Why do wars happen? One explanation is that chimpanzees and humans have an innate predisposition to kill “outsiders” when there is little risk because this behavior has provided a reproductive and resource advantage to groups of killers in the course of biological evolution. Such a theory began to be actively developed after a case when two groups of chimpanzees engaged in a “war” in Gombe National Park in 1974–1977. Researchers such as Richard Wrangham, Michael Ghiglieri, and others have stated that this phenomenon is identical to human wars and that both species have a natural predisposition to kill their neighbors, which gives the group of killers an advantage over other groups. However, there is also an alternative view that refutes this theory. Anthropologist Brian Ferguson describes it well in his 2023 book “Chimpanzees, War, and History: Are Men Born to Kill?” and we will now review some of the important evidence presented in it.

To begin with, the number of chimpanzee killings in Gombe is unreasonably exaggerated. Missing individuals are explicitly counted as killed, although they may have simply moved beyond the boundaries of the observed area. Ferguson cites the example of shifting horticulturalists of the Amazon basin, who end conflicts simply by moving farther away from each other. Human impact must also be considered, including the destruction of the chimpanzee habitat and researchers' highly intrusive observation of them. And what may have had a particular impact was the researchers' decision to provide bananas to the chimpanzees. When this led to a number of problems, including clashes between the chimpanzees at the banana distribution site, they had to abruptly stop this practice, which could have resulted in severe frustration for the chimpanzees. It is also important to consider that particular individuals were notably aggressive. For example, two females who were mother and daughter were known for frequently attacking other females and eating their infants. One male was also famous for such behavior, and it was this male, being a belligerent leader, who led the first attack in the “war.”

Observations of chimpanzees after the Gombe “war” reject a number of claims about their behavior. For example, it was thought that if a group of males met one male from another group, they would definitely kill him. But after the “war” in hundreds of intergroup encounters, this did not happen. Attacks provoked by the imbalance of power between groups did not occur anymore as well, although this imbalance was observed very often. There was also very little data to support the assumption that intergroup killings were profitable. What is confirmed, however, is the idea that the “war” was provoked by human impact, such as deforestation and the provisioning of bananas, followed by a cessation of this, causing intense intergroup competition for food.

When the chimpanzee population in Gombe recovered, competition and conflicts returned, but killings were rare. Also, most of them were intragroup, not intergroup, which does not fit with earlier assumptions. And most often, these killings were committed by males who were born and spent their first years of life at the peak of violence, including the attacks of the two previously mentioned females. Moreover, one extremely aggressive male who committed particularly brutal killings, led attacks on other chimpanzees, and even once killed a human child stood out among them. Individual differences definitely play a role in aggressive behavior. Also, chimpanzees, like humans, can carry a low-activity variant of the MAOA gene (also known as the “warrior gene”), which, according to research, predisposes humans to violent behavior, but only in the case of childhood abuse (which the male chimpanzees discussed here were exposed to). And the peak in killings in this case coincided with a new increase in human intervention in the chimpanzee habitat.

We can also mention the example of the disappearance of one group of chimpanzees in Mahale National Park, which is often explained by their extermination by another group. However, it is important to understand that disappearance does not mean killing. These chimpanzees could have simply moved to another location, and there has already been a case in Mahale where an entire population of 70 chimpanzees abruptly disappeared due to deforestation but was later found in another location. And even if they died, it could be due to many causes, such as predator attacks or diseases (which were quite common, also due to human impact).

As strong evidence in favor of the demonic perspective, the case of intergroup violence and killings in Kibale National Park, committed by the Ngogo chimpanzee community, is cited. However, although the chimpanzee habitat was described as pristine, there was, in fact, massive deforestation and growth of agricultural activity. This could eventually lead to a rapid increase in the density of chimpanzees in the remaining areas and problems with food supply. Although there was no general shortage of food and the chimpanzees were generally well fed, preferred foods such as fruits became scarce, and this was a factor causing clashes between them.

Considering all observations of chimpanzees, most adult killings occurred in just two cases: in Gombe between 1974 and 1977 and in Kibale between 2002 and 2006. Of the 27 clearly confirmed intergroup killings of adult or adolescent chimpanzees, 15 occurred in these cases, and 12 occurred in the rest of the long history of chimpanzee observations. As noted, Ngogo killings of outsiders are 23–75 times higher than the median rate suffered by individuals in nine well-studied chimpanzee communities. Overall, the two cases we cited are exceptional; they represent only 2% of the entire history of chimpanzee observations, and without them, intergroup killings of adult chimpanzees would be extremely rare. And if this were an adaptive behavior developed during biological evolution, wouldn't it occur much more frequently? There is very little evidence that such behavior is adaptive, and even if we take the strongest adaptive predictor of killings among those found, which is the population density growth, it was closely related to human impact in all cases.

It has also been claimed that it is common for chimpanzees to conduct raids in which they may exterminate other groups. But this was observed only once in Gombe. There was no “war” in Mahale, and in the case of the Ngogo community, no extermination of neighbors was observed. This means that raids don't even happen sometimes. It was just one case. The common claim that group exterminations are common is without foundation. In addition, the issue of infanticide is not clear, as many of its cases were intragroup, not just intergroup, and there were cases where males were not unlikely to kill their own infants, which clearly does not indicate such behavior as adaptive.

Finally, it is important to note that the demonic perspective, even if it had a foundation, would not apply to modern wars between states, in which leaders command rather than fight themselves, and soldiers follow orders. And in the case of “simple societies,” there is no reason to think of them as predisposed to kill members of other groups. Of course, some hunter-gatherers with complex social structures make wars, but mobile foragers (simple hunter-gatherers), which existed most of human history, are not characterized by this. Although there is individual violence and killings among the members of such societies, with rare exceptions, which some researchers like to focus on, there is no intergroup violence among them.

Upper Paleolithic archaeological evidence (dates to 50,000–12,000 years ago) does not indicate cultural differentiation across space. Everyone, across continental spaces and over long stretches of time, was making the same kinds of tools and participating in open continental-wide social networks, and all of the issues of group boundaries, “traditional enemies,” different ethnicities, and territoriality are simply incompatible with this. Simple societies were extremely flexible, and it would have been impossible if any male outsider trying to contact other groups of men would have been killed immediately. And wars between different groups of people only began to occur when higher levels of social organization emerged. An expanding state is what can introduce violence into an otherwise peaceful population of foragers or horticulturalists.

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