If Bonobos Are Aggressive, Does That Deny the "Self-Domestication Hypothesis"?

                                                Vergil, A Male Bonobo at the Cincinnati Zoo

For many years, I taught a course at Northern Illinois University on "Chimpanzee Politics," which included some comparative study of chimpanzees and bonobos, the two primate species most closely related to human beings, because all three species evolved from a common ancestor about 7 million years ago.  

Every time I taught the course, I took the students on a field trip to the Milwaukee County Zoo, which has the largest captive group of bonobos in the world (over 20 individuals).  I have also been to the Cincinnati Zoo, which has about 12 bonobos.  Of course, it would be better to study the social behavior of bonobos in the wild.  But that is hard to do because they are only found in one place in the world--in dense rainforest areas south of the Congo River in the Democratic Republic of the Congo.  Even those biologists who go there to study them find it hard to observe them as they move through the dense canopy of the rainforest.

Chimpanzees seem to be far more aggressive and violent than are bonobos.  Male chimps attack females and other males.  Sometimes these attacks are lethal.  Male chimps also form coalitions with other males to assert a male dominance hierarchy over females and other males.  These male coalitions also patrol the borders of their community, and they can launch attacks against other communities--even to the point of annihilating the whole community in war.

By contrast, bonobos have never been observed to engage in lethal attacks on other bonobos.  Female bonobos seem to be dominant over males.  And the females form coalitions with one another to attack males and mediate their conflicts.  In contrast to chimps, bonobo communities have never been observed to go to war with one another.  Bonobos from different communities can interact with one another peacefully.  All of this peacemaking depends on mutually pleasurable bisexual lovemaking--rubbing their genitals together--females with other females and with males.  This is why the bonobos have been dubbed the "hippie apes" who "make love not war."

Comparing human beings with these two ape species in working out the evolutionary links between the three species has provoked a debate among evolutionary biologists and social scientists.  The Hobbesian scientists argue that human beings are closer to chimps, which shows that the human state of nature was a state of war.  The Rousseauean scientists argue that human beings are closer to bonobos, which shows that the human state of nature was a state of peace.

In my posts on bonobos and the human state of nature, I have argued that Locke's account of the state of nature is closer to the truth than either Hobbes' or Rousseau's, and that evolved human nature combines the natural propensities of both chimps and bonobos.  As Steven Pinker would say, our human nature has both Inner Demons and Better Angels.  Lockean liberalism constructs a cultural niche of social institutions, mental attitudes, and moral traditions that tame the Inner Demons while eliciting the Better Angels to motivate voluntary cooperation and nonviolent relationships.

But in contrasting bonobos and chimpanzees, we should not assume that bonobos are utterly peaceful.  That bonobos are often aggressive in their attacks on one another is made clear in new research by Maud Mouginot and her colleagues that was just published online on Friday (Mouginot et al. 2024).

The message from this study as reported in the press--as in Carl Zimmer's report for the New York Times--is that "male bonobos commit acts of aggression nearly three times as often as male chimpanzees do."  That would seem to deny the common view that chimps are far more aggressive than bonobos.  But if you read the article carefully, you will see that the story is much more complicated than that.

Mouginot and her colleagues employed what scientists studying animal behavior call the "focal-animal sampling" method (Altman 1974).  All occurrences of specified actions of an individual, or specified group of individuals, are recorded for a pre-determined period of time.  

For their study, they had all-day focal follow data for 14 chimpanzee adult males from two communities in the Gombe National Park in Tanzania and 12 bonobo adult males from three communities in the Kokolopori Bonobo Reserve in the Democratic Republic of the Congo.  They had recorded hundreds of aggressive dyadic interactions, including contact aggression (physical contact such as hit, pull, bite, kick, or jump-on) and non-contact aggression (such as charge and chase).  They had also recorded whether the focal-male was the aggressor or the victim and whether he was interacting with another male or with a female. 

They wanted to use this data to test the "self-domestication hypothesis" of Brian Hare and Richard Wrangham.  They suggested that bonobos evolved to be less aggressive than chimpanzees just as dogs evolved to be less aggressive than wolves.  Humans selected less aggressive (or friendlier) wolves to become their companions, and over time, wolves evolved into dogs through domestication by human selection.  Similarly, if female bonobos formed coalitions to punish aggressive males, and if females preferred to mate with less aggressive males, which would tend to produce less aggressive offspring, bonobos could have been self-domesticated for being less aggressive or friendlier to one another (Hare, Wobber, and Wrangham 2012).

Moreover, Hare and Wrangham have also suggested that humans could have undergone a similar process of evolution by self-domestication to be less aggressive or friendlier towards individuals within their community (Hare 2017; Hare and Woods 2020; Wrangham 2019).  I have extended this idea of human self-domestication to explain the evolution of Lockean liberalism and the bourgeois virtues as symbolic niche-construction.

What Mouginot and her colleagues have found does not deny the self-domestication hypothesis of Hare and Wrangham, although it might require some refinement in the theory.  They found that there was a higher rate of male-male contact aggression among bonobos than chimpanzees.  And in both species, the more aggressive males had higher mating success.  But they found no evidence to contradict the observation that bonobos never kill other bonobos, while chimpanzees do kill other chimpanzees in fighting both within and between communities.

One possible explanation for why bonobo males engage in more non-lethal aggression with other males than do chimpanzee males is that since bonobo females prevent males from forming coalitions, bonobo males can attack other males without suffering reprisals from male coalitions.

As Hare told Carl Zimmer, the one dramatic difference in aggressiveness between the two species remains:  "Chimpanzees murder, and bonobos don't."

REFERENCES

Altman, Jeanne. 1974. "Observational Study of Behavior: Sampling Methods." Behaviour 48: 227-65.

Hare, Brian. 2017. "Survival of the Friendliest: Homo sapiens Evolved via Selection for Prosociality." Annual Review of Psychology 68: 155-86.

Hare, Brian, and Vanessa Woods.  2020.  Survival of the Friendliest: Understanding Our Origins and Rediscovering Our Common Humanity. New York: Random House.

Hare, Brian, V. Wobber, and Richard Wrangham.  2012.  "The Self-Domestication Hypothesis: Evolution of Bonobo Psychology Is Due to Selection Against Aggression."  Animal Behaviour 83: 573-85.

Wrangham, Richard.  2019.  The Goodness Paradox: The Strange Relationship Between Virtue and Violence in Human Evolution.  New York: Pantheon Books.

Zimmer, Carl.  2024.  "No 'Hippie Ape':  Bonobos Are Often Aggressive, Study Finds." The New York Times, April 12.

Bonobo Politics

In a previous post, I commented briefly on Ian Parker's article on bonobo research in The New Yorker. The article was slanted against Frans de Waal, arguing that de Waal has been responsible for a naive view of bonobos as "hippie primates" based on his limited observations of bonobos in the San Diego zoo, but without having ever seen a bonobo in the wild. Now, de Waal has written a response, which I find persuasive.

Here's how I would summarize the points of agreement and disagreement in this debate. The fundamental problem is that the observational study of bonobos is not as extensive as that for chimpanzees. In the wild, bonobos are found only south of the Congo River in the Democratic Republic of Congo. It's very hard to study them there because of the density and inaccessibility of their jungle habitat, and because of the political violence in the DRC. Most of the behavioral research has been done in zoos. In 2006, there were only a total of 84 bonobos in North America and 76 in Europe. The largest captive group is in the Milwaukee County Zoo, which has 21 individuals. By contrast, there are far more chimps in zoos around the world. Moreover, chimps have been studied in the wild for many decades at multiple sites.

Chimps have both male and female dominance hiearchies, but the alpha male dominates overall. Chimps are both competitive and cooperative. Their competition can be violent, even to the point of lethal violence. Until the 1970s, Jane Goodall was reporting that chimps were remarkably peaceful. But then she observed the chimps at Gombe dividing into two territorial groups that went to war, with the northern group (led by aggressive males) decimating the southern group. There have also been observations of lethal violence within chimp groups in the competition for alpha male status.

