How much of human violence is innate, and how much of it is shaped by our environments? Are we a uniquely violent species? These are questions philosophers and social scientists have tried to answer for centuries. Now researchers have done an evolutionary comparison – and conclude that the rate of lethal human violence is six times that of the average mammal…but about average for a great ape.

Gómez et al. applied comparative comparative statistical techniques to a phylogeny of mammals, including primates (monkeys, great apes, and us) to reconstruct probable ancestral rates of lethal violence at the time of the origin of our species (160,000 to 200,000 years ago). They didn’t stint on the data collection either, compiling information on more than 4 million deaths from 1,024 mammal species drawn from 137 mammalian families (80%). The human data came from 600 studies, and was derived from samples ranging from paleolithic (defined by the authors as 50,000 to 12,000 years ago), to anthropological sources from the past few centuries, from across the world.

They then calculated the proportion of deaths attributable to violence from a member of the same species out of all deaths counted for each species – a critical distinction, as clearly out-species deaths, like a lion killing an antelope, are not relevant to the question.

They calculated the rate of intraspecies lethal violence at the origin of mammals at about 0.30%, which is approximately 1 in 300 deaths. Rates of lethal violence then rose steadily over time throughout the mammalian phylogeny (Fig. 1) as the reconstructed ancestors drew closer to primates, which is fascinating in itself. It also argues for a more genetic basis; over such a length of time adaptations to lethal violence must have arisen and been selected for. Moreover, more closely related species had a more closely correlated level of violence. The incidence of human lethal violence at the origin of our species at 2%, about six times higher than the reconstructed mammalian value. We may have inherited our violent tendencies after all – but why?

violence-figure

Tree showing the phylogenetic estimation of the level of lethal aggression in mammals (n = 1,024 species) using stochastic mapping. Lethal aggression increases with the intensity of the colour, from yellow to dark red. Light grey indicates the absence of lethal aggression. Mammalian ancestral nodes compared with human lethal violence are shown in red, whereas main placental lineages are marked with black nodes. The red triangle indicates the phylogenetic position of humans. From Gomez et al, 2016, Nature Publishing.

The increases in lethal violence coincide with species having increasing amounts of group living (placing individuals in more regular close contact) and territoriality (meaning groups potentially competing for resources). The group living is interesting; other studies have shown increased brain size in more social primates. Perhaps the linking of our intelligence and violence in 2001: A Space Odyssey, pictured in the title, was not too far of the mark. I think we can rule out an alien monolith origin, however. The territoriality linkage raises another key point; often, when people think of competition in natural selection, they think of predator and prey trying to out-evolve each other. But in fact, a moment’s thought will lead you to the inevitable conclusion that you are in the greatest amount of competition with members of your own species: they want the exact same resources as you, and in addition will be in competition for mates.

An important caveat to bear in mind with this study is that it looks only at rates of lethal violence: rates of non-lethal violence are not calculated, and may be considerably higher, and/or vary widely. Considering mammals, there are numerous examples of male-on-male violence during competition for access to females; think of red deer clashing antlers. However, such fights are seldom lethal; which in a sense gives a slight paradox. You may be in most direct competition with members of your own species; however, they are likely to have the same weaponry as you, so attacking is risky as any advantage you may have is probably slight and you could be severely injured or killed. It doesn’t pay to fight to the death. (It also raises the point that nearly all of this lethal violence is perpetrated by males, something that human cultures consistently fail to deal with constructively).

What is also interesting is that, within human societies, the estimates of rates of lethal
vary widely over time, in most cases too quickly to be attributable to genetic changes. The palaeolithic samples have rates very close to the 2% predicted at the origin of our species, but rise to as much as 15–30% in samples between 3000 and 500 years ago (with a lot of uncertainty), before declining from about 100 years ago to the present day. Again, why? The rise tends to correlate with moving from early pre-societal small groups, to larger tribal groupings and then to political entities with a dedicated warrior class.

It’s interesting, learning about something like this, to reflect on the prevalence of violence in our cultural output as well; war, in particular, is often glorified, and, even when not, is used as a basis for a gripping story. I won’t claim to be immune to this; I like a heroic battle as much as anyone. I will note, however, that the more prosaic, “everyday” lethal violence that is more common, such as domestic violence, is not such an audience favourite; far harder to make that look noble. Even in my favourite genre, science fiction, violence runs throughout: it may depict humanity in a far-flung future and radically different societies and yet, in most cases, that theme of violence persists, as something we just don’t seem to get over no matter how far “advanced” we get. One of science fiction’s most successful and recognisable creations, after all, is called Star Wars. Jacob Bronowski, in The Ascent of Man, declared:

War – organised war – is not a human instinct. It is a highly planned and cooperative form of theft

I never really agreed with this (it rather depends how broadly one defines “instinct) and it seems something of a tautology. One might reasonably say that a highly planned and cooperative form of theft by violent means is called war. In pre-modern societies, war was often about resources, territory and women. In modern societies, it’s still often about all those things, to which one might add power and money. There may be a hatred of the “other” involved, and even a concerted effort to wipe out the enemy, but it is always coupled with taking something that they have. I always found it interesting that, for all the many science fiction stories (on or off screen) in which valiant humanity defeats aggressive alien invaders, as in War of the Worlds, there are quite a few in which we are the aggressor; quite often we are brutally subjugating an innately peaceful alien race (as in Le Guin’s The Word for World is Forest). Often such stories are deliberate parables for colonialism, for example, but I think they do speak to this almost subconcious knowledge of ourselves: that we are violent, and, given the opportunity and the motive, will be violent, no matter that we say it is immoral.

It is important not to use this study to justify human violence. For one thing, whatever our origins, the structure of our societies can dramatically alter the levels of violence present: rates of lethal violence in modern societies with strong legal systems and police forces, and a culture that abhors murder, are actually very low, at less than 1 in 10,000 deaths (or 0.01%), about 200 times lower than the predicted rate at the origin of our species. That is a vast difference. At some point, we reversed the trend towards increasing violence that came with increased group size and political organisation. We are perhaps the only species intelligent enough to defy the constraints of our own evolution; we do it every time we use contraception. We should use that intelligence to defy our propensity towards violence too. We can be better.

Reference

Gómez et al, 2016: “The phylogenetic roots of lethal human violence.” Nature 538, 233–237. doi:10.1038/nature19758.

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