Are We Hardwired to Kill?

By Chris Mooney | May 28, 2010 8:37 am

Vanessa Woods CoverThis is a guest post from Vanessa Woods, author of the new book, Bonobo Handshake. Vanessa is a Research Scientist in Evolutionary Anthropology at Duke University and studies the cognition of chimpanzees and bonobos in Congo.

We like to think that murderers are psychopaths, with some kind of abnormal psychology that would never appear in us, or someone we know. And yet most of us think we would kill in certain situations, like if we were at war, or someone was about to kill a person we loved.

How ‘natural’ is this instinct in us, and can we ever obliterate it completely?

In my new book, Bonobo Handshake, I talk about lethal aggression in one of our closest relatives, chimpanzees.

Chimpanzees and humans have a lot in common when it comes to killing:

esther's chimps3 164_2#1 Killers are mostly male. Though female chimpanzees can participate in killing, usually the killers are males. In humans too, the FBI reported in 2005 that 89% of killers are male


#2 Males usually attack when the ratio is 3:1. Wrangham and Wilson reported that both chimpanzees and young men in gangs attack when they outnumber their victim 3 to 1 or more. The reason for this? This is the minimum number that can safely overpower a single victim.

#3 The murder rate between chimps and humans is the same. Before we had inventive weaponry that could kill thousands with the press of a button, humans lived in hunter gatherer groups where we lived off the land like chimpanzees. Watts et al, reported that in these societies the homicide rate in humans and chimps is about the same.

#4 We kill for the same reason. Most of the killing is done over females, enemy males, and territory. Think of all the mass wars. Despite the hype of liberation and democracy, what were they really fought over?

Scary how much we have in common. Luckily, we also have a lot in common with our other closest relative, the bonobo. Bonobos, like us, can be extremely cooperative and tolerant. Bonobos don’t kill each other, and they aren’t particularly violent.

The key in bonobos seems to be tolerance. In our 2007 study, we found that tolerance makes bonobos more cooperative than chimpanzees. We’ve also found that physiologically, bonobos have a very different response to chimps in potentially tense situations.

When there is only one pile of food, bonobos experience an increast in cortisol, a stress hormone, while chimpanzees have a spike in testosterone. This means that when there’s potential for trouble, bonobos get stressed and chimps get ready to fight.

These phsyiological changes aren’t something bonobos or chimps can control. But as humans, we need to figure out the situations where we are more chimp than bonobo and correct for our behavior.

For instance, during recessions, where resources are tight, historically we see a crackdown on immigration (like the Arizona immigration strategy).

Correspondingly, if you want to see the murder rate go down, you have to figure out when murder is most likely to occur (again, over females, attacks on enemy males, and territory).

People get freaked out when you talk about a biological basis for aggression. But until you take ALL the factors into account, we’re going to keep making the same mistakes over and over again.

*My new book Bonobo Handshake is out now. It’s available on Amazon, or through my website


Comments (14)

  1. Guy

    If it came down to a fight over resources, who would win in a Chimp vs. Bonobo fight?

    This is an interesting subject.

    Thank you for sharing.

  2. Nathan

    I like this. I tend to think we are hardwired to kill, and you’re also right that almost all mass murder’s motivation we’ve commited boiled down to resources, “enemy” males, or protection or acquisition of women. As for overcoming it? Maybe someday, but more likely, there will always be an “other” that the “we” must do everything we can to kill. The “we” will just get larger, include more.

  3. Peter Morgan

    “as humans, we need to figure out the situations where we are more chimp than bonobo and correct for our behavior” — but, we’re humans. We don’t need or have to be more like bonobos than like chimps, although we may make a moral choice to pursue such a path. Our evolutionary space is not the same as that of either bonobos or chimps. Whether it turns out to be a “better” strategy for humans to be relatively more cooperative or to be relatively more aggressive is a contingent matter that partly depends on such things as our definition of “better”. If we decide that our prime objective is to minimize cortisol production, perhaps being more like chimps might be better? If we decide that our prime objective is to ensure that at least 1000 humans survive each and every calamity that ever occurs in the future, it’s hard to know whether such a prime objective is best served by cooperation or by aggression, and the answer in each different calamity may be different. Further, thinking in terms of only cooperation and aggression makes for a rather broad brush categorization of behavior, even if a biochemical analysis supports it to some extent.