Bonobos also have both male and female dominance hierarchies, but it seems that the females dominate over the males. Although the adult bonobo males are bigger and stronger than the females, the females seem to be able to intimidate the males through a sexual bonding of females, who engage in elaborate bisexual acrobatics. The situation here is unclear because some primatologists speak of bonobos as showing "co-dominance," in which the alpha male and alpha female share dominance. Richard Wrangham, Takayoshi Kano, and Chris Boehm take this position. But de Waal and Amy Parish tend to stress female dominance.

This ambiguity in the dominance structure of bonobos is evident in the group at the Milwaukee Zoo, where I have often taken the students from my "Chimpazee Politics" class. Until recently, the top of the dominance hierarchy has been held by a male-female mated pair. (A good survey of this Milwaukee group can be found in a new book--Jo Sandin's Bonobos: Encounters in Empathy.) Careful observation by keepers at the zoo, supported with meticulous daily records of behavior, suggests that the female Maringa is the "matriarch" over the whole group, although her longtime mate Lody is the alpha male. At times, Lody seemed to be just enforcing the female dominance of "the power chicks." At other times, he seemed to be acting as equal to Maringa, as if they were the "royal couple."

Bonobos seem to use sexual pleasure--homosexual as well as heterosexual--as a way of pacifying conflict and releasing tension. Thus, sexuality is not just for reproduction but also for nonreproductive bonding. This pacifying effect of sexual coupling also seems to be used when bonobo groups meet in the wild. While chimps have been observed using lethal violence in group competition, bonobos have never been seen to do this. This might be because we haven't yet observed bonobos long enough. After all, Goodall had to wait for almost 15 years before she saw warfare among her chimps. But at least as of now, the evidence suggests that bonobos are better at pacifying conflict than are chimps.

Still, this does not mean that bonobos are totally peaceful. As de Waal says, the very fact that bonobos have to work hard at reconciliation shows that there is competition and conflict that needs to be controlled.

And if bonobos seem in some ways to be more egalitarian than chimps, this does not mean that hierarchy is completely absent among bonobos. There is a dominance structure among both females and males, and it's ultimately a matter either of female dominance over the males or co-dominance of alpha female and alpha male.

If human ancestry is traceable back to a common ancestor with bonobos and chimps, then one would expect that human beings show a political nature that is ambivalent in combining competition and cooperation, a propensity to dominance and a propensity to resist dominance. In his newest book Our Inner Ape, de Waal concludes that human beings are the "bipolar ape," combining chimp and bonobo traits, "being both more systematically brutal than chimps and more empathic than bonobos."

In any case, I see nothing here to support the Marxist notion that social engineering in a socialist society could make human beings so selflessly cooperative that they would have no interest in private property or social hierarchy. The failure of the Marxist regimes in the 20th century confirms the conclusion of primatologists like de Waal that Marxism is contrary to human nature.

We are neither selfless cooperators nor rational egoists. We are neither purely egalitarian animals nor purely despotic. We are a complex mixture of natural propensities that balance self-interest against cooperation and dominance against equality.

This goes against the utopian belief of the radical left in human perfectibility as supporting a revolutionary transformation of the human condition through social re-education. Rather, it supports the realistic belief of libertarian conservatism in human imperfectiblity as showing the need for regimes based on traditional morality, family life, private property, and limited government.

Other posts on bonobos can be found here and here.

Bonobos--The Politically Correct Primate

Although bonobos resemble chimpanzees, bonobos are now considered a separate species. They have passed into popular culture as the "hippie chimps." They are sexual swingers who shift easily from heterosexual to homosexual activity. They resolve conflict through sexual pleasure rather than aggressive violence--make love not war! Although bonobo males are bigger and stronger than bonobo females, the females seem to be dominant over the males, which seemed to be enforced by females bound together by lesbian sex attacking any male who becomes too assertive. And while chimps have shown what looks like warfare between distinct territorial groups, bonobo groups seem to mix easily without warfare. So while bonobos would appear to be political animals in the sense that they must manueuver their way through a complex social community, bonobo politics would seem to be more peaceful, egalitarian, and matriarchal than is chimp politics.

But all of this is based on remarkably skimpy observational evidence. Wild chimpanzees are found only in the dense tropical jungle south of the Congo River in the Democratic Republic of Congo. The few primatologists who have tried to study them in the wild have found them so hard to track that some observers have seen them for only a few hours after years of seeking them out. Most of the popular picture of bonobo behavior comes from Frans de Waal, who has seen bonobos in zoos, but who has never seen a bonobo in the wild.

I have regularly taught a course at Northern Illinois University on "Chimpanzee Politics," which includes some reading on bonobos. Each time I teach this course, I take my students on a field trip to the Milwaukee County Zoo, which has the largest group of bonobos in captivity (18-20 individuals). Each time that we go, I am struck by how we don't see the behavior that we have read about in de Waal's books (particularly, the 1997 book Bonobo: The Forgotten Ape). For example, we go there with the voyeur's expectation of seeing lots of sexy encounters, but we're usually disappointed. This experience as well as my reading of the primatology research comparing chimps and bonobos has made me skeptical of de Waal's story of bonobo life and the popular acceptance of this story as showing the potential for confirming Rousseau's image of the noble savage.

In the July 30th issue of The New Yorker, Ian Parker has an article on bonobo research. He surveys the debate among primatologists over de Waal's claim that bonobos are radically different from chimps. One can see here the moral and political implications of these debates. Feminists, gay activists, and those on the political Left would like to see bonobos as the closest living evolutionary relative of human beings, and they like de Waal's account of bonobo life as suggesting that human beings could potentially adopt the "bonobo way" of sexual equality and peaceful hedonism. Darwinian conservatives are suspicious of this as motivated by utopian longings.

As Parker indicates, the direct observations of bonobos in the wild are leading some primatologists to question de Waal's account. Bonobos are probably more violent than de Waal is willing to admit. They are not more bipedal than chimps. They are not more sexually active than chimps. And whether bonobos show female dominance--in contrast to the male dominance of chimps--is open to doubt. Even in de Waal's Bonobo book, there are some occasional suggestions that bonobo males are "slightly dominant" or maybe "co-dominant" with females (60, 74-81).

The Cincinnati Bonobos' Answer to the Creation Museum: In the Image of God or Created from Animals?

 

                            Vergil, A Male Bonobo in the Cincinnati Zoo, Born January 24, 1994

                      Vim, a Male Bonobo in the Cincinnati Zoo, Born September 5, 1995

                            Zanga, a Female Bonobo in the Cincinnati Zoo, Born January 9, 1999

After touring the Creation Museum and the Ark Encounter in northern Kentucky for two days, my wife and I spent a day at the Cincinnati Zoo.  I was particularly interested in seeing the bonobos.  The Cincinnati Zoo is one of the few zoos in the United States with bonobos.  They have about 12 individuals.  When I taught my course at Northern Illinois University on "Chimpanzee Politics," I used to take my students on a bus trip to the Milwaukee County Zoo, which has the largest group of bonobos in the U.S.--about 21 individuals.

I have written a series of posts on bonobos and how bonobos and chimpanzees are the primates most closely related to human beings, which suggests that the last common ancestor of these primates and human beings probably combined the traits that we see in bonobos and chimpanzees.  Certainly, at the Cincinnati Zoo, the signs posted around the bonobos clearly suggest that human beings evolved from some bonobo-like primate.  

