This is the third installment in the series, Why We Fight, about the origins of warfare.

This post was chosen as an Editor's Selection for
Has warfare been handed down to us through millions of years of evolution? Is it part of who we are as a species? At the heart of this question is whether humans have a natural capacity to kill other humans. Some social scientists have concluded that evolution has in fact left us with this unfortunate ability.

Primatologist Richard Wrangham, a major proponent of this idea, developed the “Imbalance of Power Hypothesis” to explain how evolution could produce a propensity for warfare in humans. The idea is that our primate ancestors could have gained access to additional food and other resources by attacking and killing their neighbors. Of course, these deadly attacks would have only been worthwhile if the attackers could ensure their own safety. So, Dr. Wrangham reasons, our ancestors would have carried out deadly attacks only when they severely outnumbered their victims. The conclusion is that our ancestors who were psychologically predisposed to cooperatively pick off their neighbors would have had a distinct evolutionary advantage. Or, in Dr. Wrangham’s words, ”there has been selection for a male psyche that, in certain circumstances, seeks opportunities to carry out low-cost attacks on unsuspecting neighbors.” This trait would have been amplified and passed down through the generations until it was eventually inherited by modern humans, who presumably took this predisposition and ran with it, inventing more and more efficient ways to kill each other.

The “Imbalance of Power Hypothesis” is largely based on evidence of violence in the animal world, particularly observations of violent behavior among chimpanzees, our closest animal relatives. But other social scientists have instead studied modern humans in an attempt to discover whether warfare is rooted in evolution. And, contrary to the predictions of the “Imbalance of Power Hypothesis,” many of these scientists have concluded that humans have an innate aversion to killing others.

Evidence of a powerful resistance to killing has popped up in unexpected places. Many people assume that soldiers in a firefight instinctively respond to enemy fire by shooting back, and that soldiers in a kill-or-be-killed situation will choose to kill. But informal interviews conducted with thousands of American combat soldiers during World War II by army historian S.L.A. Marshall revealed that as many as 75% of soldiers never fired their weapons during combat. In recent years the rigor of Marshall’s research methods has been called into question, but his basic conclusion that the majority of soldiers will not return fire during combat if left to their own devices has been corroborated by evidence and accounts from other wars, including the American Civil War, World War I, and the Falklands War.

So why didn’t these soldiers use their weapons? Lt. Col. Dave Grossman, a psychologist and professor of military science, looked at this evidence and concluded “that there is within most men an intense resistance to killing their fellow man. A resistance so strong that, in many circumstances, soldiers on the battlefield will die before they can overcome it.” In some ways this isn’t all that surprising. Very few people would seek out an opportunity to kill others. At the same time, you may find it hard to believe that it is sometimes impossible for soldiers to kill others even when their own lives are at risk.

And yet despite this apparent aversion to killing, we still manage to kill each other with alarming frequency. How can this be? For anthropologist Paul Roscoe the answer is that we’re simply too smart for our own good. Humans excel at overcoming our biological limitations using technological innovation: if your arms aren’t long enough to reach an apple in the upper branches of a tree, use a stick to knock it down. Can’t take the square root of large numbers in your head? Write a computer program to do it for you. Similarly, we can find ways to get around our natural aversion to killing if we decide that it’s in our best interest.

Throughout history and around the world people have come up with ways to overcome an aversion to killing, such as dehumanizing the victim, placing distance between the killer and the victim, and using drugs or loud music to induce a trance-like state in a killer. In fact, following publication of Marshall’s findings in the aftermath of World War II, the U.S. military embarked on a campaign to more effectively prepare soldiers for combat by employing realistic training exercises. New recruits began to practice shooting at pop-up, human-shaped targets rather than the traditional, stationary bull’s-eyes. More and more elaborate and realistic combat simulation exercises and ’war games’ were implemented. The point of this new training was to make killing an automatic response under combat conditions. And it worked. Interviews with American soldiers during the Vietnam War revealed that somewhere between 80 and 100 percent of soldiers shot at enemies during firefights.

