War is often seen as a defining feature of human history, but its origins remain hotly debated. Some scholars argue that warfare is deeply rooted in our evolutionary past, while others claim it is a recent by-product of agriculture and complex societies. At the center of this debate lies a crucial question: why do men in particular engage in war?
If war is defined narrowly as organized armies under central command, our early ancestors could not have fought one. But if war is understood more broadly as groups banding together to kill members of other groups, then it becomes possible to see evidence of it deep in human history. This form of conflict, called intergroup coalitionary killing, occurs in other species as well. Chimpanzees conduct lethal raids, wolves attack rival packs, and even ants wage territorial battles. Humans fit within this broader pattern, suggesting that organized violence predates agriculture.
Our closest relatives offer important clues. Chimpanzees are notorious for their raids, in which coalitions of males ambush members of neighboring groups. These attacks are not random acts of aggression. They bring rewards in the form of new territory, access to females, and enhanced status for the participants. The risk of injury or death is spread across the group, while the benefits are concentrated among the males who succeed. Bonobos, in contrast, are often described as peaceful. They display tolerance across groups, share food, and resolve conflict through social bonding and sexual behavior. Yet even bonobos engage in aggression, with injuries documented in many intergroup encounters. Their peaceful reputation may stem from strong female alliances and reduced feeding competition, conditions that limit the benefits of lethal violence. Taken together, these two species show that coalitionary violence is a real possibility, but not an inevitable outcome.
Why men rather than women? Evolutionary theory points to differences in reproductive stakes. In most societies, men’s reproductive success is highly variable. Some men father many children while others leave none. War can dramatically alter those odds. Victorious men can seize resources, captives, and prestige, all of which can increase their reproductive opportunities. Even trophies or symbols of success can elevate social standing and attract mates. Women, who bear the higher biological costs of reproduction, are less likely to gain from the risks of warfare. This asymmetry helps explain why men are the primary participants in raiding and war across cultures.
Evidence from hunter gatherer societies supports the idea that men have long engaged in organized violence. Far from being uniformly peaceful, surveys show that the majority of forager groups practiced some form of warfare. Their conflicts were not large battles but raids and ambushes, tactics that minimized risk while still providing the possibility of killing rivals or taking captives. In pre-colonial Australia, warfare was common across the continent. In the Andaman Islands, raiding was routine, but peace ceremonies allowed groups to restore relationships and form alliances. These cases suggest that conflict was not an aberration but part of the rhythm of life.
One explanation for warfare is resource scarcity. Some studies find that when food or land becomes scarce, violence increases. But the evidence is inconsistent. In some regions with dense populations, violence was low, while in others with sparse populations, it was high. Low population density did not prevent warfare. Chimpanzees often live at lower densities than humans yet still carry out lethal raids. The critical factor seems to be whether the benefits of attacking outweigh the costs. When men believe that raiding will bring rewards in the form of land, mates, or prestige, they may choose violence regardless of ecological conditions.
At the same time, humans have always sought ways to cooperate across group boundaries. Marriage alliances tied families together. Trade networks carried goods over thousands of kilometers long before agriculture. Archaeological evidence shows that stone tools and beads were exchanged across vast distances tens of thousands of years ago. Rituals also played a role. In the Andaman Islands, hostile groups came together in ceremonies that expressed anger through symbolic gestures before ending in reconciliation. Similar practices existed in Australia and Tierra del Fuego. These traditions show that while war was present, peace was equally possible and often actively pursued.
Men engage in war because it has often provided rewards that outweighed the risks. By joining coalitions, men could increase their chances of survival, gain status, and improve their reproductive success. Yet war was never the only path. The same capacities that allowed men to coordinate violence also allowed them to build alliances, trade, and forge peace. Human evolution left us with both capacities. The persistence of war reflects one side of that legacy, but the possibility of peace reflects the other.