Here, then, the Zoo is challenging the claim of the Creation Museum and the Ark Encounter that there is no evolutionary connection between humans, bonobos, and chimpanzees.  As part of the "Starting Points" exhibits at the Creation Museum, bonobos are grouped with gorillas, orangutans, and chimpanzees as "great apes" (CMS, 35-36).  But while evolutionary scientists place the great apes in the same family with human beings--Hominidae--the Creation Museum insists that the great apes should be placed in their own family--Pongidae--so that there is a separation between the "Ape Kind" and "Human Kind."  This is required for their claim that God originally created these two "kinds" as separate, and endowed only the human kind with the moral dignity of being created in God's image.  To teach that the great apes and human beings belong to the same evolutionary lineage, they argue, would destroy the foundations of human morality by denying the moral uniqueness of human beings as the only animals aware of and subject to God's moral law.

But it's not clear that this teaching is grounded in the Bible.  First of all, the Bible never identifies human beings as belonging to the "human kind."  The word "kind" (Hebrew min) appears in Genesis, Leviticus, Deuteronomy, and Ezekial 30 times, but it is never applied to humans.  So, the Bible does not clearly separate the "Ape Kind" from the "Human Kind," as claimed by the Creation Museum.  Moreover, the Bible refers to "apes" only twice (1 Kings 10:22 and 2 Chr. 9:21), and the Bible does not identify the various species of apes.  Everything that the Creation Museum says about the great apes being in their own family has no Biblical basis.  This confirms my point that the Bible is not a science text about natural history.

But surely, Ham and the Creation Museum would say, the Bible does clearly teach that human beings are the only animals created in God's image, with a humanly unique moral awareness, and surely this must mean that God did not allow the human species to evolve from primates, which denies the Darwinian account of human evolution from animals.

But if we compare behaviors in bonobos and chimpanzees, we can see that they are more similar to humans than to each other (Brian Hare and Shinya Yamamoto, "Minding the Bonobo Mind," in Hare and Yamamoto, eds., Bonobos: Unique in Mind, Brain, and Behavior [Oxford: Oxford University Press, 2017], 1-13).  Chimpanzees share at least five traits with humans not manifest in bonobos:  extractive foraging, lethal aggression between groups, infanticide/male coercion of females, cooperative hunting, and male-male alliances.  Bonobos share at least five traits with humans not manifest in chimpanzees:  non-conceptive sexual behavior, mother's importance to adult offspring, high levels of adult play, sharing between strangers, and female gregariousness.

And yet we can also see that human beings are unique animals in at least three respects.  In The Descent of Man, Darwin noted that self-consciousness is uniquely human: "It may be freely admitted that no animal is self-conscious, if by this term it is implied, that he reflects on such points, as whence he comes or whither he will go, or what is life and death, and so forth" (105).  Morality is also uniquely human: "A moral being is one who is capable of comparing his past and future actions or motives, and of approving or disapproving of them.  We have no reason to suppose that any of the lower animals have this capacity. . . . man . . . alone can with certainty be ranked as a moral being" (135).  And language is uniquely human: "The habitual use of articulate language is . . . peculiar to man" (107).  Is the Bible pointing to these three uniquely human traits in saying that humans were created in God's image?

As I have argued previously, we can explain human evolution from primate ancestors through a growing expansion of the number of neurons in the cerebral cortex of the primate brain.  Humans are unique intellectually and morally because of the 16 billion neurons in the human cerebral cortex.  That gives them mental capacities for self-conscious reflection, language, moral reasoning, and symbolic thinking generally, which includes the symbolic capacity for imagining supernatural realities and believing the Bible to be a supernatural revelation of God's creative history and His promise of human immortality with divine judgment in the afterlife.  The Creation Museum and the Ark Encounter are manifestations of that evolved symbolic capacity of the human mind.

A Chimpanzee Civil War: The Inner Demons and Better Angels of Primate Nature

               Ngogo Chimpanzees from the Western Group Attacking the Central Group

As I have indicated in previous posts, evolutionary anthropologists studying the evolution of war and peace have been divided into Hobbesians who believe that our hunter-gatherer ancestors were naturally violent and warlike and Rousseauans who believe that hunter-gatherers were naturally peaceful.  But in setting up this debate as Hobbes versus Rousseau, they have overlooked the position of Locke.  This is a serious mistake, because the weight of the evidence and argumentation on this issue today is on the side of Locke's account of the state of nature as a state of peace that tended to become a state of war.  It seems now that while Hobbes was partly right and partly wrong about the state of nature, Rousseau was mostly wrong, and Locke was mostly right.

THE GOMBE CHIMPANZEES

I have also indicated, how this debate extends to chimpanzees.  The Hobbesians see chimpanzees as showing an evolved propensity to war, and the clearest evidence for this is the chimpanzee civil war that Jane Goodall observed in Gombe in the 1970s.  At the beginning of 1973, she and her colleagues noticed that the chimps had formed two separate communities--the northern or Kasakela community, based on the valleys of the Kakombe and Kasakela rivers, and the southern or Kahama community, based on the valley of the Kahama river.  By early 1974, they saw the first of a series of attacks by the Kasakela community on the Kahama community, which led over four years to the complete annihilation of the Kahama community, which allowed the Kasakela chimps to expand their territory over the area once claimed by the Kahama chimps.  Since human beings are closely related to chimps phylogenetically--chimps and humans evolved from a shared common ancestor some 5-6 million years ago--chimpanzee war could show the evolutionary roots of the human propensity to war.

But the Rousseauans insist that in normal circumstances, chimpanzees are naturally peaceful, and they explain the Four-Year War in Gombe as provoked by the abnormal circumstances created by human impacts on the chimps--particularly, their being fed bananas at Goodall's camp.  During her first two years at Gombe, Goodall was frustrated by her failure to closely observe the chimps because if she attempted to follow them, they ran away from her.  She spent a lot of her time sitting on a hill trying to see the chimps through binoculars.  But then in the summer of 1962, she discovered that if she set out bananas every day, the chimps would crowd into her camp.  Soon they were eating 30, 40, or even 60 bananas per day.

Fighting over the bananas caused increased aggression among the chimps.  They even threw rocks at humans who got too close.  Goodall was shocked by the violence that came from competition for bananas.  Eventually, she tried to lower the violence by restricting the supply of bananas.  This did reduce the violence.  But it also drastically reduced the nutritional benefits that the chimps had gained from the bananas.  Some of Goodall's Rousseauan critics--particularly, R. Brian Ferguson (2023)--have argued that the restriction of the banana feeding created the frustration that prompted the violence of the Four Year War.  When Kasakela males sought food in the southern areas of their range, Kahama males chased them away.  But when Kahama chimps came into Goodall's camp, they were always fed bananas.  So it's not surprising that the Kasakela chimps developed a deep animosity toward the Kahama chimps, although they had once belonged to the same community.

THE NGOGO CHIMPANZEES

Now we have a new report--in the journal Science--of a long-running chimpanzee civil war that has divided the Ngogo chimpanzees in the Kibale National Park in Uganda (Sandel et al. 2026).  The scientists who have studied this group since 1995 have never fed them bananas or any other food.  So artificial provisioning cannot be a factor in explaining their violence.

The Ngogo group is the largest group of chimpanzees ever studied by scientists.  The size has increased from 118 individuals in 1998 to 201 in 2016.  The number of mature males (over 12 years old) has fluctuated around 40 individuals.

The Ngogo chimpanzees range over a large territory, which they have expanded through war with their neighbors.  From 1999 to 2008, they ranged over a territory of about 29 square kilometers (18 square miles).  During this time, they engaged in intensive border patrols, particularly along their northeastern border.  In 2009, they engaged in numerous lethal attacks on their neighbors to the northeast, which allowed them to expand their territory, adding about 6 square kilometers (3.7 square miles) to their territory.  This seemed to confirm the "resource acquisition hypothesis" for explaining chimpanzee warfare as adaptive for territorial expansion (Mitani, Watts, and Amsler 2010).

