behavior.

What ends up happening is that chimpanzees from the territory with a more scarce supply will sometimes split up into groups to look for fruit, or a lone chimpanzee from a larger group will go off on his or her own to look for fruit. When chimpanzees split off like this, they are vulnerable to attack from a group of chimpanzees from another territory that might be making an incursion into the territory of the vulnerable group or patrolling the border between the two territories. For chimpanzees, an imbalance of power coupled with other species-typical behaviors – namely territoriality and hostility to outsiders – spells bloodshed. If a relatively large group of chimpanzees encounters a solitary chimpanzee or a small group of chimpanzees from another territory “scrambling” for food, the larger group is likely to attack. This usually occurs when one group clearly outnumbers the other (with a mean ratio of 8:1).

The result? Anthropologist Richard Wrangham writes: “By wounding or killing members of the neighboring community, males from one community increase their relative dominance over the neighbors. According to the imbalance-of-power hypothesis, the proximate benefit is an increased probability of winning intercommunity dominance contests (nonlethal battles); this tends to lead to increased fitness of the killers through improved access to resources such as food, females, or safety.”

Since this sort of coalitionary killing has clear evolutionary advantages, why isn’t it observed in bonobos, who are the closest relative to both chimpanzees and humans? There are a number of possible answers. First, bonobo social hierarchies work differently from those of chimpanzees and humans: female-female coalitions dominate, and males gain social status by aligning themselves with high-ranking females. This could lead to an overall reduction in male aggression or the likelihood for males to form violent coalitions. But this theory is problematic, since there are species such as lions that are similarly matriarchal and peaceful within their own group, but whose females are aggressive to members of outside groups. And since there is a clear evolutionary advantage to intergroup aggression (improved access to resources), something else has to explain bonobos’ peacefulness.

It turns out that bonobos don’t face the same ecological pressures that chimpanzees face. Resources in bonobo habitats are usually abundant, so bonobos tend to form groups of roughly equal size and rarely travel alone or in small groups in search of food. In other words, bonobos don’t face the imbalances of power that chimpanzees do. So there may have been evolutionary pressure against coalitionary violence in bonobos: the relatively equal sizes of different groups means that one group of bonobos can’t easily dominate another, and the relative uniformity of food supply in bonobo habitats means that there isn’t much reason to attack. In bonobos, an instinct for aggressiveness or violence to outsiders would work against evolutionary fitness.

Bonobos are an extraordinarily peaceful species.
Bonobos are an extraordinarily peaceful species.

How does all this apply to humans? First off, we know from archaeological data that early humans frequently died at the hands of other humans. We can also infer from archaeological data that this violence was often coalitionary, where groups of humans would raid the territories of other humans and kill some of their members. Even among present-day humans, raids are the most common form of war in small-scale independent societies. It is also reasonable to presume that prehistoric humans participated in scramble competition like chimps do, given their diets and relative lack of resources. Finally, the sheer ubiquity of prejudice, hatred of outsiders, and the formation of biases against members of an “out-group” in developed, contemporary human societies suggests that intergroup hostility was common in human prehistory as well. The imbalance-of-power hypothesis, which describes violence between groups of chimpanzees, is therefore likely to apply to early humans, barring future archeological evidence against this hypothesis.

But understanding the evolutionary pressures acting on chimpanzees and prehistoric humans doesn’t clearly explain modern warfare. Humans who live in modern, industrialized states don’t engage in simple raids, and the scope and definition of “in-group” versus “out-group” look nothing like those of chimpanzees or our prehistoric ancestors. Militaries also look nothing like the violent coalitions of chimpanzees and early humans: soldiers, often reluctantly, are simply following orders, whereas participation in raids in pre-state cultures is usually voluntary.

Almost certainly, our instincts to be hostile to humans who are not in our in-group and our propensity for attacking them when we have enough to gain intermingle with the complexities of modern culture and politics to produce the complexities of modern warfare. Anthropologist Richard Wrangham writes:

The imbalance-of-power hypothesis may explain why culturally derived information is used in certain ways. For example, as a result of cultural beliefs or social pressure, individuals can either broaden or contract their concept of where an in-group/out-group boundary falls, or of how important it is. Idealogues can persuade their followers that sufficient power asymmetry exists to make attacks on an outgroup worthwhile. Culture can thus manipulate the information an individual uses to assess whether an attack is desirable. (Wrangham, 1999). 

