Is there violence in our genes?
(appeared in Oct 2016)

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It is difficult to resolve whether violence in humans is hard-wired or arises from circumstances, says S.Ananthanarayanan.

The 17th century, English philosopher, Thomas Hobbes held that men and women were acquisitive and grasping and orderly society was impossible without a sovereign power, or a political community, to impose a set of laws. In the “state of nature”, he said, all persons would lay claim to everything, they would be in “continual fear and danger of violent death,” and their life would be ”solitary, poor, nasty, brutish and short.”

A century later, Jean Jacques Rousseau of Geneva thought differently, that “….nothing is so gentle as man in his primitive state” and that a sense of morality was ‘natural’ and ‘innate’ and had to be nurtured in isolation from decadent civilization. Hobbes’ view could be understandable in the context of the violence in western society during the preceding centuries and as a premise to explain the ‘social contract as the basis of the state. Rousseau, on the other hand, was the forerunner of enlightened ideas and methods of education, which have helped moderate savagery in human affairs and even and lessen disastrous wars in recent times.

José María Gómez, Miguel Verdú, Adela González-Megías and Marcos Méndez, from institutes and universities in Almeria, Grenada, Valencia and Madrid, in Spain, describe in the journal, Nature, a study of whether there were evolutionary bases for the incidence of lethal violence among human beings. The Nature paper analyses the records of lethal violence in over a thousand animal species and six hundred human populations, ranging from the Paleolithic era to the present, to look for patterns, over the progression of species, in the incidence of death caused by violence of a member of the same species.

From the perspective of evolutionary bases for human violence, the paper says, violence can be viewed as an adaptation that leads to reproductive success through more mates, status and resources. But in the human context, a brace of ecological, social and cultural conditions affect the value of violence as a means to promote fitness and survival, and it is difficult to separate the different factors, the paper says.

Same species violence, however, is not excusive to humans, it is there among monkeys, chimps and gorillas and carnivores like tigers or wolves, for instance, kill others of different packs or even in the same pack when there are leadership struggles. Even apparently peaceful animals like hamsters or horses are known to kill others of their species, the paper says. For this reason, that violence appears to be there in all animal species, the study estimated what level of violence humans could be expected to show, given their position in the evolutionary tree.

The study collected information of more than four million instances of death and death due to violence by a member/s of the same species in 1,024 different species. While different kinds of records were relied upon, in the case of archeological instances the nature of injury of the skeletal remains was the indicator. While this could lead to some cases of death, where there was no damage to bones, being excluded, verification in the cases where there were also written records, like statistical yearbooks, shows that there is no serious difference, the study says.

In this way, the levels of lethal violence were calculated as the percentage of cases of death by same species violence, out the total number of deaths. In the case of humans, the same species killing included infanticide, cannibalism, inter-group aggression, war, homicide, execution and any other kind of intentional killing. Six hundred human communities, starting from stone-age tribes and bands of over 30,000 years ago were studied, using hundreds of published data sources.


The findings are that almost 40% of the mammal species studied shows some degree of lethal violence. Overall, including the species with little violence, the average comes to 0.3 % of all deaths, or three out of a thousand deaths are due to intra-species violence. “These findings suggest that lethal violence, though infrequent, is widespread among mammals,” the paper says.

Next, the team examined whether related species seemed to have similar levels of lethal violence. The tendency of related species to resemble one another is measured by the ‘phylogenetic signal’, which is related to how closely a shared trait is restricted to related species. The signal for lethal violence over a field of 5,020 extant mammals and 5,747 extant and recently extinct mammals was at more than 60%, which is a high figure. The figure not being even higher indicates that inherited traits are not shared by all members of species, for example, the bonobo (Pan panicus) a close relative of the chimpanzee (Pan troglodytes),is markedly less aggressive, even gentle, compared to the chimp, at least in captivity.

This variability suggests that other factors, like territoriality and social behavior modify aggression, the paper says. When these factors were considered in the statistical treatment of the cause-of-death data, it was found that the level of lethal violence was higher in social and territorial species than in solitary and non-territorial species.

This measure of the extent of expression of traits in related branches of the genetic tree enabled an estimation of the level of lethal violence that should be expected in humans, who evolved from primates and share a great part of the primate genome. The level of lethal violence inferred comes to 2%, or two of every hundred deaths arising from conspecific violence. The path of variation of this level, over the course of evolution, is that it has been generally low but rising to 2.3% in evolutionary branch points near the beginnings of primates (apes, monkeys, lemurs, etc), dropping to 1.8% in the ancestral ape (gorilla, orangutan). “These results suggest that lethal violence is deeply rooted in the primate lineage,” the paper says.

Actual levels

This inferred level of lethal violence was then compared with what is actually observed in human populations. While the levels at prehistoric periods was little different from those predicted, the levels were considerably higher during most historic periods. Violence, however, seems to reduce when we come the modern and contemporary times.

Changes in the cultural and environmental conditions may be responsible for the pattern, the study says. It is notable that population density, a factor that raises levels of lethal aggression in animals, was lower during periods of high violence in human societies, compared to the modern and contemporary ages, the study says.

As for the effect of socio-political organization, it is seen that violence levels in prehistoric ‘bands and tribes’ which were comparable with inferred levels, were higher in the present day. While the reasons for this may be the competition for resources, which may have promoted the formation of the groupings, a significant observation is that in state societies, the violence is less than the inferred levels. “It is widely acknowledged that monopolization of the legitimate use of violence by the state significantly decreases violence in state societies,” says the paper.

This may be a somewhat cynical explanation, but Steven Pinker, professor of psychology at Harvard University, in his book, ‘The better angels of our nature’, connects the rise in rationality in modern times with the decline in violence. Current levels of education and communication and interdependence are clearly environmental conditions that diminish the value of violence as a means of getting on.


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