    “we’re going to keep making the same mistakes over and over again” — situations are always slightly different, and our responses are always slightly different. It’s never absolutely clear that a situation is exactly enough the same that we should avoid anything similar to the strategy that didn’t work last time in a slightly different situation. Considering chimps and bonobos as a guide to how cooperatively or aggressively we should act when the stock market crashes seems problematic.

    I look forward to the details of your insights into the similarities and differences between chimps, bonobos, and humans, but stepping over into philosophy, ethics, and morality needs care.

  4. Jared Diamond talks about this in his Pulitzer-Prize winning book Guns, Germs, and Steel. He suggests that the reason so many groups like the Neanderthals, and animals like the mammoths survived through ice age after ice age and then went extinct shortly after meeting us humans is because we are a particularly violent and deadly species.

  5. Hank Fox

    If you look at the data in another way, it proves that killing is actually rare, that mostly, we’re hardwired to be peaceful. No other species with a brain large enough to notice lives peacefully in such large numbers as we.

  6. Wow, I really love the intellectual standard on the Intersection. On my other blog, I get more of the ‘gay people are bad’ posts in response to bonobos.

    1. yes, i agree, murder is complicated. And there are many, many contributors to why someone would kill, and why we have war. But the biological basis for aggression is one of these contributing factors, and therefore important to understand.

    i don’t think we should be evolving towards chimpanzee or bonobo. I just think that as a super intelligent species, we can learn something from both, and maybe adapt our behavior to emulate.

    i do think that the most dangerous thing we can do is pigeon hole murderers, mass murderers and the partakers of genocide into an ‘abnormal’ box that has nothing to do with ordinary people. Nazi Germany could happen anywhere. And has happened anywhere, repeatedly, all over the world.

    The only way to prevent such things occurring is to understand why they occurred. From every angle, including why we would have a biological disposition to do them in the first place.

  7. Chris the Canadian


    Thank you for the article. Such studies of primate behavior fascinate me because of the similarities that are found between our species and our primate cousins. The kill instinct in humans was probably very important for the survival of the species in the evolution of Homo Sapien Sapien. The human instinct to kill. It’s rooted in the need to survive. The difference in primates and humans versus other species is that we are cognicant of what killing means. To kill another of our own kind is to rid oneself of a competitor, a potential threat, or another user of finite resources.

    Now, with our ever evolving lives, would it be a good thing to rid humans in some way, shape, or form of our kill instinct? I say that would also be a dangerous proposition. To turn a whole species into a group of docile pacifists leaves them open for extinction if a new or more aggresive species tries to take hold. Rather, by learning about the instinct and the mechanisms that turn it on and off, we as a species can better understand those anomalies and help prevent unfortunate acts of violence from occurring in the future.

    My opinion is that the instinct to kill is part biological, part chemical and part psychological. Chemical meaning the body releasing certain hormones and chemicals in reaction to a situation the brain perceives as threat or danger, similar to an adrenalyne rush. Finally, it is my opinion that our ability to comprehend and understand murder, genocide, war, and death is what stops many people from committing these acts. For most of us our ‘conscience’, for lack of a better term, stops us from performing these acts. We understand the consequences of these actions and stop short of performing them.

    So the real question to me is, why can most people stop themselves whereas others cannot? Where is the disconnect?

  8. ponderingfool

    Doesn’t the environment in which the bonobos live also matter (genes in context)? That is one of the reasons why is is important not to save just bonobos from extinction but also their ecosystem.

    Have bonobos always been limited to the Congo? My understanding is that the working model on their evolution is they split from common chimps about 1 million years ago possibly due to the formation of the Congo River. This has limited contact in the wild between bonobos and the various subspecies of common chimps.

    Could the differences seen even at the hormonal level be outcomes of founder cultural effects? Learned behavior passed along (enhanced by differences in diet)?

  9. Paul

    Sadly, I think natural selection has shown that *something* about the bonobos’ love-in isn’t working. What’s their history? Unless their numbers once were comparable to chimps, but humans preferentially invaded the habitats of bonobos? Evolution doesn’t minimize deaths–it maximizes survival. Apparently, the violent social mechanisms of humans and chimps are more successful in the wider world than the obviously-enviable social mechanisms of the bonobos. I’d say Genghis Kahn proved that rape and pillage are pretty good ways to propagate ones genes. It’s not an ideal world, this one we live in.

  10. Albert Bakker

    #7 – “Finally, it is my opinion that our ability to comprehend and understand murder, genocide, war, and death is what stops many people from committing these acts.”

