There’s a new paper in Nature about the level of intraspecific violence in humans and other species, written by José Maria Gómez et al. (free reference and download below). The question is how often members of single species kill each other in the wild, and whether humans are outliers. It’s already gotten a lot of attention in the press, including an Atlantic summary by Ed Yong, but I’ve avoided reading the journalism until I read the original paper. Now that I have, I’ll summarize the Nature paper briefly for those who haven’t seen other pieces about it.

First, the authors used data from the literature to estimate the level of lethal violence in 1024 species of mammals from 137 families. The question was this: what percentage of individuals who die within a species do so after interacting with members of their own species? That’s the measure the authors take as the degree of “lethal violence” within species. It does not include lethal violence from members of other species, like rabbits getting nicked by raptors.

When you impose that data on the known phylogeny (family tree) of mammals from genetic and morphological data, you can then, using techniques known for a while, estimate what degree of lethal violence existed in various species’ ancestors. As a hypothetical example, imagine a group of ten related birds, nine of which have crests and one of which was uncrested. Assume further that we know from genetic data that those birds all had a single common ancestor and were all each other’s closest relatives (i.e. they’re a “monophyletic group”). If that’s the case, then it’s a reasonable assumption that that common ancestor also had a crest. (It’s more parsimonious to assume that the crest was lost once than that it evolved nine or so times independently in the descendants of an uncrested bird.) That’s a simple example, but you can use techniques like that to make quantitative estimates, too, and that’s what the authors did for lethal violence.

They first imposed measured levels of lethal violence on the known phylogeny of mammal species. Here it is; the caption comes from the paper, and the color in a branch indicates the estimated level of violence in that branch, ranging from light yellow (peaceful) to dark red (violent). Click to enlarge, and notice the redness around carnivores and, especially, primates; more on that later:

The authors also found that related species tended to have related levels of violence. That’s what I would have expected, and when I read that sentence I thought, “Well of course: violence is more common in species that are more territorial as well as those that are more social, for territoriality and sociality breeds inter- and intragroup competition for mates, food, and territory. And of course if a species is social or territorial, its relatives are likely to be social and territorial.”

And, sure enough, there was a strong correlation between both sociality, territoriality, and violence among the species. Here’s a graph showing that, with territoriality seemingly inciting more violence than sociality:

Now what about our own lineage? Information about lethality was obtained from 600 human populations dating from the Paleolithic to the present, using both fossil (bone) and historical evidence. Lethal violence included homicide, cannibalism, war, infanticide, execution, and so on. Information was also available from H. neanderthalensis. There are two main results:

The proportion of individuals in the genus Homo killed by lethal violence was about 2%, and this estimate is robust to things like the uncertainty of phylogenies. This is pretty high compared to some other animals (see below), but is explained by the fact that hominins are both social and territorial. This is the ancestral condition before we became civilized. These levels persist in many non-“civilized” groups, though, and from this the authors conclude that there is an evolved, genetically-based propensity for humans to be violent at a level that causes roughly 1 in 50 humans to be killed violently by other humans. That baseline level can be reduced by the imposition of law and “civilized” societies.

Violence is correlated with human social organization. The authors divided human groups into four types: “bands” (hunter-gatherers and the like), “tribes” (small groups that live in semipermanent places, with egalitarian societies composed of hunter/horticulturalists), “chiefdoms” (hierarchical non-industrial societies pervaded by kinship ties), and “states” (“politically organized complex societies”). Here are the data, which show that “historic” bands and tribes didn’t differ significantly from the phylogenetic “ancestral” level of violence, while historic chiefdoms and contemporary bands and tribes have significantly higher levels of violence than presumed in our ancestors. In contrast, both historic and contemporary states have considerably lower levels of violence than the ancestral estimate, probably (as the authors note) because in such societies the state takes over the imposition of violence. That, in fact, is one of Steve Pinker’s hypotheses in Better Angels for the historical decline in violence over the last five centuries.

Finally, I still haven’t read Ed Yong’s piece, though I will now, but I will reproduce a figure from his piece that someone put on Twi**er. It shows the level in violence among many species, and you’ll be surprised at the most violent:

Yes, the primates are up there, but Jebus, the most violent species is the MEERKAT, with over 19% of individuals killed by other meerkats. Who knew?

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Gómez, J. M., M. Verdú, A. González-Megías, and M. Méndez. 2016. The phylogenetic roots of human lethal violence. Nature, doi:10.1038/nature19758, Published online, 28 September 2016.