Although we’ve found ways to short-circuit an aversion to killing over the years, this aversion seems deeply rooted in our minds. But where did it come from? Is it an evolved trait inherited from our distant ancestors? Or is it something that we learn and absorb from our social surroundings? Or perhaps it has emerged from an inseparable combination of both biological and cultural evolution. Regardless, this aversion to killing exists, and it reassures us that warfare is not an inescapable part of human life, and gives us hope that one day we might stop fighting wars.


ROSCOE, P. (2007). Intelligence, Coalitional Killing, and the Antecedents of War American Anthropologist, 109 (3), 485-495 DOI: 10.1525/aa.2007.109.3.485

Wrangham, Richard W. Evolution of Coalitionary Killing, in Yearbook of Physical Anthropology, 42:1-30 (1999) [pdf]

Grossman, Dave. On Killing: The Psychological Cost of Learning to Kill in War and Society. (1995)

  29 Responses to “The Psychology of Killing and the Origins of War”

  1. [...] This post was mentioned on Twitter by Flipboard Science and Frank Aldorf, ScienceSeeker Feed. ScienceSeeker Feed said: The Psychology of Killing and the Origins of War [...]

  2. [...] Smells Like Science, Dan Bailey tries to understand why humans kill each other and engage in war. Research suggests we’re actually averse to killing each other. Why do we do it? Perhaps [...]

  3. An innate aversion? Well, that’s possible, but in the Austronesian world (including my area of interest, eastern Indonesia), from Taiwan all the way through the Philippines, Indonesia, island Melanesia, Madagascar, Micronesia, to Polynesia, some form of deliberate and planned head-hunting, cannibalism, or human sacrifice was practised. Head-hunting was particularly ruthless: children’s heads were fair game in a lot of areas. In Hawai’i, David Malo, a mo’o'olelo (historian) who was born in the early nineteenth century and experienced the last days of the Kingdom of Hawai’i, reports that coughing, sneezing, or otherwise making noise during a ritual could lead to instant death.

    It clearly comes from an Austronesian source, but this extends over literally thousands of years, right up to the late twentieth century in some areas, and it indicates that maybe this activity must have some basis in innate human inclinations, or it wouldn’t have started at all, let alone continued for so long and in so many areas. Peer pressure and the power of institutionalisation are strong forces, but they aren’t strong enough to explain this spread and time-depth if it is indeed so counterintuitive and contrary to human instinct.

    So too in Mesoamerica. People seem to think that human sacrifice and cannibalism were part of the downfall of the Maya civilization and of Mesoamerican civilization in general. But human sacrifice was practised in Mayan lands and at Teotihuacan long before the Spanish arrived, and was conducted for at least a thousand years in the Valley of Mexico. In Peru as well, the Inkas, and probably the civilizations of the Andes preceding them, had a system of sacrifice of children, known as capacocha, who were sent from the provinces to Cusco for rituals to be performed whereupon they were sent back to the provinces and killed by their own people.

    I’m not saying that there’s no innate empathetic response or an innate basis for compassion. Simply that one study indicating that mostly Christian soldiers socialised with certain anti-murder dispositions will prefer not to kill people is not enough evidence on which to base a theory of innate aversion to killing when there is plenty of evidence that people will kill each other quite deliberately, and not with particularly technologically advanced means (bearing mind that head-hunting, even after the introduction of rifles, was undertaken with matchets, sabres, keris, and even obsidian and bamboo weapons in Melanesia and Polynesia). An innate aversion would suggest avoidance of killing whenever possible, but this isn’t the pattern we see in life. Unfortunately. I would suggest a fusion of cultural and biological evolution in both human sacrifice and the low rate of deadly fire among US soldiers.

    • Thanks for the comment – this is a subject that has been intensely debated for centuries, probably because it says so much about who we are and whether we can justify war and other collective violence.

