Since September, I've posted three columns, including two written by others, on whether lethal chimpanzee raids--and by implication, human warfare—are adaptive and hence innate. In the first, I critique a widely reported study in Nature: “Lethal aggression in Pan is better explained by adaptive strategies than human impacts.” In the second, anthropologist Brian Ferguson criticizes the Nature paper. In the third, authors of the Nature paper, led by Michael Wilson, respond to Ferguson and me. In the guest column below, Ferguson volleys back, pointing out that data of Wilson et al. contradict their claim that chimp violence is "adaptive." I hope all those who believe that war is innate--perhaps because of media reports like those mentioned below by Ferguson--read this dialogue and reconsider their views. —John Horgan

Brian Ferguson replies to “Human impacts are neither necessary nor sufficient to explain chimpanzee violence (or bonobo non-violence),” by Michael L. Wilson et al.

I want to express my thanks to John Horgan and Scientific American for hosting this dialogue, and to the distinguished authors of “Lethal aggression in Pan is better explained by adaptive strategies than human impacts” (Wilson et al. 2014a), for devoting so much attention to the question of whether deadly chimpanzee violence is fostered by the effects of human activity. I agree with their response to my previous posting (Wilson et al. 2014b) that there are points of common ground in our respective approaches. I also agree that we may learn significant lessons about human warfare by critical examination of chimpanzee and bonobo behavior–though the lessons I draw are often quite different from those drawn previously. On issues about the measures they use to assess both adaptive strategies and human impacts, and thus the significance of their statistical findings, I see no reason to modify my original response (Ferguson 2014), and refer the reader there.

One question raised in their reply is whether my approach, which in some cases identifies resource competition as a cause of intergroup violence, is adaptationist. “Ferguson appears to agree with us that violence is an adaptive strategy for resource competition” (Wilson et al. 2014b:2). Direct conflict over key resources is an important part of my explanation of chimpanzee violence–as it is part of my explanation of some tribal warfare (e.g. Ferguson 1984), although it must be thoroughly embedded in its sociocultural context (Ferguson 2009). But in my analysis of chimpanzees, intense resource conflict generally is linked to human disruption, what I have labeled the Resource Competition + Human Impact Hypothesis (Ferguson 2011: 250-251). The importance of human impact is, of course, the main bone of contention in the current debate, something which I am obligated to substantiate in my forthcoming book, Chimpanzees,“War,” and History: Are Men Born to Kill?

But whether to label such conflict over resources “adaptive behavior” is a complicated question. In a broad, loose sense, sure it is. Resources are needed to survive and reproduce. Seeking food, chimpanzees act “adaptively” in all kinds of ways, which can include inflicting violence on neighbors. That is mainstream behavioral ecology. But in evolutionary biology, the “special and onerous concept” of adaptation (Williams 1966: 4) has a more precise meaning, that of a specific functional trait, designed by natural selection, to contribute to the survival and reproduction of individuals. Such an adaptation has been proposed.

In the chimpanzee literature and its popular expositions, this adaptation is often characterized as an inborn tendency to kill outsiders whenever that can be done at low cost. That is where I disagree. Some primatologists might prefer the former sense, seeing potentially lethal violence as one way chimpanzees may react to immediate situations of intense resource competition–and there is our common ground. But it is the latter meaning which has been theoretically elaborated and widely presented as the adaptive theory of deadly chimpanzee violence. Its most prominent advocates are two of the Nature article’s main organizers and authors, Michael Wilson and especially Richard Wrangham (writing with John Mitani and David Watts) (Wilson et al. 2014a: 417). That article’s closing words are: “killing is a means to eliminate rivals when the costs of killing are low” (Wilson et a. 2014a: 416).


The developed theory is that chimpanzees have an innate predisposition to kill outsiders, even if they are not engaged in immediate, sharp resource competition, because such killing at anytime reduces “rival” male strength relative to their own group. All that is required is a marked numerical imbalance, as when a multi-male party encounters a solo male from another group. That is held to be a regular occurrence because of chimpanzees’ fission-fusion pattern of association, something they share with human hunter-gatherers. The selective advantage is that this makes it more likely for the killing group to have comparatively more males in non-lethal, group confrontations over resources. “In theory, killing might be a response to competition: but there’s no indication that it happens more when resources are in short supply–more likely, it happens when food is abundant... the hypothesis [is] that chimpanzees kill rivals whenever they can do so safely, because killing raises the likelihood of winning future battles” (Wrangham 2006: 51). This is known as the Imbalance of Power Hypothesis, or with a slight variation in meaning, the Rival Coalition Reduction Hypothesis (Wilson and Wrangham 2003:381; Wilson et al. 2004:525). My book argues against the existence of that evolved predisposition, the adaptation to kill whenever it is cheap.