Prior to 2015, the Ngogo chimpanzees belonged to a single group, although there was a social substructure of two to four "clusters" that could be identified each year.  They were generally divided into a Western cluster and a Central cluster, but this cluster membership was fluid, because individuals could move freely between these clusters.  But then on June 24, 2015, when members of the Western and Central clusters approached each other near the center of their territory, the Western chimpanzees ran away, and the Central chimpanzees chased them.  For at least six weeks, the two groups avoided one another, which had never previously been observed.

In 2016, the Western males started territorial patrols directed toward the Central chimpanzees.  In 2017, what was once the center of a shared territory became a border.  In 2018, the Western group initiated lethal attacks on the Central group.  These aggressive attacks by the Western group have continued up to the present.  Remarkably, the Western group is smaller than the Central group.  In 2018, the Western group had a population of 83 (including 10 adult males and 22 adult females), while the Central group had a population of 107 (including 30 adult males and 39 adult females).  The death toll from the war drove the population of the Central group down to 80 in 2024.

This Ngogo civil war differs from the Gombe civil war in one striking way.  At Gombe, the larger group (the Kasakela chimps) attacked and defeated the smaller group (the Kahama chimps).  This led some primatologists like Richard Wrangham to develop the "imbalance-of-power hypothesis"--that larger groups (particularly, in the number of adult males) will attack smaller groups.  The Ngogo civil war is evidence against that hypothesis.

Actually, the scientists who have studied the Ngogo civil war have not been able to agree on any alternative hypothesis for explaining the war.  Some of them have wondered, however, whether shifting social bonds could explain the rupture that led to the war.  In 2014, five adult males died, perhaps because of disease.  Dr. Aaron Sandel, one of the leaders of the Ngogo Chimpanzee Project, has suggested that these five individuals might have been social bridges that held the Ngogo groups together into one community.  So that the contingency of their deaths in one year created the conditions for the polarization of the community that then led to the war.

Similarly, if Abraham Lincoln had died before or shortly after the beginning of the American Civil War, the history of that war would probably have been radically altered.  Or if Thomas Crooks had assassinated Donald Trump on July 13, 2024, American politics might be very different from what it is today.  America might be less polarized.  And perhaps there would have been no war with Iran.  Such unpredictable contingencies often determine the history of war and politics.

INNER DEMONS, BETTER ANGELS, AND LOCKEAN LIBERAL NICHE CONSTRUCTION

So what, if anything, does the history of chimpanzee wars teach us about the evolutionary history of human war and peace?  How we answer that question may depend a lot on whether we think humans are more closely related to chimpanzees or to bonobos.

Chimpanzees seem to be far more aggressive and violent than are bonobos.  Male chimps attack females and other males.  Sometimes these attacks are lethal.  Male chimps also form coalitions with other males to assert a male dominance hierarchy over females and other males.  These male coalitions also patrol the borders of their community, and they can launch attacks against other communities--even to the point of annihilating the whole community in war.

By contrast, bonobos have never been observed to engage in lethal attacks on other bonobos. Female bonobos seem to be dominant over males.  And the females form coalitions with one another to attack males and mediate their conflicts.  In contrast to chimps, bonobo communities have never been observed to go to war with one another.  Bonobos from different communities can interact with one another peacefully.  All of this peacemaking depends on mutually pleasurable bisexual lovemaking--rubbing their genitals together--females with other females and with males.  This is why the bonobos have been dubbed the "hippie apes" who "make love not war."

Comparing human beings with these two ape species in working out the evolutionary links between the three species has provoked a debate among evolutionary biologists and social scientists.  The Hobbesian scientists argue that human beings are closer to chimps, which shows that the human state of nature was a state of war.  The Rousseauean scientists argue that human beings are closer to bonobos, which shows that the human state of nature was a state of peace.

In my posts on bonobos and the human state of nature, I have argued that Locke's account of the state of nature is closer to the truth than either Hobbes' or Rousseau's, and that evolved human nature combines the natural propensities of both chimps and bonobos.  As Steven Pinker would say, our human nature has both Inner Demons and Better Angels.  Lockean liberalism constructs a cultural niche of social institutions, mental attitudes, and moral traditions that tame the Inner Demons while eliciting the Better Angels to motivate voluntary cooperation and nonviolent relationships.

But in contrasting bonobos and chimpanzees, we should not assume that bonobos are utterly peaceful.  That bonobos are often aggressive in their attacks on one another is made clear in new research by Maud Mouginot and her colleagues (Mouginot et al. 2024).

The message from this study as reported in the press--as in Carl Zimmer's report for the New York Times--is that "male bonobos commit acts of aggression nearly three times as often as male chimpanzees do."  That would seem to deny the common view that chimps are far more aggressive than bonobos.  But if you read the article carefully, you will see that the story is much more complicated than that.

Mouginot and her colleagues employed what scientists studying animal behavior call the "focal-animal sampling" method (Altman 1974).  All occurrences of specified actions of an individual, or specified group of individuals, are recorded for a pre-determined period of time.  

For their study, they had all-day focal follow data for 14 chimpanzee adult males from two communities in the Gombe National Park in Tanzania and 12 bonobo adult males from three communities in the Kokolopori Bonobo Reserve in the Democratic Republic of the Congo.  They had recorded hundreds of aggressive dyadic interactions, including contact aggression (physical contact such as hit, pull, bite, kick, or jump-on) and non-contact aggression (such as charge and chase).  They had also recorded whether the focal-male was the aggressor or the victim and whether he was interacting with another male or with a female. 

They wanted to use this data to test the "self-domestication hypothesis" of Brian Hare and Richard Wrangham, who have suggested that bonobos evolved to be less aggressive than chimpanzees just as dogs evolved to be less aggressive than wolves.  Humans selected less aggressive (or friendlier) wolves to become their companions, and over time, wolves evolved into dogs through domestication by human selection.  Similarly, if female bonobos formed coalitions to punish aggressive males, and if females preferred to mate with less aggressive males, which would tend to produce less aggressive offspring, bonobos could have been self-domesticated for being less aggressive or friendlier to one another (Hare, Wobber, and Wrangham 2012).

Moreover, Hare and Wrangham have also suggested that humans could have undergone a similar process of evolution by self-domestication to be less aggressive or friendlier towards individuals within their community (Hare 2017; Hare and Woods 2020; Wrangham 2019).  I have extended this idea of human self-domestication to explain the evolution of Lockean liberalism and the bourgeois virtues as symbolic niche-construction.

What Mouginot and her colleagues have found does not deny the self-domestication hypothesis of Hare and Wrangham, although it might require some refinement in the theory.  They found that there was a higher rate of male-male contact aggression among bonobos than chimpanzees.  And in both species, the more aggressive males had higher mating success.  But they found no evidence to contradict the observation that bonobos never kill other bonobos, while chimpanzees do kill other chimpanzees in fighting both within and between communities.

One possible explanation for why bonobo males engage in more non-lethal aggression with other males than do chimpanzee males is that since bonobo females prevent males from forming coalitions, bonobo males can attack other males without suffering reprisals from male coalitions.

As Hare told Carl Zimmer, the one dramatic difference in aggressiveness between the two species remains:  "Chimpanzees murder, and bonobos don't."

Frances White, one of the leading scientists studying bonobos in the wild, has explained that in the wild bonobo females serve a policing function, in that they intervene in fights to moderate conflicts through impartial mediation, because they benefit from living in a stable social order that is not disrupted by violence.

She has also observed that bonobos--like all primates--show a range of personality types, so that some individuals have more violent temperaments than others, and consequently the occurrence of violence can depend on the contingency of whether there are such violent individuals in the group.  She said that many of the deaths of the males comes from "testosterone poisoning"--young males vigorously displaying their virility in the forest canopy can kill themselves by slamming into a tree.