      You’re quite right that evidence of an aversion to killing in one culturally specific context is not evidence of a universal aversion to killing. I hinted that there is evidence of this aversion in other cultures around the world and throughout history, but didn’t go into specific examples for the sake of brevity. In socieities where killing is/was common (or at least culturally sanctioned at frequent intervals), including your examples of the head-hunter societies of Melanesia and the ancient Mesoamerican cultures, elaborate cultural practices were established that served to short-circuit an apparent aversion to killing. Killing can yield advantages, and these cultural adaptations amounted to psycho-technological innovations that allowed people to kill. Paul Roscoe argues this point in his 2007 American Anthropologist paper that I cited above, and gives particular attention to the head-hunter societies of New Guinea:

      “In New Guinea, Waropen warriors consumed palm wine before head-hunting to “muster courage” and overcome their “fear of and revulsion against” intentionally killing and beheading another human being (Held 1957:199, 220). Elsewhere in New Guinea, warriors consumed betel nut, tobacco, ginger, wild aroids, or tree bark prior to battle to induce a “trance state” or render themselves “deaf” to the entreaties of their victims (e.g., Haberland and Seyfarth 1974:349, 351; Lewis 1995:34; Telban 1998:193).”

      “[Another] technique is to distort the reality of killing by displacing responsibility for the act onto a spiritual or secular authority. In New Guinea, ancestral or totemic spirits may be represented as the real authors of a kill, the warrior acting merely as the vehicle of their desires (Haberland and Seyfarth 1974:351; Telban 1998:193).”

      “The most common way to overwhelm an aversion to killing, however, is to combine dehumanization of the enemy, which denies him or her conspecific status, with an image that elicits killing responses appropriate toward nonhuman species. In New Guinea, enemies were commonly styled wild pigs, and battle was a pig hunt rather than a homicide (Bateson 1958:140; Bowden 1983:110–111; Harrison 1993:102–103; Roscoe n.d.).”

      As for human sacrifice in pre-columbian Mesoamerica, it occurred in a highly ritualized, religious context. Sacrifice victims were considered food demanded by the gods. Again, this practice transferred responsibility for the killings to a spiritual authority, making it possible for priests to carry out their sacrificial duties. Significantly, Maya and Aztec warriors generally sought to capture enemies alive during battle for later sacrifice – it appears that they did not often kill during battle.

      These cultural practices in New Guinea, Mesoamerica and elsewhere allowed people to kill. If killing were an innate predisposition, we would expect it to be easy, and these practices would be unneccessary. The existance of an aversion to killing doesn’t necessarily predict that humans would “avoid killing whenever possible,” just that we would struggle to do it.

      Also, the fact that head-hunting and human sacrifice persisted for thousands of years says more about the longevity of a single cultural tradition than it does about an innate predisposition to kill. These practices ultimately ceased when European culture was introduced as the ruling culture through conquest and colonization. Europeans viewed head-hunting and human sacrifice as repugnant, although Europeans had often participated in equally brutal killing. The difference was the cultural context of the killing. If a predisposition to kill were innate, we’d expect the cultural context of the killing not to be so significant.

      So, I guess you can tell which side of this debate I fall on – I don’t think there’s evidence of an innate predisposition to kill. As for an innate aversion to killing? I agree with you that it may not be possible, or even productive, to separate biological and cultural evolution since the two are hopelessly intertwined. But I think there is significant evidence to suggest that an aversion to killing is extremely widespread, if not universal. Unfortunately, we’ve come up with ways to overcome this aversion when we think it’s in our best interest.

  4. I agree, except to say that I don’t think the aversion to killing is particularly strong; it is mutable in a wide range of circumstances, and there are cultural practises that manage to circumvent it. Alcohol may be required, but the mere fact that conscious dehumanisation and rage are enough to make someone kill someone else speaks of the strength of the aversion. I’m not disagreeing in substance, just in degree. I would also suggest that a killer instinct isn’t so far-fetched, given that some psychopaths – indubitably humans – relish killing. This could of course be due to a lack of aversion rather than a positive instinct. In any case, I the strength of the aversion, which is definitely present, seems to be fairly weak.

    These practices ultimately ceased when European culture was introduced as the ruling culture through conquest and colonization.

