Humans Are Genetically Predisposed to Kill Each Other

humanskull-lowresA new study of 1,024 mammal species has determined which animals are the most vicious killers of their own kind. Killer whales perhaps? Pit bulls maybe? For the answer, just look in the mirror.

“Step back and view our species objectively from the outside, the way a zoologist would carefully observe any other animal, or see us the way every other creature perceives human beings. The brutal reality could not be more evident or more horrifying. We are the most relentless yet oblivious killers on Earth.

“Our violence operates far outside the bounds of any other species. Human beings kill anything. Slaughter is a defining behavior of our species. We kill all other creatures, and we kill our own. Read today’s paper. Read yesterday’s, or read tomorrow’s. The enormous industry of print and broadcast journalism serves predominantly to document our killing. Violence exists in the animal world, of course, but on a far different scale. Carnivores kill for food; we kill our family members, our children, our parents, our spouses, our brothers and sisters, our cousins and in-laws. We kill strangers. We kill people who are different from us, in appearance, beliefs, race, and social status. We kill ourselves in suicide. We kill for advantage and for revenge, we kill for entertainment: the Roman Coliseum, drive-by shootings, bullfights, hunting and fishing, animal roadkill in an instantaneous reflex for sport. We kill friends, rivals, coworkers, and classmates. Children kill children, in school and on the playground. Grandparents, parents, fathers, mothers–all kill and all of them are the targets of killing…” -R. Douglas Fields, Why We Snap, p. 286, 2016.

After writing those words in my new book Why We Snap, I have been frequently challenged for being overly harsh on the species that has chosen to christen itself “sapiens,” (the wise one). But I was not offering social commentary. I was providing an objective, zoological description of this species. This week Maria Gomez and colleagues, Zoologists working in Spain, published the results of their in-depth research in a report in the journal Nature on the evolutionary roots of the human propensity to kill their own kind. The researchers compiled data on lethal violence within 1,024 species of mammals, and the results verify my description of us. The analysis shows that deaths caused by other members of the same species is responsible for 0.3% of all deaths on average for all mammals, but the rate of lethal violence among Homo sapiens is 7 times higher. Together with our primate ancestors we stand out as aberrations in our penchant to kill our own kind.

The reasons can be traced back to our primate ancestors, which are exceptionally violent creatures, killing each other at a rate of 2.3% like we do. These data indicate that the incessant repetition throughout recorded history and in prehistoric times of murder and war among all cultures of human beings has its roots in our evolutionary stalk. In part, the reasons for this rampant self-killing appear to relate to our big brains and the conscious awareness and conniving that big brainpower makes possible, but primarily because of two other key aspects of Homo sapiens and other primates: fierce territoriality and living in social groups. Across all mammalian species, conspecific deadly violence is highly correlated with these two factors. A double hit of both factors compounds the violence. Whales and bats are highly social, but not territorial, for example, and they have very low rates of killing their own kind. Human beings are highly social but extremely territorial–“Trespassers will be shot!” “He cut into my lane!”

When researchers examined how different types of social groups of humans affect the rate of killing, they found that lethal violence was common in present-day societies organized into bands or tribes, and severe violence is frequent in chiefdoms because of territorial disputes, population and resource pressures, and competition for political reasons, but violence decreased in state-run societies. Presumably, the authors conclude, this is because socio-political organizations of populations in state-run societies that are designed to suppress violence and respond to it, act to inhibit the innate, genetically-predisposed propensity of people to kill each other. Consider, for example, if there were no police to call, what would you have to do?

Missing from the analysis, but unquestionably the most important factor in violence among humans and other mammals, is sex. Males (boys, men, and the males of other mammalian species) are inherently violent, and they are responsible for the vast majority of violent death. This is a relic of the traditional male role in defending territory and social organization that our human and non-human mammalian ancestors practiced.

We are evolutionarily and genetically predisposed to snap in deadly violence, but in comparison to other animals, biology has indeed endowed our species with extraordinary “sapiens.” The problem is that the neural circuits of violence that cause us to explode in rage and violence are deep in the brain beneath the cerebral cortex where consciousness arises. The frontal lobes of the brain can squelch these circuits of rage that we share with other violent mammals, but this “top-down” conscious control of our violent impulses is slower to act than the circuits of explosive violence deep in our brain. Understanding this neurocircuitry is vital. Territoriality and social interactions are the “E” for environment, and “T” for tribe, triggers of sudden aggression in the mnemonic “LIFEMORTS,” which is a convenient way to learn to recognize the 9 triggers of rage. Each of these triggers of rage activates different neural circuits in the brain’s threat-detection mechanism. Learning about these biologically and genetically embedded triggers of violence can enable us to engage the part of the human brain that distinguishes it from all other mammalian brains–the forebrain. Circuits from the forebrain to the brain’s threat detection circuits can squelch sudden aggression and violence if there is even a split second to think, be it on the road, in domestic life, within societies, or nations at war. Everyone should learn the LIFEMORTS; especially in an election year.

The nomenclature, sapiens, seems an Escher-like ambiguity shifting freely between science and sarcasm. Perhaps this species would be more aptly named Homo nudus, the naked ape, not sapiens. So it seems when miles on the road are marked in incidents of rage–clocked at one every 20 min on average. One wonders when Orlando is no longer Disney. When sun and glitz cease to be French Nice. When children don’t build sand castles on the beach, but wash up lifeless like dead seaweed and broken bottles. When skyscrapers are scraped up, as airplanes are mutated into missiles. When houses of worship become slaughterhouses of hate. When millions flee across the globe from their homes in Syria bombed into a manmade hell on Earth. When a child sits stunned in an ambulance having watched his home and family destroyed by deliberate attack. When a boy trades a handgun for a father and projects his personal pain upon playmates, preferring to become a child murderer. When police in moments of fear fire first and question later, and when they are picked off by a sniper like points in a pointless video game. Violence is in our genes and in our environment, but so too are territoriality and society. These things we will not change. Genes change at a glacial pace. But territory and society shift constantly and they are molded by man. There is hope through understanding the science of human violence, as we can see. Some men do deliberate and decide to forfeit their life if necessary to rush into a war zone wearing white helmets to dig a broken child out of rubble.


This article was first published in Psychology Today:


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