She has also said that if dominant males are grouped together in zoos without females who can moderate their male conflicts, then nasty fighting is likely to break out.  She has explained then that what the females are doing in the wild groups in pacifying conflicts is "niche construction"--behavior that creates a social environment in which stable and peaceful cooperation is adaptive.

In most of evolutionary theory, we think of organisms as carrying genes, and the inheritance of these genes by the next generation depends on organisms surviving and reproducing according to chance and natural selection in their environments.  But there is another process of evolution that arises from organisms changing their environments, which modifies the natural selection pressures in their environments.  This is evolutionary niche construction (Odling-Smee, Laland, and Feldman 2003).  Among many animals, evolutionary niche construction includes the transmission of culturally learned traditions.  And among human beings, it includes the transmission of culturally learned symbolic systems such as art, science, religion, and philosophy.

I have suggested that the history of classical liberalism is evolutionary niche construction, and that this is a big part of Pinker's argument: the history of classical liberal philosophy has created a cultural moral environment of liberalism in which peaceful cooperation and declining violence are adaptive.  (This is what Deirdre McCloskey would identify as the work of rhetorical entrepreneurs in the marketplace of ideas who have used moral persuasion to create a liberal culture that honors the bourgeois virtues.)

As Pinker argues, human nature is a mixture of Inner Demons and Better Angels.  Human beings are innately predisposed to violence by their Inner Demons, but the expression of those predispositions is not "hydraulic"--that is, a drive that must necessarily be satisfied--but "strategic"--that is, a propensity that is responsive to environmental triggers.  Classical liberalism constructs a cultural niche of social institutions, mental attitudes, and moral traditions that tend to elicit the Better Angels to motivate voluntary cooperation and nonviolent relationships.

In this way, human nature constrains and enables but does not determine human culture and individual judgment.  Within the constraints of human nature as a mixture of Inner Demons and Better Angels, classical liberalism can foster those cultural traditions and individual judgments that limit the Inner Demons and channel the Better Angels towards a system of liberty and voluntarism.

REFERENCES

Altman, Jeanne. 1974. "Observational Study of Behavior: Sampling Methods." Behaviour 48: 227-65.

Ferguson, R. Brian. 2023. Chimpanzees, War, and History: Are Men Born to Kill? Oxford, Oxford University Press.

Goodall, Jane. 1986. The Chimpanzees of Gombe. Cambridge: Harvard University Press.

Hare, Brian. 2017. "Survival of the Friendliest: Homo sapiens Evolved via Selection for Prosociality." Annual Review of Psychology 68: 155-86.

Hare, Brian, and Vanessa Woods.  2020.  Survival of the Friendliest: Understanding Our Origins and Rediscovering Our Common Humanity. New York: Random House.

Hare, Brian, V. Wobber, and Richard Wrangham.  2012.  "The Self-Domestication Hypothesis: Evolution of Bonobo Psychology Is Due to Selection Against Aggression."  Animal Behaviour 83: 573-85.

Mitani, John C., David P. Watts, and Sylvia J. Amsler. 2010. "Lethal Intergroup Aggression Leads to Territorial Expansion in Wild Chimpanzees." Current Biology 20:R507-R508.

Odling-Smee, F., Kevin Laland, and Marcus Feldman. 2003. Niche Construction: The Neglected Process in Evolution. Princeton University Press.

Pinker, Steven. 2011. The Better Angels of Our Nature: Why Violence Has Declined. New York: Penguin Books.

Sandel, Aaron A. et al. 2026. "Lethal Conflict After Group Fission in Wild Chimpanzees." Science 392(9 April):216-220.

Wrangham, Richard.  2019.  The Goodness Paradox: The Strange Relationship Between Virtue and Violence in Human Evolution.  New York: Pantheon Books.

Zimmer, Carl.  2024.  "No 'Hippie Ape':  Bonobos Are Often Aggressive, Study Finds." The New York Times, April 12.

The Aristotelian Prudence of Bonobos

In the Department of Political Science at Northern Illinois University, we have "Politics and the Life Sciences" as a field of study at both the undergraduate and graduate levels. Many of our Ph.D. students have combined "biopolitics" with political theory and other traditional areas of research in political science. Andrea Bonnicksen, Rebecca Hannagan, and I are the three faculty members who teach in this area. One of the things we do is compare human politics with the political behavior of other primates--particularly, chimpanzees and bonobos.

The Milwaukee County Zoo has the largest group of bonobos in captivity. And so I have taken some of my students to visit the Milwaukee Zoo to see the bonobos. This semester Bonnicksen will be taking some of her students there.

Many people find it weird, if not ridiculous, that political scientists would be looking for political behavior among apes. But for me, this is an extension of Aristotle's biological science of political animals.

Last year, Jo Sandin published a book--Bonobos: Encounters in Empathy--on the Milwaukee bonobos. Dr. Harry Prosen is a psychiatrist who has worked with the bonobos there for some years. Sandin reports that Prosen's colleagues in the psychiatric community have been impressed by his accounts of the practical judgment shown by some of the bonobos. In particular, Lody--until recently, the alpha male--is said by Dr. Prosen to show "evidence of wisdom, in the Aristotelian sense: the ability to see life in all its aspects and to act in a way that benefits others." According to Prosen, "Lody's empathetic behaviors and ability to use good judgment in parenting skills, discipline and, in many instances, the demonstration of altruistic behaviors have had a powerful impact on the development of the juvenile males in the bonobo group" (Sandin, pp. 51-52).

Many people would dismiss as silly the idea that apes might exercise prudence or practical judgment, which Aristotle regarded as the primary intellectual capacity for moral and political life. But Aristotle's biological writings would suggest that he himself would take this seriously. After all, he often attributes prudence (phronesis) to nonhuman animals--and particularly, to those he identifies as political animals. Although he does not speak of apes as political, he does recognize their remarkable similarities to human beings and suggests that they are the animals most closely related to human beings.

As I have suggested in previous posts, the importance of prudence--judging what is best in the particular circumstances of particular individuals--across many animal species shows the complexity and contingency of animal behavior, so that we cannot predict animal behavior with precision. The failure to achieve predictive power in the scientific study of human politics shows a pattern that holds across all animal behavior. A biological science of political animals would be a historical science of particular individuals and groups with complex cultural traditions.

Two previous posts on bonobos can be found here and here. Some of my posts on animal prudence can be found here, here, and here.

The Evolution of War and Lethal Violence

Over the years, I have written a long series of posts on whether evolutionary science can adjudicate the debate between Thomas Hobbes and Jean-Jacques Rousseau over whether our earliest human ancestors were naturally violent (as Hobbes argued) or naturally peaceful (as Rousseau argued).  Many social scientists have been vehement in taking one side or the other in this debate.  But I have argued that John Locke took a third position that is closest to the truth--that our foraging ancestors lived in a state of peace that tended to become a state of war.  Hobbes is partly right. Rousseau is mostly wrong. And Locke is mostly right.

Now a new study published in Nature has stirred up this debate again.  This research shows how human lethal violence against members of our own species is part of our evolutionary history as mammals. 

Jose Maria Gomez and his colleagues have compiled and analyzed the sources of mortality from a comprehensive sample of more than 4 million deaths from 1,024 mammalian species drawn from 137 mammalian families, which is 80% of the total number of mammalian families.  The data for humans was from over 600 studies, ranging from Paleolithic samples (50,000 to 12,000 years ago) to anthropological studies of the last few centuries. (J. M. Gomez, M. Verdu, A. Gonzalez-Megias, & M. Mendez, "The Phylogenetic Roots of Human Lethal Violence," Nature 538 [13 October, 2016]: 233-37). 

