The propensity for humans to kill one another may be hot-wired into our DNA but culture also has the power to change our behaviour, a new study has found.
The research, led by José María Gómez at the Estación Experimental de Zonas Áridas in Spain and published in the journal Nature, explored the human tendency to act violently towards members of our own species.
This topic has been hotly debated among experts, with scores of studies questioning whether this vicious trait is an evolutionary development or the result of our cultural or social dynamics.
The research team proposed a new approach to this question – comparing our rate of violence to that of our prehistoric ancestors.
Violent and aggressive behaviour is exhibited by a range of species in the animal world, but our closest primate cousins are also guilty, which suggests interpersonal violence may play an evolutionary role, with sexual victory presumably at the forefront of such clashes.
“It is widely acknowledged that evolution has shaped human violence,” the researchers write.
“Violence can be seen as an adaptive strategy, favouring the perpetrator’s reproductive success in terms of mates, status or resources.”
The team assessed a whopping amount of data on lethal interpersonal violence – killing within a species – among hundreds of different mammals throughout history.
With these numbers they drew parallels to the evolutionary tree to see where the behaviour may have originated.
Their data included four million deaths dating back as far as 50,000 years, spanning 600 human populations and including more than 1,000 mammal species.
The team calculated the percentage of deaths caused by members within the same species, which might include aggression, infanticide or cannibalism; or, among humans, war, homicide or execution.
According to the findings, interpersonal violence represents about 2% of all deaths across the history of humans. This number was close to the estimates for our evolutionary ancestors, which may suggest a certain amount of our tendency to kill each other is built into us from way back when our species first evolved.
Importantly, the findings show that the killing of other humans varies across different populations, suggesting that social and cultural factors also play a role.
To further investigate these differences, the research split human populations into four distinct types: bands, tribes, chiefdoms and states.
In the ‘states’ group, levels of lethal violence were lower than the standard set by our evolutionary ancestors, which the researchers attribute to the laws about killing in state-based populations.
“It is widely acknowledged that monopolisation of the legitimate use of violence by the state significantly decreases violence in state societies,” the paper explains.
The researchers suggest that as well as having evolved among a particularly violent group of mammals, our social behaviour and territorial tendencies only serve to increase the propensity for violence towards one another.
“We were, at the dawn of humankind, as violent as expected considering the common mammalian evolutionary history,” the paper claims.
Luckily, this isn’t the end of the story. Our level of violence “has changed as our history has progressed, mostly associated with changes in the socio-political organisation of human populations”.