    Sadly this is not or at best partly the case. In situations of war, almost exclusively a prerequisite for genocide, usually a good comprehension of the situation, being well versed in the situation politically and historically and most importantly being a socially well adapted individual increases your chances of partaking in or initiating these acts. Conversely people who – actively – refuse to take part in rape, mass executions or torture for example are much more likely to be (mild) social outcasts, they fail to properly dehumanize the category of people that form the enemy at the time. They are often a little bit indifferent, a bit difficult, maybe interact superficially with the other members of the group and such, which fortunately allows for the possibility to dispose of them quickly by having them declared insane or get rid of them even more efficiently.

    That seems to me to be the really problematic behavior which is much more complex (-ly rooted) than can be explained by hormonal disparities only. Exactly the qualities that help achieve success in normal (peaceful) social life is the behavioral basis for become a beast in an (abnormal) wartime situation.

  11. I an a cultural anthropologist who lives in hope that humans can learn to be more like bonobos…but that may take some work. Just wanted to chime in here with a question–It seems that most of the commenters are male (I am just going by the names provided and making assumptions) and possibly *not* one female commenter. Is there a gender thing going on here?

  12. sHx

    1- I don’t know if we are hard-wired to kill, but I am damn certain we are conditioned NOT to kill. From cradle to grave, we are conditioned not to kill our fellow human beings. This is something neither chimps nor bonobos can speak of. In fact, chimps and banabas can’t speak at all, which makes it rather impossible for them to communicate to each other why killing is bad.

    2- The best of the human killers, soldiers, must be trained for weeks and months to be de-conditioned so as to bring out the ‘killer instinct’. Even then, it is safe to say that an overwhelming majority of soldiers retire from their profession, live long afterwards, and die in peace, without firing a single shot in anger, let alone killing an enemy. The best killers among chimps and bonobos rise to become the leaders of the pack.

    3- Firearms, from musket to hydrogen bomb, have certainly made killing much, much easier physically and psychologically. They have also enlarged the battlefield and made it deadlier. Yes, at the press of a single button, one may kill anywhere between 1 to 10.000.000 people. But, contrary to common perception, the deadlier the weapon, the less likely it’ll be used in war. Some would argue that wars between major powers have been avoided since World War II precisely because of the fears that nukes might be used. We don’t need any insights from chimps and bonobos to develop a sense of proportion and to acquire an ability for restraint; not in nuclear age.

    4- I am not sure what chimps and bonobos think about all the ‘murders’ and violence that are being committed in their respective societies and what they do about it, but I am 100 percent certain that for all homo sapiens murder is a serious crime regardless of the reasons. Murder has been a crime in all human civilisations in all recorded human history, and laws impose very severe punishments for such crimes. Fortunately, we have moved away from the rather primitive ‘eye for an eye’ type of retribution for such crimes. We now have much better laws. Thanks to the Enlightenment and the spread of humanism, we now consider trial, conviction and incarceration for a certain period of time as more humane action than killing the killer on the spot.

    We are humans, you see? We can think, we can reason, we can empathise; these are the qualities that makes us distinct from, and better than, other animals. There is nothing, absolutely nothing, that other animals can teach homo-sapiens that can make our lives any better.

    Sorry for the rant. Today, I am not in the mood to let chimpist/bonoboean romantic delusions that’s often inflicted on my fellow humans go unchallenged. I am high on testostrone; civility be damned.

  13. Sean McCorkle

    If I can believe Steven Pinker on this subject, we seem to be getting less violent over time:

  14. Pearl

    My question is:

    Are bonobos meat-eaters?

    Chimps are. Humans are. If you need to eat animal products to survive, you have to be able to kill. (I know there are vegan arguments that we are not meat-eaters, but I don’t think they stand up well.) From my study of the “rise” of humans, it seems we definitely have left a bloody trail behind us, but as sHx points out, we have a lot of conditioning going on in both directions.

    I do, however, have to seriously disagree with sHx on his last point: Other animals, plants, the whole world have a lot to teach us about how to live on this planet without ripping the fabric of the biosphere to shreds. In that area, there is hardly an organism that can match our destructive capacity.


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About Chris Mooney

Chris is a science and political journalist and commentator and the author of three books, including the New York Times bestselling The Republican War on Science--dubbed "a landmark in contemporary political reporting" by and a "well-researched, closely argued and amply referenced indictment of the right wing's assault on science and scientists" by Scientific American--Storm World, and Unscientific America: How Scientific Illiteracy Threatens Our Future, co-authored by Sheril Kirshenbaum. They also write "The Intersection" blog together for Discover blogs. For a longer bio and contact information, see here.


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