They applied comparative statistical techniques to the phylogeny (the family tree) of mammalian violence to reconstruct the rates of lethal violence, defined as killing by members of the same species.  They calculated that the rate of lethal violence at the phylogenetic origin of mammals was about 0.30%, which is approximately 1 in 300 deaths.  Rates of lethal violence increased across the family tree leading to primates--2.3% for the common ancestor of primates and tree shrews, declining slightly to 1.8% for the ancestor of the great apes (gorillas, orangutans, chimpanzees, and bonobos), and then increasing to 2% at the origin of the human species, which is about six times higher than the rate at the origin of mammals.  Increases in lethal violence were correlated with increasing group living and territoriality.  Apparently, lethal violence increases when individuals are living close together and competing for territorial resources.

Gomez and his colleagues compared the phylogenetically inferred levels of human lethal violence with the levels observed in archaeological and anthropological records.  They found that the phylogenetic prediction of about 2% for prehistoric foragers was about the same as what emerged from their calculations from the archaeological records.  But they also found that levels of lethal violence rose far above 2% in historic times when human beings began to live in chiefdoms and states.  Levels of deaths by lethal violence did not begin to decline until about 500 years ago.  Finally, in the last 100 years, these levels fell to below 2% for the first time in human history.

In his commentary in Nature, Mark Pagel claims that this Gomez study supports the Hobbesian position that human beings are innately violent (Pagel, "Lethal Violence Deep in the Human Lineage," Nature 538 [13 October, 2016]: 180-81).  In an article in The Guardian, Steven Pinker is quoted as claiming that this study does support the Hobbesian position and Pinker's argument in The Better Angels of Our Nature about how modern societies have brought a dramatic reduction in violence from the high levels in prehistoric foraging societies.

On the contrary, John Horgan and Brian Ferguson have claimed that the Gomez study actually supports the Rousseauean position that war is not innate for human beings but largely a product of the cultural environments that arose after human beings left the ancient foraging life.

There are a number of points in dispute here.

The first point is that in measuring how often humans and other mammals kill members of their own species, the Gomez study makes no distinction between individual killing and killing in war.  So they include all forms of human-on-human killing, which includes individual homicides and infanticide.  Rousseaueans like Brian Ferguson and Douglas Fry agree that prehistoric foragers often killed one another, but almost all of this homicidal killing arose in personal disputes or feuding, and so, they argue, this was not warfare.  But notice what this means: the Rousseaueans are conceding that our foraging ancestors were not utterly peaceful, as Rousseau claimed, because they did sometimes kill one another, although this was not killing in war between groups.  One can find high rates of individual killing among ancient foragers, but without war.

The second point is that the Gomez study's estimate of 2% of deaths among ancient foragers being due to lethal violence is much lower than the estimates by Pinker and others of 15-25% of deaths due to warfare.  While Pagel claims that the Gomez study shows "hunter-gatherer societies as being engaged in constant battles" (181), the Gomez study actually concludes that the 2% rate "contrasts with some previous observations," such as that of Pinker (235).

The third point is that the Gomez study shows that rates of lethal killing increase (from 2% to 9%) with the move from ancient foraging bands to chiefdoms.  These high rates do not drop below 2% until the last 100 years.  This looks like the pattern suggested by Douglas Fry--like the letter n--low violence in the ancient foraging past, then increasing violence in more complex societies, followed by declining violence in contemporary states.  Pinker might argue that this conforms to the Hobbesian pacification thesis, because the only form of social order that reduces violence is the modern centralized state.  Ferguson and Fry seem to concede this.  This shows that both the Hobbesians and the Rousseaueans can agree that violence is not genetically determined, because the expression of the natural capacity for violence depends on the social environment.  As Azar Gat has said, "war is innate, but optional."

The fourth point is that the rates of lethal violence in prehistoric bands and tribes is much lower than for bands and tribes studied by anthropologists over the past few centuries.  This suggests that foraging societies become more violent after having contact with colonial societies, and that such high levels of violence cannot be found in the archaeological record for ancient prehistoric foragers.

The final point is that the two living ape species most closely related to humans--chimps and bonobos--show conflicting evidence as to whether the primate phylogeny of humans favors an innate propensity to violence.  Chimpanzees show male coalitional raiding leading to killing that looks like primitive warfare similar to what people like Richard Wrangham see among human foragers.  But there is no clear evidence for even a single case of conspecific killing among bonobos.  Pagel writes: "Even the usually peaceful bonobo Pan paniscus can sometimes display violent behavior" (181).  But he does not cite any evidence for bonobo lethal violence.

In the Supplementary Material for their article (found online at the Nature website), Gomez and his colleagues provide the data for mammalian lethal violence.  For chimpanzees, they say there is documentation for 734 deaths and for 4.49% of these being deaths from conspecific killing.  For bonobos, it's 145 deaths and 0.68% conspecific deaths.  For the bonobo data, they cite five articles.  Apparently, they are relying on one "suspected" case of conspecific killing for bonobos.  But when I contacted Frances White, the leading observer of bonobos in the wild, and asked her about this, she said that this one case is very dubious, and that there is no clear case of conspecific killing among bonobos.  She said that there were at least three cases of bonobo cannibalism, but there was no evidence that these were cases of killing rather than eating already dead carcasses.

It's easy to understand why the Rousseaueans love the bonobos--there're the hippie apes who make love not war.

On the other hand, it's also easy to understand why the Hobbesians love the chimps and not the bonobos, because the chimps are closer to Hobbesian expectations for a evolutionarily close human relative.  Frances White once observed:

"As we have found out more about how bonobos behave in the wild, they have declined in favor as a model for our ancestor . . . This makes me wonder how much our use of models is influenced by how we would like our ancestor to have behaved--clearly the bonobo has fallen from grace because it shows what, for many, is behavior that is socially unacceptable for a close relative of ours."

Some of my other posts on the evolution of war and violence are here, here, here, here, here, here, here, here, here, here, here, here, here, here, here, and here.

Classical Liberalism as Evolutionary Niche Construction for Declining Violence: Bonobos and Humans

At the Liberty Fund conference on "Liberty and Violence," one of the participants was a primatologist who studies bonobos in Africa.  At one point in our discussion of Steven Pinker's Better Angels of Our Nature, she complained about how Pinker unfairly dismisses the evidence for peaceful cooperation among bonobos, in contrast to the violence of chimpanzees.  She noted that Pinker relies on primatologists who study chimps or who have only studied bonobos in zoos, and who want to make bonobos look like chimps in their tendency to violence.

She explained that in the wild bonobo females serve a policing function, in that they intervene in fights to moderate conflicts through impartial mediation, because they benefit from living in a stable social order that is not disrupted by violence.

She also observed that bonobos--like all primates--show a range of personality types, so that some individuals have more violent temperaments than others, and consequently the occurrence of violence can depend on the contingency of whether there are such violent individuals in the group.  She said that many of the deaths of the males comes from "testosterone poisoning"--young males vigorously displaying their virility in the forest canopy can kill themselves by slamming into a tree.

She also said that if dominant males are grouped together in zoos without females who can moderate their male conflicts, then nasty fighting is likely to break out.  She explained then that what the females are doing in the wild groups in pacifying conflicts is "niche construction"--behavior that creates a social environment in which stable and peaceful cooperation is adaptive.

Here she was appealing to an idea developed by F. John Odling-Smee, Kevin Laland, and Marcus Feldman in Niche Construction: The Neglected Process in Evolution (Princeton University Press, 2003).  In most of evolutionary theory, we think of organisms as carrying genes, and the inheritance of these genes by the next generation depends on organisms surviving and reproducing according to chance and natural selection in their environments.  But there is another process of evolution that arises from organisms changing their environments, which modifies the natural selection pressures in their environments.  This is evolutionary niche construction.  Among many animals, evolutionary niche construction includes the transmission of culturally learned traditions.  And among human beings, it includes the transmission of culturally learned symbolic systems such as art, science, religion, and philosophy.