    While I agree with the rest of what you’ve said, I would say that this is a tiny bit wrong. In fact, Europeans managed to mute their opposition to head-hunting when it was in their best interests. British-sanctioned head-hunting raids happened early in the taking of Vanuatu and the Solomon Islands, for instance, and also in Borneo during the Second World War. It wasn’t a hard-and-fast rule – the repugnance didn’t extend to absolute moral principle, it seems, although yes, they did end the practice.

    • It sounds like we’re more or less on the same page on this (although you can probably guess that I’d side more with your reasoning that psychopaths/sociopaths are considered to have pathological conditions precisely because they lack the empathy and an aversion to killing that most other people have.)

      I actually didn’t know that the British encouraged head-hunting under certain circumstances, but that makes sense – if there’s one thing European colonists were good at, it was exploiting local violence to their advantage.

      Anyway, thanks again for playing devil’s advocate on this one – you got me to bring up some points that I didn’t include in the original post.

  5. I like your post – I find that my sneaking suspicion that most people just want to live their lives in peace is completely undermined by reading / watching the news, which would make you think that most people are violent and destructive.

  6. This topic always cracks me up. People kill because people can. Then poeple who don’t kill cry about the ones that do without considering why they do it. I can honestly tell you I don’t even know how many people I have killed. Don’t care either. Get paid to pull a trigger. Funny that people decry killing when it is because of thier own demands that make it happen. Sure you never pulled a trigger, but when you complained about the price of gas, guess what just happened. When you buy a 7 dollar shirt at walmart, guess what just happened. When you purchase diamonds, emeralds, rubies, gas, t-shirts, fruits, vegetables, anything. You not only ask for it, but demand it. You may not tell us to kill, but the society that you insist on living in demands it. Killing is living.

  7. I don’t really agree with this article in a lot of ways. I understand the research involved but from my history classes I can tell you that the people that fought in WW1 and 2 and many wars prior and shortly after that did not have training. 75% may not have fired their weapons but being a psychology major and the fact that this very own article says that we are “Smart,” I would have to say that not returning fire was because they did not know how to handle themselves in combat and knew they would most likely be killed if they stood to return fire. The civil war should not be used because you had naighboring towns fighting each other, they knew their “Enemies” as friends and for many it is hard to kill someone you know. I think this article would have no basis given some of the arguments made in the comments (Including mine) along with more research into the subject.

    • Hey Fred, thanks for the comment. Sorry for being blunt, but your claim that American combat soldiers did not receive training prior to WWII isn’t true. In earlier conflicts combat training was approached differently than it is today, but was still extensive. And as a result of this training soldiers routinely showed extraordinary discipline under fire. But even so, this training often didn’t result in high rates of fire. And that’s sort of the point, most soldiers must be taught to kill with extensive conditional training, meaning that killing isn’t something that comes naturally to people.

      I’m not sure if you disagree only with Lt. Col. Grossman’s interpretation that the historical rate of fire data indicates a reluctance to kill, or if you also disagree with the basic hypothesis presented by anthropologist Paul Roscoe and others that humans are not predisposed to kill one another. Your point that American Civil War soldiers may have found it difficult to fire at their former countrymen (and in rare instances someone they knew personally) supports the idea that killing is not something that comes easily to humans. If you’re interested in this debate you should check out Dave Grossman’s book, “On Killing,” where he discusses some of the points you brought up.

  8. [...] for yourself.) Lt. Col. Dave Grossman, who is a psychologist and professor of military science, looked at Marshall’s evidence and concluded that “there is within most men an intense resistance to killing their fellow man. A [...]

  9. I’ve been the victim of road rage and I say I’d gladly kill the man that spit my face. Some drivers almost show criminal intent toward me riding a bicycle when they should give us space. But the system itself provokes animal instincts to take over road manners by not restraining the predators among us.

  10. [...] animal instincts to take over road manners by not restraining the predators among us."…#comment-25485 *** It may just be that the cage makes the beast to go insane. But that's only my humble opinion. [...]