The proposed predisposition to kill by the numbers so as to shift the balance of male members, is frequently extrapolated to human warfare. “Chimpanzees and hunter gatherers, we conclude, share a tendency to respond aggressively in encounters with members of other social groups... and to seek, or take advantage of, opportunities to use imbalances of power for males to kill members of neighboring groups... This means that whenever the costs are sufficiently low it pays to kill or damage individuals from neighboring groups” (Wilson and Wrangham 2003:384). This application to human warfare is what compelled me to take up the study of chimpanzee intergroup violence.

An article by Richard Wrangham (2004) expounds the theory, and its application to people. I quote it at some length to clarify what is at issue. It begins with the typically non-lethal large group encounters, and the importance of male numbers. “Most encounters between chimpanzee communities involve males... If they are numerous, they advance. If not they retreat toward the heart of their territory... In the end, the party with fewer males generally retreats. The result can be important. For several weeks, the losing community tends to avoid an area that would otherwise have provided access to a preferred food; this could mean the difference between a few weeks of eating from a rich fruit crop, and being forced onto a poor diet that causes delayed response and threatens infant survival” (pp. 28-29). “A strong evolutionary rationale for killing derives from the harsh logic of natural selection. Every homicide shifts the power balance in favor of the killers, giving them an increased chance of outnumbering their opponents and therefore of winning future territorial battles. Bigger territories mean more food, and therefore more babies” (pg. 32). Data is claimed to support this postulated predisposition to kill members of other groups in order to reduce their relative number of males. “Between 1963... and 2002, a total of about 145 data-years of observation were logged across the five long-term sites. During that time, forty-six intercommunity kills were observed or suspected. Thirty-one involved members of the study communities (twenty-four adult males, one adult female, six infants” (pg. 29).

The lessons for humankind are clear. After mentioning raiding among a few hunting and gathering peoples, Wrangham writes: “The principle that underlies the mayhem is simple, then. When the killing is cheap, kill. In any particular instance it may or may not lead to a bigger territory, but from the perspective of natural selection, the specific case is less important than the average benefit. The integrating effect of selective pressures on emotional systems requires only that killing should lead to benefits sufficiently often. Just as the first male fig wasp that emerges from pupation will immediately attempt to kill any other males he finds in the same fig, so the defenders of territory benefit by taking advantage of the opportunity... What matters, it seems, is that in future battles the neighbors will have one less warrior. So those who killed will become a little more powerful as a result” (pg. 34). “[S]election has favored a human tendency to identify enemies, draw moral divides, and exploit weaknesses pitilessly across boundaries. As a result, our species remains specially predisposed to certain types of violent emotion. That selection operated in the context of a hunter-gatherer world that has all but disappeared. But if its legacy is that we are biologically prepared by natural selection to be killers, an understanding of the neural basis of intergroup violence should be a research priority” (pg. 35). (Also see Wrangham and Glowacki 2012. For contrary ethnographic evidence about the prevalence of war among hunter-gatherers, see Fry and Soderberg 2013; 2014. For archaeological evidence that war does not go back endlessly in time, see Ferguson 2013a; 2013b; Haas and Piscitelli 2013.)

This prominent position complicates a response to the Nature article, and its authors’ subsequent posting. Some of the signatories may disagree with this specific adaptive hypothesis, and/or its application to humans. The potential application to human warfare is not even mentioned in Wilson et al. (2014a), although that connection is the opening question raised in Joan Silk’s (2014:321) commentary in the same issue of Nature. In the reply to my own web posting, regarding implications for human warfare, the primary authors note: “We expect that among our 30 co-authors some diversity of opinion exists on the topic” (Wilson et al. 2014b:9). Yet they close with that very connection: “It also raises the intriguing possibility that humans and chimpanzees share similar patterns of violence due to our shared evolutionary history; we may have inherited these patterns of behavior from our common ancestor” (Wilson et al. 2014b:9).