In response to this discussant's comment about social niche construction among bonobos, I suggested that the history of classical liberalism is evolutionary niche construction, and that this is a big part of Pinker's argument: the history of classical liberal philosophy has created a cultural moral environment of liberalism in which peaceful cooperation and declining violence are adaptive.  (This is what Deirdre McCloskey would identify as the work of rhetorical entrepreneurs in the marketplace of ideas who have used moral persuasion to create a liberal culture that honors the bourgeois virtues.)

As Pinker argues, human nature is a mixture of Inner Demons and Better Angels.  Human beings are innately predisposed to violence by their Inner Demons, but the expression of those predispositions is not "hydraulic"--that is, a drive that must necessarily be satisfied--but "strategic"--that is, a propensity that is responsive to environmental triggers.  Classical liberalism constructs a cultural niche of social institutions, mental attitudes, and moral traditions that tend to elicit the Better Angels to motivate voluntary cooperation and nonviolent relationships.

In this way, human nature constrains but does not determine human culture and individual judgment.  Within the constraints of human nature as a mixture of Inner Demons and Better Angels, classical liberalism can foster those cultural traditions and individual judgments that limit the Inner Demons and channel the Better Angels towards a system of liberty and voluntarism.

A few of the people at this Liberty Fund conference were such fervent critics of the Pinker argument that they resolutely rejected this claim that there has been a historical trend towards declining violence as fostered by classical liberal culture.  But by the end of our discussions, I had a strong impression that many--and maybe most--of the people there found the Pinker argument persuasive, even if one could dispute some of Pinker's evidence and argumentation.

Nevertheless, even those of us who were persuaded by Pinker were left with the unsettling conclusion that while the general historical trend of declining violence and increasing liberty was encouraging, the contingencies of history make it impossible to predict that this trend will continue unbroken into the future.

There was a lot of discussion of whether Pinker's science is a falsifiable and predictive science.  I argued that Pinker's science is a historical science, as opposed to nonhistorical sciences like physics and chemistry.  In a historical science, one can make retrospective  predictions about the past and make only broad pattern predictions about the future.  One cannot precisely predict the future.  And, indeed, Pinker stresses that he is only claiming that we can see declining violence in past history, and that we cannot be sure that this will continue into the future.  His entire book, as he indicates in the first paragraph (xxi), is looking backward, so that he does not, because he cannot, make precise predictions about the future.

Pinker suggests that there are four possible historical trends in war--escalation, cyclical, random, and declining sawtooth (191-92).  He is arguing for seeing the trend as a declining sawtooth.  The way he draws this pattern through the Second World War as one data point shows that in a declining sawtooth pattern, there can be generally decline in violence, while still allowing for a sudden jump in violence towards the greatest single atrocity in human history--55 million violent deaths in the Second World War--which indicates the radical contingency in Pinker's historical science.

This contingency is made evident in Pinker's book by his noting how three individuals acting under the influence of illiberal ideologies were responsible for the greatest atrocities of the 20th century.  The Second World War would not have occurred without Hitler.  The Great Purge in the Soviet Union would not have occurred without Stalin.  The Great Famine and the Cultural Revolution in China would not have occurred without Mao.  So "tens of millions of deaths ultimately depended on the decisions of just three individuals" (343).

It will always be possible for such individuals to appear in circumstances that allow them to rise to positions of power that enable them to perpetrate mass atrocities of violence.  But this can be made unlikely insofar as classical liberalism as evolutionary niche construction continues to spread, making us more like bonobos and less like chimpanzees.

Some of these points are elaborated in my series of posts on Pinker (in October to December of 2011) and in my post on "The Behavioral Ecology of Chimpanzee War and Liberal Peace."

A New Study of Chimpanzee Warfare: Hobbes Versus Rousseau

 A chimpanzee war party of males on patrol typically moves in a single-file pattern.

I have written a series of posts on the debate over chimpanzee warfare, which can be found here and here. 

As I have indicated, this is ultimately a debate in political philosophy between Hobbesians who argue that war is rooted in human nature and Rousseaueans who argue that war is a purely cultural trait and that the original state of nature was peaceful.  Since chimpanzees and bonobos are the living primates most closely related to human beings, the dispute over whether chimps and bonobos are naturally warlike or peaceful becomes part of the dispute over the evolutionary roots of human warfare.

A new article favoring the Hobbesian position has just been published in Nature:  Michael L. Wilson, et al., "Lethal Aggression in Pan Is Better Explained by Adaptive Strategies than Human Impacts," Nature 513 (18 September 2014): 414-417.  Here's the abstract:

"Observations of chimpanzees (Pan troglodytes) and bonobos (Pan paniscus) provide valuable comparative data for understanding the significance of conspecific killing.  Two kinds of hypotheses have been proposed.  Lethal violence is sometimes concluded to be the result of adaptive strategies, such that killers ultimately gain fitness benefits by increasing their access to resources such as food or mates.  Alternatively, it could be a non-adaptive result of human impacts, such as habitat change or food provisioning.  To discriminate between these hypotheses, we compiled information from 18 chimpanzee communities and 4 bonobo communities studied over five decades.  Our data include 152 killings (n = 58 observed, 41 inferred, and 53 suspected killings) by chimpanzees in 15 communities and one suspected killing by bonobos.  We found that males were the most frequent attackers (92% of participants) and victims (73%); most killings (66%) involved intercommunity attacks; and attackers greatly outnumbered their victims (median 8:1 ratio).  Variation in killing rates was unrelated to measures of human impacts.  Our results are compatible with previously proposed adaptive explanations for killing by chimpanzees, whereas the human impact hypothesis is not supported."

This is impressive evidence for the Hobbesian argument.  But as indicated by a news story in The New York Times and a  blog post by Marc Bekoff, the Rousseaueans are not ready to surrender.  Four points need to be made here.

First of all, notice that after 50 years of studying 22 communities, there have been only 58 directly observed killings and 41 inferred.  Even if one concedes that intraspecific killing occurs, one can see that it is remarkably rare, and generally these animals are peaceful and cooperative.  The Hobbesians have to admit this.

The second point here for the Rousseaueans is that the bonobos still look even more peaceful than the chimps.  And that's why the hippie bonobos are so loved by the Rousseaueans.

The third point is that Rousseaueans like Brian Ferguson will argue that this study does not go deeply enough into the long history of human disturbance of chimpanzee habitats to see its effects.  Ferguson is writing a book--Chimpanzees and War--that will elaborate his case for the Rousseauean position.  Meanwhile, Ferguson has written a good response to the Nature article.

The final point is that even if lethal coalitional raiding is a natural adaptation for chimpanzees and human hunter-gatherers, complex human warfare as it arose first with bureaucratic states is a largely cultural invention that can be changed through cultural evolution.  Richard Wrangham, one of the senior authors of the Nature article, has stressed this.  And, indeed, if Steven Pinker is correct, the cultural history of recent centuries has shown declining violence.

Monogamy, Polyamory, and the Primate Myth

That we can understand human nature by comparing ourselves with our closest living primate relatives--chimps and bonobos--has been a recurring theme in my blog posts over the years.  So it might seem that Jonathan Leaf's new book--The Primate Myth--is a powerful critique of my position, because Leaf argues that we cannot learn who we are by studying chimps and bonobos, assuming that the underlying patterns of their behavior are the same as ours, which is what Leaf calls the Primate Myth.  Even if we share some common genetic ancestry with primates--particularly, the great apes--we are radically different from apes, Leaf insists, in our genetic profile, in the configuration of our brains, in our anatomy, and in our social behavior.  To explain those differences, we need to see how our evolutionary adaptations for hunting and language made our earliest human ancestors less like apes and more like dolphins, dogs, rats, and elephants.  

Leaf stresses that human beings are unique in their extreme sociality--their social awareness and their capacity for social collaboration.  While primates are social animals, they are not as highly social as the animals that live in packs or herds.  Humans are more a herd or pack animal than a primate, because our earliest evolutionary ancestors depended on collaborative hunting in packs and herding together to repel predators.