  11. Survival of the fittest.

  12. I think there is only one answer, in terms of mass killings and orchestrated suffering that happens in industrialized wars, conflicts, or social instability…

    It is the old men at the top of the chain, who condition and brainwash the younger generations into butchering one another. Those in power, whether it is the US military, the old jihadists that run Al Qaeda, the Warlords in Africa, or the bosses of the Cartels in Mexico, they use the tactics mentioned in the article, dehumanization, sadism, dropping bombs, and propaganda to brainwash the masses into accepting this bloodshed day after day. All for their own benefit.

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  15. I’m interested how this applies to terrorists. So would it be social engineering like “dehumanizing America, Israel, the West” or the cult of violence in many of those societies, plus enticements of financial rewards for the family of the suicide bombers in question that would help over come terrorists from their natural inclination not to kill.

    It also seems they usually use mostly young, angry frustrated men (in fact usually the kind of people who engage in shooting sprees in the U.S.) to do so such attacks most of the time vs. more stable middle age men or women.

    In fact regarding such shooting sprees in the U.S. would it be social isolation, exposure to violent media, and the fact many of the perpetrators get caught up in the glory of American gun control that leads them to overcome such an inclination not to kill either?

    While I believe, as in it is just an opinion so I cannot claim it is necessarily correct, that most people are not predisposed to kill there are certainly portions of the general population, including psychopaths and narcissists, who probably have little problem doing so.

    Finally, has there ever been any link to humans predisposition to rationalize everything and thus somehow always find a way to justify killing others (for examples racist ir extremist ideologies).

    Just a few thoughts, what do you peeps think?

  16. [...] War is a fascinating topic of the mind and brings together the two greatest primordial forces in our psychic makeup; the force of sex and the force of kill. There are very special individual’s who within the chaos of death somehow control their fear and are able to weave their way through to a place of momentary safety. They kill. The moment could be as short as only one minute or even shorter. In a fight the measure is miniscule, but those few seconds could be enough to survive the fight. [...]

  17. French archives on the murder of Joan of Arc by men who both supported and dreaded war
    opens many doors of learning.

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  20. Isn’t war a bit out of date?

    The Holistic way to end war

    I hope one day we do stop war. It is become so gross and unbearable . It is the negative in a creative loving and positive world. Why are working class men around the world manipulated to fight wars. In a war one man or a group of men take each others lives? that is the reality of it. Is it sexual ? maybe if men cant fuck ( because or the main religions of today) they then kill? is part of it sexual frustration? Men need to release that energy whether gay straight or bisexual this is well understood today. Maybe people are more bisexual then we know and need to bond sexually; is seeing a man or anyone die some form or ritual power kill? Remember before the main religions we have now people were free to just be themselves. They were wars but mainly individual fights between people within a culture ie fights! Along came those religions about 4000 years ago and endless war! in the name of religion ( so they can not be religions or spiritual they can not be So new armies formed called Religion?.

    In Hilter’s case self hatred that he acted upon ; as new evidence coming now suggest that he and his wife may have had roots belong to the religion and people which he claimed to loath.

    Also there is some suggestion now that Gay men are created by nature to help create peace: if that is true interesting how all the main religions banned free adult sexuality including Hilter ( thereby avoiding peace or peace makers). It is said in this article that because of the brain of many gay men they are able to mediate between men and womenand between groups of men in a diplomatic way ( don’t under estimate this) . We are searching for missing links to the question of why we have war.

    What is happening? to me are beautiful as a gay man I can only hope for their well being happiness to keep being fit sexy and attractive and I will support many male friend when he is down or ill; Because I love my fellow men so much. Emotional yes and we should all let our inner self out more maybe the energy or surge would stop war globally.

    In the ancient past before Egypt there were raids on neighboring people for food; so I am told by anthropologists! So it seems that when the so called empires evolved one had to be as strong as the other. So a pattern of war evolved leading to Nuclear weapons as the ultimate message to stop!

    “People are so precious all they really want to do is Love care live then move to next incarnation I am sure”

    When I watched a film about World War 1 recently; I cried. Those innocent men dying because general Haig said so? and because The German army might have invaded from French shores the Uk.