In other words, cultural mechanisms might warp our perception of factors that are normally entered into the calculus of warfare, but what ultimately drives us to war isn’t so dissimilar from what drove our prehistoric ancestors to coalitionary violence.

Moving beyond explanations that rely on evolutionary biology, political theorists are beginning to develop an ecology of modern human warfare. This branch of research has shown us that much like prehistoric human or chimpanzee conflict, contemporary human conflict usually boils down to resource scarcity and how we deal with it. Geographer Philippe Le Billon suggests that although few wars are sparked by resource control conflicts, resource allocation has a part to play in almost every contemporary war. He argues that while “it would be an error to reduce armed conflicts to greed-driven resource wars, as political and identity factors remain key, the control of local resources influence the agendas and strategies of belligerents.” And while many current wars aren’t directly about resources, they are often products of earlier mercantile wars that were explicitly about resources. And if we are to believe anthropologists, resource allocation is an integral part of human and chimpanzee violence. When we want the resources of another group, and when we think we have a clear advantage over that group, we attack.

But, of course, the picture of modern warfare is a complex one. Not all resource-rich countries go to war with weaker resource-poor neighbors. As Le Billon suggests, whether countries go to war is largely dependent upon the history of those countries’ relations. As such, to understand why states go to war requires both anthropological and political analysis.

Modern warfare is far more complex than prehistoric intergroup violence
Modern warfare is far more complex than prehistoric intergroup violence

While scientists and political theorists don’t yet fully understand why we go to war, it should be clear from what we’ve considered that unfortunately violent conflict is part of our nature. Research in primatology and anthropology largely corroborates Thomas Hobbes’ conviction that the life of early humans was “nasty, brutish, and short.” And the contemporary landscape of human relations can sometimes seem as or more bleak.

But there is reason to be optimistic about our future. Psychologist Steven Pinker convincingly argues in The Better Angels of Our Nature that there has been a marked decrease in human violence over the course of history. The likelihood of a human dying a violent death now is far less than it was hundreds or thousands of years ago. We may look at the horrors of the 20th century and surrender ourselves to despondency, but in fact even the 20th century was statistically less violent than previous centuries.

While the reason for this decline in violence is unclear, Pinker offers a number of theories. One is that the modern state has a monopoly on power and can thus maintain peace within its borders; another is that globalization through commerce and technological progress is bringing more and more of humanity into our “in-group”; another is increasing respect for women and women’s values (I’m reminded here of bonobos); yet another could be increasing literacy rates and media consumption, which could allow people to see and take the perspectives of others; and finally, it could be the increasing application of reason and rationality to human affairs, which can, as Pinker writes, portray violence as “a problem to be solved rather than a contest to be won.”

But if we are to believe the anthropological story of how we evolved a war instinct, or if we accept the notion that resources play a crucial role in the ecology of contemporary warfare, I would like to suggest that increasing universal access to resources is as good an explanation for the millenia-long decrease in violence as what Pinker proposes. And if that’s the case, then perhaps we should think seriously about how to move humanity toward a fully post-scarcity society, or at least a proto-post-scarcity society. That, I think, may be a surer path to peace.

Regardless of what the right answers are – what the true nature of human violence is and how we can bring about peace – what I have written here has been in the spirit of Pinker’s last theory for why humans are becoming more peaceful, which is that applying reason to human behavior can help us think more rationally about violence. Though we do not yet fully understand human violence, the continuing march of rational inquiry makes me think that one day we will. And this makes me hopeful, because the better we understand war, the better we can predict and prevent it.


Le Billon, P. (2001). The political ecology of war: natural resources and armed conflicts. Political geography, 20(5), 561-584.

Pinker, S. (2011). The better angels of our nature: The decline of violence in history and its causes. Penguin UK.

Wrangham, R. W. (1999). Evolution of coalitionary killing. American journal of physical anthropology, 110(s 29), 1-30.

Wilson, M. L., Boesch, C., Fruth, B., Furuichi, T., Gilby, I. C., Hashimoto, C., … & Wrangham, R. W. (2014). Lethal aggression in Pan is better explained by adaptive strategies than human impacts. Nature, 513(7518), 414-417.


Cover image: Fury of Achilles by Charles-Antoine Coypel

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