Because of the Primate Myth, humans have been classified as belonging to the Order of Primates and the Family of Hominidae (humans and the other great apes--chimpanzees, bonobos, gorillas, and orangutans).  But if we recognized how radically different human beings are from primates, Leaf argues, we would classify humans as belonging to their own order (Homo), or at least in their own family, separated from the great apes (245).

Leaf is certainly right in saying that to understand human evolution, it is not sufficient to look for human similarities to those other primates with whom we share common descent, because we must also see how humans differ radically from those primates.  By convergent evolution our human ancestors evolved adaptations for filling an evolutionary niche--collaborative hunting--that made them more like the herd or pack animals than like the other primates with whom they shared genetic ancestry.

But Leaf is wrong in saying that most evolutionary scientists today deny this because they are so committed to the Primate Myth that humans are most like primates--particularly, chimps and bonobos--in their behavioral and psychological propensities and unlike all non-primate animals.  The Primate Myth is a straw man, because in attacking the Primate Myth, Leaf distorts what most evolutionary scientists say about human evolution to make it easier to refute.

In a few places in his book, Leaf comes close to admitting this.  One example is a passage about Frans de Waal, who is the primary villain in Leaf's book (38-40). Leaf says that many readers of de Waal's first book--Chimpanzee Politics--mistakenly assumed that he was saying "that humans were almost exactly the same as chimps." In fact, Leaf observes, de Waal in that book points to "a number of critical differences" between humans and chimps. And in his later writings, de Waal "emphasized the kinship that humans bear to an array of non-primate animals." "In other words, what de Waal was thought to be arguing in his first and most influential book and what he was actually proposing aren't the same."  But then Leaf devotes most of his book to attacking what he here recognizes to be a mistaken interpretation of de Waal--the belief that humans are almost the same as chimps and other primates.

I can imagine, however, that Leaf could respond by saying no, this is not a straw man, because I point to good examples of people assuming the Primate Myth in explaining human evolution.  The best example might be the argument of Christopher Ryan and Cacilda Jetha in their popular bestseller Sex at Dawn: How We Mate, Why We Stray, and What It Means for Modern Relationships (2010).  Under the influence of the Primate Myth--"the idea that human behavior is very much like ape behavior"--they say that since chimps and bonobos are sexually promiscuous, with no enduring pair-bonding of sexual mates, that must mean that our earliest human ancestors were just as promiscuous, and therefore monogamous pair-bonding is contrary to our evolved human nature.  Human beings would be happier, Ryan and Jetha insist, if married couples were "polyamorous"--if they felt free to have extramarital sexual affairs without feeling shame or fearing that this must break up their marriage: couples can happily live together and rear their children together but sleep around without harming themselves or their children, because primate promiscuity is natural for human beings.  

Leaf devotes over eight pages of his book to explaining and refuting their argument (168-177).  He has also taken excerpts from this part of his book for an article on "The Polyamory Delusion" for National Review that will be the cover story for the February 2026 issue.  His preoccupation with Sex at Dawn makes me think that his reading of this book some fifteen years ago was what originally motivated him to write The Primate Myth to counter the harmful consequences of teaching people that imitating the behavior of chimps and bonobos would make them happy.

Leaf is correct in criticizing Ryan and Jetha for their refusing to see that ape sexuality is different from human sexuality, because in contrast to ape promiscuity, the evolutionary function of human sexuality is to strengthen monogamous pair-bonds and the biparental care of children.  He is mistaken, however, in his claim that evolutionary biologists agree with Ryan and Jetha in their primate model of human sexuality as naturally inclined to polyamorous promiscuity.  The Primate Myth embraced by Ryan and Jetha is a fringe position that is outside the mainstream thinking of most evolutionary scientists, and that's why I say the Primate Myth is a straw man.

Previously, I have written about how the evolution of monogamous pair-bonding among our prehistoric foraging ancestors gave birth to the unique structure of human society that sets it apart from the social life of chimps and bonobos as based on sexual promiscuity in mating and reproduction.  Even in cultures that allow polygamy, most people are in monogamous marriages.  And there is plenty of evidence that polygamous marriages tend to be full of conflict: the cowives fight amongst themselves, men fight with one another over access to women, and children often suffer from parental neglect.

It is true, of course, that even in societies where monogamy is the norm, there is sexual promiscuity that can create some uncertainty about paternity.  But remarkably, recent research shows that the rate of "extra pair paternity"--where a woman bears offspring from a man other than her spouse--is usually below 3 percent (Larmuseau et al. 2013, 2019; Zimmer 2016).  By contrast, extra-pair fertilization in socially monogamous bird species can range as high as 20 percent (Brouwer and Griffith 2019; Griffith et al. 2002).

Leaf cites this kind of evidence as showing that while promiscuity is a problem for human beings, it is not as pervasive as it is for primates because human beings have an evolved natural desire for monogamous pair-bonding.  Leaf rightly concludes that this refutes Ryan and Jetha's primate model of human sexuality as naturally inclined to polyamorous promiscuity rather than sexual monogamy.

But Leaf is wrong when he says that evolutionary biologists agree with Ryan and Jetha (Leaf 2025, 171, 173).  In fact, Ryan and Jetha repeatedly insist that they are attacking the "standard narrative of human sexual evolution" that upholds monogamous pair-bonding as crucial for human evolution (Ryan and Jetha 2010, 7-8, 11, 25, 34-35, 38-39, 46-60, 99, 137, 142, 149, 223, 266, 300).  Leaf is silent about this.

For example, Ryan and Jetha criticize Frans de Waal for embracing the "standard narrative."  They quote various remarks by de Waal as illustrating the "standard narrative" of human sexual monogamy that they reject.  De Waal argued that the nuclear family is "intrinsically human," and the pair-bond is "the key to the incredible level of cooperation that marks our species," because "our success as a species is intimately tied to the abandonment of the bonobo lifestyle and to a tighter control over sexual expressions." He observed: "The intimate male-female relationship . . . which zoologists have dubbed a 'pair bond,' is bred into our bones.  I believe this is what sets us apart from the apes more than anything else."   He also said: "Both chimps and bonobos are far more promiscuous than we are. Our testicles reflect this: they are mere peanuts compared to our ape relatives' coconuts" (de Waal 2005, 108, 113, 124-25; Ryan and Jetha 2010, 76, 115, 225).

Leaf does not allow his readers to see this.  Because if his readers saw that the Primate Myth as embraced by Ryan and Jetha is rejected by de Waal and most other evolutionary scientists, they would see that when Leaf attacks the Primate Myth, he is attacking a straw man.

REFERENCES

Brouwer, Lyanne, and Simon Griffith. 2019. "Extra-Pair Paternity in Birds." Molecular Ecology. 28: 4864-4882.

de Waal, Frans. 2005. Our Inner Ape: A Leading Primatologist Explains Why We Are Who We Are. New York: Riverhead Books.

Griffith, Simon, Ian Owens, and Katherine Thuman. 2002. "Extra Pair Paternity in Birds: A Review of Interspecific Variation and Adaptive Function. Molecular Ecology 11: 2195-2212.

Larmuseau, M. H. D., et al. 2013. "Low Historical Rates of Cuckoldry in a Western European Human Population Traced by Y-chromosome and Genealogical Data." Proceedings of the Royal Society B. 280:20132400.

Leaf, Jonathan. 2025. The Primate Myth: Why the Latest Science Leads Us to a New Theory of Human Nature. New York: Bombardier Books.

Ryan, Christopher, and Cacilda Jetha. 2010. Sex at Dawn: How We Mate, Why We Stray, and What It Means for Modern Relationships. New York: Harper.

Zimmer, Carl. 2013. "Monogamy and Human Evolution." New York Times, August 2.

Zimmer, Carl. 2016. "Fathered by the Mailman? It's Mostly an Urban Legend." New York Times, April 8.