Although it’s unclear whether genetics or other factors are responsible, new study suggests that lethal violence is part of our evolutionary history

Humans are predisposed to murder each other, new research suggests, although it remains unclear if it’s down to genetics or other factors.

Researchers from Spain have found that a tendency to bump off members of the same species is particularly common among primates, and have estimated that around 2% of human deaths at the origin of our species were down to such lethal spats.



“What it is saying, in the broadest terms, is that humans have evolved strategies for solving problems with violence,” said Mark Pagel, professor of evolutionary biology at the University of Reading, who was not involved in the research.

But, the authors add, the impact of society can greatly modify how aggressive humans are, with the proportion of human deaths due to people fighting between themselves fluctuating over mankind’s history.

“Lethal violence is part of our evolutionary history but not carved in stone in ‘our genes’,” said José María Gómez, first author of the study from the Estación Experimental de Zonas Áridas (EEZA) in Spain. “At least to some extent, the way humans organise in societies influences our levels of lethal violence.”

The question of human violence has puzzled thinkers for centuries, from Thomas Hobbes in the 17th century, to contemporary psychologists such as Jared Diamond and Steven Pinker, the Johnstone professor of psychology at Harvard University and author of The Better Angels of Our Nature.

Pinker has previously argued that humans engage in lethal violence as a “natural condition,” but that deaths from such violence have decreased with the rise of modern societies with sophisticated institutions and laws. But the idea has proved controversial.



The new research is likely to add further fuel to the debate.

Writing in the journal Nature, the team of researchers from four Spanish institutions sought to unpick the evolutionary contribution to lethal human violence by looking at how commonly a range of different mammals kill members of the same species.

To do so, they examined the evolutionary family tree of mammals, looking at data from more than 1,020 species, as well as 600 human populations stretching as far back in time as the Stone Age, drawing on evidence from human remains found in archaeological excavations.

For each species of mammal, the team looked at the proportion of deaths that were down to violence between members of the same species. The findings showed that such lethal violence is more common in some mammals, such as primates, than in others, such as bats or whales, and is more common among mammals that exhibit social and territorial behaviour that those that go it alone.

Using statistical models, the researchers then estimated the likely levels of such lethal violence for evolutionary ancestors at various points in the family tree of mammals.

The results revealed that for the ancestor of all mammals, around 1 in every 300 deaths was down to lethal violence between members of the same species. But, the authors note, for evolutionary ancestors of the primates and apes, the figures were higher.

Around 1.8% of deaths are thought to have been down to lethal violence for the ancestor of the great apes, and around 2% for the first humans - a figure more than six times higher than at the origin of mammals.

“We cannot tell that 2% of violence is due to genetic factors,” said Gómez. “Not only genes are inherited from ancestors, also environmental conditions and ecological constraints. Those are also probably influencing the human lethal violence in our evolutionary past.”

But Pagel believes it is important to emphasise that genetic adaptations could be at play. “Humans emerged from a very long lineage of species - great apes and before them the primates - that all expressed relatively high levels of lethal violence,” he said. “When you immerse an animal in a particular environment, it evolves genetic-based strategies for dealing with that environment. There is good reason to believe this reflects a real genetic or innate tendency to solve problems with violence.”

The research also shows that, according to observed evidence, levels of lethal violence among humans have fluctuated over time. While the figure was around 2% for prehistoric man, matching the expected level from the calculations, the proportion of deaths down to such violence rose during other eras such as the iron age and the post-classic period (around 800-500 years ago), falling in contemporary times to below 2%.

“From the empirical figure of 2% of deaths by lethal violence in primitive hunter-gatherers, different historical times have had different levels of lethal violence,” said Gómez.

Pinker told the Guardian the new research is impressive and supports his views that humans have a natural tendency to engage in lethal violence, that rates of such violence were high in prehistoric societies and are higher in tribes and chiefdoms than hunter-gatherer bands, and that such impulses are dampened in modern societies. “Though I made all these points in The Better Angels of Our Nature in 2011, [the authors] have demonstrated them with much greater precision, rigour, and depth,” he said.

But Douglas Fry of the University of Alabama at Birmingham in the US, disagreed, saying that: “Gómez and colleagues demonstrate that recent assertions by Steven Pinker and others that violent death in the Paleolithic was shockingly high are greatly exaggerated. To the contrary, the findings show that social organization is critically important in affecting human violence.”

Gómez says the research highlights the control humans have over their situation in life and death. “The main message of our study is that no matter how violent or pacific we were in the origin, we can modulate the level of interpersonal violence by changing our social environment,”he said. “We can build a more pacific society if we wish.”