    Has the hunter gatherer evolved into solders and armies. One Anthropologist felt this might be correct ; google. It was said that stooping the raiding men of the past evolved into professional armies. So Men taking from one village turned into countries taking from each other on mass scale (resources).

    Surely the answer is to Open Borders which Tony Blair did; in some ways that was very radical when looking at history. If all countries followed suite we may find that countries share goods and wealth and if they don’t they should. The Uk is helping with the Ebola situation . If all countries helped each other I think there would be no need for war. This would separate out those who wish for war due to some perversion.

    If taking each others things has always been the problem; then share! Or is there some other reason for modern war like dictatorship; power Heterosexual power over all else! Because generally Gay men avoid war and there is proof of this and there are millions of Gay men globally and peaceful Bisexual men. Question; So do some many; Straight men see war as a way to assert Heterosexual values onto everyone else and another man has no value to a straight male. Is this true? Can it be true?

    What else are the answers: I wish to see a new Loving and peaceful religions of total peace and respect form We deserve this…. I wish so much for this

    My advise is avoid old religions believe in yourself and your task and presence on earth. Love yourself and your friends and family treasure them each day. Help others

  21. Anecdotes are fraught with pitfalls, but given that I’ve had direct experience with this issue I wanted to comment for what its worth. I fought in combat with Marines on multiple occasions. What I noticed over time was that the vast majority of guys I fought with did indeed do strange things. Yes, we did train for realism, and the guys did shoot their weapons at the enemy. But what was strange was that they always seemed to shoot “around” people. They would shoot at vehicles, at buildings, at objects, but not as much at people. And it wasn’t just fear. Yes, they were anxious, and that made their shooting less accurate. But it was more than that. They always seemed to “miss” in such a way as to NOT hit people. I’m not saying it was always like that, just that there was a tendency to see that. They were firing to keep the enemies head down and make them disengage. In military jargon, they didn’t seem to always be firing for “effect” even when they were supposed to be. Now, to the articles point.

    There were exceptions to this rule and this is what I think the article overlooks (maybe). I wasn’t particularly popular or unpopular, but when the shooting started they started to call me “dead eye dick” (which I do not like). I was always chosen to be the preferred man on first contact. But I wasn’t gung-ho, wasn’t a particularly good marksman (some were much better than I) and didn’t particularly sense anything different about what I was doing and what they were doing … except that I always fired for effect, with the same giddiness that a target shooter has when they want to hit the target, and see its reaction (see it “blow up”). To me, the people were no different than the objects, and I received greater satisfaction from hitting them, not vehicles and buildings. I can only guess, but I think maybe around 5% of us were more like me, and the rest were as described before.

    All of us I think felt a huge drop of inhibition when our lives were directly threatened. Being shot at does this and I think that is normal. What I sensed was different between some of us was that some of us had different thought processes on the back end that were inhibited normally. Only under the rarest of conditions where our lives felt genuinely and seriously threatened, did those thought patterns materialize to behavior. So, most guys, even when the inhibition was gone, had a thought pattern that YES, strongly resisted the notion of killing and it was deeply innate. And I say that because the most efficient way to defend themselves was to shoot the PEOPLE. They felt a DIFFERENCE between people and inanimate objects that I don’t. And all the guys knew this, but no one ever talked about it.

    Afterward we all had counseling more or less, and I told them about this. But I am not crazy and nor were they, because we all had normal inhibition control (outside combat, it’s always there, so you don’t know, but I can tell you it is really STRONG and you can tell when it goes away, very surreal). So, speaking to the comment about head-hunters and all that, I would bet you a million dollars, and I’m sure of it, that those that ran those circus shows were like me, but for some reason their inhibition was aberrant. I think the thing psychologists should try to understand is why and how inhibition gets removed when it is naturally turned ON all the time, very consistently, and very powerfully, and should only turn OFF when a legitimate, very imminent and real threat to your life is present. I think otherwise it is abnormal. So, yes, the question is, why and how is it getting turned off outside of the most extreme situations?

    I think it naive to suppose that there are not people who want to take advantage of others, and that they want to disinhibit useful minds for their own gain. I’ve wondered about this in my case and it still bothers me.

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