By: Helen Beilinson
Why are humans violent?
This is a dense question that has been heavily debated for centuries. There are, very simply, two camps—nature and nurture. The former has been popularized by the seventeenth-century philosopher Thomas Hobbes, who argued that the natural state of man is one of violence and independent perpetuation, that humans are naturally, inherently violent. On the other hand, Jean-Jacques Rousseau retorted, nearly a century later, that individuals are not born violent or peaceful, but instead, are molded by their environments. Outside of philosophy, social scientists have tackled this distinction by focusing on the nurture side, analyzing how sex, age, race, and socio-economic status can influence an individual’s propensity towards violence. In a recent issue of Nature, Spanish scientist José Mariá Gómez and colleagues took to answering this question from a different, unique angle—evolution.
Lethal conspecific violence, or violence occurring between members of the same species, is not unique to humans, from infanticide in primates and dolphins to horses and hamsters attacking their own. The prevalence of aggression throughout mammals, and its high inheritability, questions whether evolution has shaped human violence due to intraspecies violence being an adaptive strategy for survival. To address this question, the authors of this study used comparative methods from evolutionary biology to quantify the levels of violence in 1,024 mammalian species, including those that are currently extinct. They assessed causes of death in over 4 million instances, defining the level of lethal violence in a species as the probability of dying from intraspecific violence compared to other causes.
Out of the analyzed species, nearly 40% had instances of conspecific lethal violence, with, on average, 0.30% of deaths within a population occurring at the hands of those in the same species. The authors calculated phylogenetic signals of related species to analyze the evolution of lethal aggression in mammals. This signal is essentially a measure of how lethally violent a particular species is in comparison to other closely related species. They found that lethal violence was entirely absent from some species, like bats and whales, and was more frequent in other groups, such as primates. Lethal violence was at similar levels in closely related species, speaking to its heritable aspect.
Within primates, one of the most notoriously violent groups of animals, there were differences in levels of violence, indicating violence’s evolutionary flexibility. While chimpanzees were highly violent, bonobos were tamer. This observation drove the authors to ask whether other factors could influence violence within a species. The authors subsequently scored the analyzed species for territoriality and social behavior, two traits that could drive aggression. They found that social, territorial animals had high levels of lethal violence than solitary, non-territorial species.
Studying the evolution of and phylogenetic signals for lethal violence in mammals as a whole provided a basis for studying the violence in humans. In addition to the animal species studied, 600 human populations were analyzed, ranging in time across human history, from the Paleolithic era (~2 million to 10,000 years ago) to the present.
We emerged from the primate line, with a long evolutionary history of higher-than-average levels of conspecific lethal violence, so it is unsurprising that at the origin of our species, human lethal violence accounted for 2% of all deaths, six times higher than the reconstructed mammalian value. Additionally, we as Homo sapiens are both social and territorial, trains with stronger tendencies towards lethal violence in mammals.
Over human history, the estimates of lethal violence vary greatly. Although Paleolithic estimates were close to 2% of deaths were due to lethal aggression, estimates rose as high as 15-30% in various times throughout history, peaking about 3,000-5,000 years ago. Today, levels of lethal violence have decreased markedly. The authors claim that socio-political organization was a significant factor in the changes in violence. They found that there was a correlated rise in violence when human moved from pre-societal organizations, including bands and tribes, to more modern organizations like chiefdoms and states. However, although high population densities in most mammalian species drive lethal aggression, in humans, population increases were consequences of successful pacification, leading to less violence.
Although the news is packed daily with stories of human-on-human violence, today, less than 1 in 10,000 deaths (about 0.01%) are due to lethal violence. Based on the model put forth by Gómez, this translates to humans being about 200 times less violent today that can be predicted by our evolutionary past. Even a lethal violence rate of 0.01% is too high; there is a lot of social and political work that needs to be done to lower this incidence as much as possible, ideally to zero. A violent past and phylogenetically inherited lethal violence have set up modern humans to be naturally violent creatures, nevertheless, it is clear that culture, be it social or political, can strongly influence and modulate levels of aggression in a population.