Public Representations

There is no doubt that this extrapolation to humans is the intended take-away message in media coverage of the Nature article. NBC (2014) posted “As one of humanity’s closes living relatives, chimps can shed light on the evolution of people, such as when humans adopted warlike behaviors, Wilson said.” In The Los Angeles Times (Science Now 2014), “‘Because chimpanzees are so closely related to us, it raises the possibility that maybe these patterns are something that we share because we share them from our common ancestor,’ Wilson said” (Morin 2014). In The Boston Globe, “‘The chimp model is a very helpful model for understanding how some of humans’ really challenging kinds of behavior have been favored by natural selection,’ said Richard Wrangham” (Johnson 2014). In The Washington Post, “The research feeds into a lengthy debate over the nature of chimp violence, and what it means for humanity’s own propensity for murder. ‘We’re trying to make inferences about human evolution,’ [said] lead researcher Michael L. Wilson” (McCoy 2014).

In USA Today, “The study’s authors, led by Wilson, argue that the data show chimpanzees are natural-born killers, not peaceniks goaded into killing by human interference–a finding that some scholars believe has implications for the origins of human violence. Chimps live in territories that they patrol in large groups, a social system that leads to a ‘tendency... to kill neighbors,’ says study co-author Richard Wrangham, a primatologist at Harvard University. ‘We’re talking about something that has been very much applicable to human ancestors’” (Watson 2014). And in The Independent (2014) “Chimpanzees and humans have one key trait in common–both are natural born killers, scientists have shown. Evidence suggests our closest animal relatives have an almost psychopathic tendency towards violence and slaughter that is not the result of human interference.” (The New York Times was considerably more skeptical [Gorman 2014]). Although it is apart from the science, one may wonder what effect this portrayal will have on efforts to protect wild chimpanzees, which across Africa today suffer assaults by humans that threaten the survival of the species. “‘We’re all interested in conservation. We want chimps to do well,’ Wilson says. ‘And then they go and kill each other’” (Watson 2014).


Does Wilson et al. (2014a) support the adaptive hypothesis discussed above? In my previous posting, I made a few points based on the data about both male and female killings, as compiled in Wilson et al. (2014a), a (nearly) comprehensive data set which is a major accomplishment of that article. Although general discussions about external killings often refer to “individuals” or “victims” without specifying sex, the central contention is that they reduce the coalitionary strength of “rival” males, the protagonists in group confrontations and the patrollers who attack outsiders. “[T]hey direct their attacks almost entirely to males, the sex that alone defends the territory... Even infants killed by adults are mostly male. Those who die, therefore, are mainly the present and future defenders of the territory. By killing rival males when it is cheap to do so, chimpanzees shift the balance of power between their own community and the neighbours” (Wrangham 2006:52). The killing of males is the clear focus in Wilson and Wrangham’s discussion of lethal attacks on adults (2003:372-375) Extended Data Tables 1 through 4 enable evaluation of the proposition that intercommunity killings of males has been selected for because they reduce the number of neighboring males relative to their own group.

Unlike my prior discussion of male and female victims, what follows is limited to males only. I will not consider bonobos, why they do not kill, and what that says about male violence across three related species, though this is a major topic in my book. Nor will I include the 53 “suspected” killings. Although these inflate by half the headline of 152 “killings,” suspected killings are omitted from Wilson et al.’s (2014a:414) statistical analyses. That is good. In many of those instances, notably the adult male “killings” at Mahale and several at Gombe, direct evidence is negligible, and suspicions are based on debatable assumptions and expectations. Discussion here is restricted to “observed” or “inferred” killings of male “weaned victims” (all but a handful being adults) and infants, from 18 long-term research sites, over a total of 426 years of observation.

For the total sample (inter- and intracommunity), 32 weaned males are reported as observed or inferred killings. 15 of them come from the years of “Four Year War” at Gombe (1974-1977) and the Ngogo expansion (2002-2006). Those 9 years account for 2.1% of observation years, and 46.9% of all weaned male killings. That is a kill rate of 1.67 weaned males per year. The other 417, or 97.9%, of observation years, with 17 weaned male killings, have a rate of .04 killings per year. These exceptionally bloody 9 years are true outliers, with a weaned male kill rate over 40 times higher than the rest of the record, and require particular explanation.

The question, however, is not how many males are killed in total, but do the killings reduce the number of rivals relative to the killers’ own group? If the 9 outlier years are removed from this sample, across 18 research sites and 417 observation years, there are 9 observed or inferred intercommunity killings of weaned males, vs. 8 intracommunity killings. Net rival reduction over 417 years = 1.

