The roots of lethal violence between humans can be found in their evolutionary tree, researchers have said. Violent behaviour directed towards members of the same species has been documented in other primates, and may have been inherited by humans throughout their evolution. The consequences of such behaviours can however be mitigated by society.
The philosophical question about whether humans are inherently violent, or if murder, conflict and violence depend on cultural factors is an ancient debate. Already in the 18th century, French thinker Jean-Jacques Rousseau argued that violence was shaped by our environment.
Since then, social scientists have attempted to explain how human interpersonal violence is linked to external cultural forces rather than to human nature.
Undoubtedly, cultural factors are in part to blame when violence surges within communities. Take the latest US murder rates, which have gone up by 10.8% since last year, according to data released by the FBI - the biggest percentage increase since 1971. This increase is likely due to the socio-economic context more than to evolutionary processes.
Yet, it has previously been shown that aggressiveness in humans has a genetic component with high heritability which suggests that evolution has also shaped human violence.
The recent study published in the journal Nature agrees that levels of violence between humans vary with changes in socio-political organisation of populations, but it also reinforces the idea that lethal violence has a biological basis, which can be traced back on the human evolutionary tree.
More than four million deaths
The researchers - led by José María Gómez from Estación Experimental de Zonas Árida in Spain -point out that violent behaviour directed towards members of the same species is common in primates, but is less frequent in other mammals, such as whales. They wanted to find out how humans compared with other mammals.
Compiling data from over four million deaths, the researchers calculated the proportion of deaths attributable to violence from a member of the same species, in 1,024 mammal species from 137 taxonomic families and in about 600 human populations, ranging from about 50,000 years ago to present.
Using this data and with the help of evolutionary trees, the authors reconstructed the rate of lethal violence caused by members of the same species at the origin of all mammals at about 0.30%. Rates of lethal violence then increased steadily over time, as the reconstructed ancestors became more closely related to primates.
At the origin of our species, the scientists predicted that the proportion of human deaths caused stood at 2%. This figure is similar to the percentage of lethal violence inferred for our prehistoric ancestors and for the evolutionary ancestor of primates and apes. It is higher than for other groups of mammals.
This suggests that humans, like other primates, are intrinsically more violent than the average mammal, and that they inherited this trait over the course of their evolution.
That is not to say we cannot do anything to fight our nature. The study concludes by showing that society can modify these innate tendencies. Levels of lethal violence have changed throughout history, often in parallel to socio-economic changes - these levels have not remained constant at 2%. Social, economic and cultural factors may thus influence levels of inherited lethal violence in humans. The rule of law may mitigate naturally violent behaviours.