We could, as Wrangham and others suggest, add in the killings of infants which were identified as males, future rivals or defenders of the group. If that is done, for those 417 years, even that infinitesimal advantage is reversed. 4 between-group male infant killings are recorded, vs. 8 intracommunity (although the victim’s sex was unknown in most external infant killings). The grand total of male rival reduction, weaned and infant = negative 3. Over 417 years of field observations, total chimpanzee killings did not reduce “rival” males in the slightest.

If we bring in the 9 outlier years, that adds 14 weaned and 1 infant males dead by external killings, and 1 weaned and 2 infants by internal killings, a net gain over extra-community males of 12. Adding that to the total for external male rival reduction, produces the grand total of 9 rival males down, over 426 years. That is a rate of 1 rival reduction every 47 years of observation.

The evidence compiled in Wilson et al. (2014a) documents less than zero rival male reduction over 98% of the observation years, and 1 rival male reduction about every 2 chimpanzee generations with the outlier years included. Does this support, or rather does it challenge, the postulate that it is an innate predisposition to kill outside males whenever killing is cheap, that such behavior is “normal” or “pervasive” among chimpanzees, that such killing is a specific response designed by natural selection to increase survival and reproduction?

That is all I have to say at the present time. The Nature article and response to my posting raise many other issues, and these will be addressed in my forthcoming book. Rather than present the evidence and arguments of Chimpanzees, “War,” and History in piecemeal fashion, I ask my colleagues for a little patience until it is published. But I do look forward to a response from my colleagues, should they care to. They can have the last word for now.

References cited:

Ferguson, R. Brian (1984). “A re-examination of the causes of Northwest Coast warfare.” In Warfare, Culture, and Environment, R. Brian Ferguson, ed. pp. 267-328. Orlando: Academic Press.

Ferguson, R. Brian (2009). “Ten points on war.” In The anthropology of war: Views from the frontline, Alisse Waterston, ed., pp. 32-49. New York: Berghahn.

Ferguson, R. Brian (2011). “Born to live: Challenging killer myths.” In Origins of Altruism and Cooperation, Robert W. Sussman and C. Robert Cloninger, eds. pp. 249-270. New York: Springer.

Ferguson, R. Brian (2013a). “Pinker’s list: Exaggerating prehistoric war mortality.” In War, Peace, and Human Nature: The Convergence of Evolutionary and Cultural Views, Douglas P. Fry, ed., pp. 112-131. New York: Oxford University Press.

Ferguson, R. Brian (2013b). “The prehistory of war and peace in Europe and the Near East.” In War, Peace, and Human Nature: The Convergence of Evolutionary and Cultural Views, Douglas P. Fry, ed., pp. 168-190. New York: Oxford University Press.

Ferguson, R. Brian (2014). "Response to Wilson and colleagues." "Cross-Check," (John Horgan blog) (accessed 9/18/2014)

Fry, Douglas P., and Patrik Soderberg (2013). “Lethal aggression in mobile forager bands and implications for the origins of war.” Science 341: 270-273.

Fry, Douglas P., and Patrik Soderberg (2014). “Myths about hunter-gatherers redux: Nomadic forager war and peace.” Journal of Aggression, Conflict, and Peace Research 6:255-266.

Gorman, James (2014). “Lethal violence in chimps occurs naturally, study suggests.” The New York Times, September 17.

Haas, Jonathan and Matthew Piscitelli (2013). “The prehistory of warfare: Misled by ethnography.” In War, Peace, and Human Nature: The Convergence of Evolutionary and Cultural Views, Douglas P. Fry, ed., pp. 168-190. New York: Oxford University Press.

Johnson, Carolyn Y. (2014). “Is war innate or a modern invention?” Boston Globe, September 17, 2014. (accessed 9/17/2014)

McCoy, Terrence (2014). “Chimpanzees are natural born killers, study says, and they prefer mob violence.” The Washington Post, September 18.

NBC News (2014), “Chimps are naturally violent, study suggests.” (accessed 9/18/2014)

Silk, Joan B. (2014). “The evolutionary roots of lethal conflicts.” Nature 513: 321-322.

Watson, Traci (2014). “Chimp-on-chimp violence: A study in the roots of brutality.” USA Today, September 17 (accessed 9/17/2014)

Williams, George (1966). Adaptation and Natural Selection: A Critique of Some Current Evolutionary Thought. Princeton: Princeton University Press.

Wilson, Michael L., and Richard W. Wrangham (2003). “Intergroup relations in chimpanzees.” Annual Review of Anthropology 22:363-392.

Wilson, Michael L., William R. Wallauer, and Anne E. Pusey (2004). “New cases of intergroup violence among chimpanzees in Gombe National Park, Tanzania.” International Journal of Primatology 25:523-549.

Wilson, Michael L., Christophe Boesch, Barbara Fruth, Takeshi Furuichi, Ian C. Gilby, Chie Hashimoto, Catherine L. Hobaiter, Gottfried Hohmann, Noriko Itoh, Katerlijne Koops, Julia N. Lloyd, Tetsuro Matsuzawa, John C. Mitani, Deus C. Mjungu, David Morgan, Martin N. Muller, Roger Mundry, Michio Nakamura, Jill Pruetz, Anne E. Pusey, Julia Riedel, Crickette Sanz, Anne M. Schel, Nicole Simmons, Michel Waller, David P. Watts, Frances White, Roman M. Wittig, Klaus Zuberbuhler, and Richard W. Wrangham (Wilson et al. 2014a). “Lethal aggression in Pan is better explained by adaptive strategies than human impacts.” Nature 513:414-417, plus supplemental material.

Wilson, Michael L., Christophe Boesch, Takeshi Furuichi, Ian C. Gilby, Chie Hashimoto, Catherine L. Hobaiter, Gottfried Hohmann, Katerlijne Koops, Tetsuro Matsuzawa, John C. Mitani, David Morgan, Martin N. Muller, Roger Mundry, Anne E. Pusey, Julia Riedel, Crickette Sanz, Anne M. Schel, Michel Waller, David P. Watts, Frances White, Roman M. Wittig, and Richard W. Wrangham (Wilson et al. 2014b). “Human impacts are neither necessary nor sufficient to explain chimpanzee violence (or bonobo non-violence).” "Cross-Check" (John Horgan blog) (accessed 10/1/2014)

Wrangham, Richard W. (2004) “Killer species.” Daedalus, Fall, 25-35.

Wrangham, Richard W. (2006). “Why apes and humans kill.” In Conflict, Martha Jones and A.C. Fabian, eds., pp.43-62. New York: Cambridge University Press.

Wrangham, Richard W., and Luke Glowacki (2012). “Intergroup aggression in chimpanzees and war in nomadic hunter-gatherers.” Human Nature 23:5-29.

Postscript: While completing this response, I had the good fortune to see the documentary film, The Hadza: The Last of the First, currently in theatrical release, in which Richard Wrangham plays a prominent role. I want to take this opportunity to note what a beautiful, interesting, and important film this is, about a hunting-gathering people whose lives illuminate our own, and who are presently imperiled by 21st century forces. See it if you can.

Editor's Note: I recently received via email the note below, which I thought merited posting here. John Horgan

John Horgan: I recently read your posts on the article by Wrangham et al. about chimpanzee violence, and Brian Ferguson’s response. In their response to his response, I was dismayed to find that they, all the authors who signed, include a reference to Steven Pinker’s new book, the Better Angels of our Nature, as if it had established that violence has indeed decreased. I recently wrote a book called Beasts: What Animals Can Teach Us About the Origins of Good and Evil. I have an appendix in that book in which I criticize Pinker’s book. For Wrangham and others to take Pinker’s book at face value is astonishing. I wonder if they read it? If they did, how could they take it seriously? It puzzles me. I cannot go into detail here, but there are so many holes in his reasoning it is just mind-boggling. He also makes careless errors that relate to his central argument, getting figures wrong to such an extent that they are made to say the opposite of what they actually say (buttressing his own position – I cannot tell whether this is deliberate or just sloppy). It is very disturbing that these authors so blithely cite Pinker as if he had proven once and for all that there is less violence today than ever. Of course he is only able to do this by excluding data he does not like (e.g., economic violence). His comments on the Second World War just sickened me, e.g., he says something to the effect that “nobody but Hitler and a few henchmen thought it was a good idea to kill Jews.” What? Has he read nothing on he Holocaust? Does he really believe that you can kill 6 million people by accident? What an astonishing statement! Jeffrey Moussaieff Masson, Ph.D.

Photo: Wikimedia Commons,