The Distinctiveness of Human Aggression

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The “self-domestication hypothesis” is the idea that in the ancestral environment, early human communities collectively killed individuals prone to certain forms of aggression: arrogance, bullying, random violence, and monopolizing food and sexual partners.

Over time, our ancestors eliminated humans—typically males—who were exceedingly aggressive toward members of their own group.

If there was a troublemaker, then other less domineering males conspired to organize and commit collective murder against them.

Women too were involved in such decisions involving capital punishment, but men typically carried out the killing.

Humans tamed one another by taking out particularly aggressive individuals. This led us to become relatively peaceful apes.

But if humans are “self-domesticated,” then why are there so many violent people among us today?

The fact is, humans are not nearly as violent as our nearest evolutionary relatives.

Comparing the level of within-group physical aggression among chimpanzees with human hunter-gatherer communities, chimps are 150 to 550 times more likely than humans to inflict violence against their peers.

We humans are far nicer to members of our own group than chimps are. Thanks to our ancestors and their ability to plan organized murder. And tear overly dominant males to shreds.

Many people are familiar with the findings that bonobos are more peaceful than chimpanzees.

This is true.

Male bonobos are about half as aggressive as male chimpanzees, while female bonobos are more aggressive than female chimpanzees.

Bonobos are “peaceful,” relative to chimps. But bonobos are extremely aggressive compared to humans.

The eminent Harvard biological anthropologist Richard Wrangham explores these findings at length in his fascinating 2019 book The Goodness Paradox: The Strange Relationship Between Virtue and Violence in Human Evolution.

This is a review and discussion of Wrangham’s book.

The book highlights a particularly ugly observation from primatologists: “One hundred percent of wild adult female chimpanzees experience regular serious beatings from males.”

Chimp males commonly beat up on females, often in surprise attacks. A grim passage from the book:

“One attack at Kanyawara, in western Uganda…lasted for a full eight minutes, during which a male grabbed sticks and beat the female intermittently with them, when he was not slapping, punching, and kicking her. The male’s aim in such attacks is to intimidate the chosen female into readily acceding to his future demands for sex. For each female, one male distinguishes himself from other males by being the one who most frequently attacks her. The tactic is often successful. Over subsequent weeks, a female’s most frequent aggressor tends to be her most frequent sex partner, and eventually, even though she is likely to mate several times with every male in her community, he will be the most likely father of her next baby. This stomach-churching practice is part of the reason why, as males become adult, they go through a ritual of beating up on every female. A male’s ability to intimidate females is a vital component of his strategy for having as many offspring as possible.”  

Glimpsing into the behaviors of humans as well as our nearest evolutionary relatives can be disturbing.

Still, Wrangham is careful to note:

“Whether a behavior evolved because it was directly selected for, or because it was a by-product of another adaptive feature, or indeed, whether it evolved at all, should not color our moral judgments. Many tendencies that we regard as morally reprehensible clearly evolved, including numerous kinds of sexual coercion, lethal violence, and social domination. Equally, many morally delightful tendencies did not evolve, such as charity to strangers and kindness to animals. Our decisions about which behaviors we like or dislike should never be attributed to our understanding of their evolutionary history or adaptive value.”

Ever since the Enlightenment, as religion gradually fell by the wayside, people have been trying to ground their moral compasses in another prestigious entity—science. Sadly, this hasn’t really worked.

At the outset, Wrangham states that most of the material in his book is so new that it has only been published in scientific papers. His aim is to translate the technical research into a more accessible form.

Still, although the main idea is straightforward enough, much of the text in The Goodness Paradox is rather dense for a general audience. I’ll try to communicate Wrangham’s core points while not diluting the message.

Humans present a puzzle. We are the only species capable of horrific cruelty as well as extraordinary kindness. Think of the Nazis who existed in the same society as those who risked their lives hiding strangers from being killed by them.  

This incongruity gives rise to the perennial question: Are humans naturally good or evil?

In the introduction of the book, Wrangham provides his answer: both.

Informed by evolutionary science, the book states that our biology gives rise to contradictory motives and behaviors.

We are innately good and innately bad.

Wrangham then posits a more useful question: What is the significance of primate behavior for understanding human nature?

In chapter one, the book reports observations of early anthropologists who would encounter “primitive peoples.” These western observers were surprised at how little fighting there was within these communities.

Their peacefulness, however, extended only to members of the same society.

Wrangham writes:

“In 1929, the anthropologist Maurice Davie summarized a consensus understanding that remains true today: people were as good to members of their own society as they were harsh to others. There are two codes of morals, two sets of mores, one for comrades inside and another for strangers outside, and both arise from the same interests. Against outsiders it is meritorious to kill, plunder, practice blood revenge, and steal women and slaves, but inside the group none of these things can be allowed because they would produce discord and weakness.”

In everyday life, humans tend to experience relatively low levels of within-group violence.

But conflict between groups creates conditions for catastrophic death rates. Humans are good at getting along with their peers. And good at extinguishing their adversaries.

How can this be explained?

First, by understanding the psychology of aggressive behavior.

The book defines aggression as “a behavior intended to cause physical or mental harm.” This falls into two major types:

• Reactive aggression: Impulsive, uncontrolled, reflexive

• Proactive aggression: Deliberate, calculated, premeditated

Reactive aggression is an impulsive response to a threat. It is an immediate reaction that is a consequence of anger, fear, or both.

It is displayed far more among men than women. And is linked with high levels of testosterone.

Think of a person you have met who is prone to outbursts of explosive rage in response to relatively minor inconveniences. The person is most likely a young male.   

Reactive aggression is associated with losing your temper in response to perceived insults, embarrassment, physical danger, or momentary setbacks.

This kind of impulsivity aims to eliminate the source of the anger, which is usually the person responsible for eliciting the aggression. Reactive aggression is especially pronounced among young men fighting over status or the attention of potential romantic partners.

In contrast, proactive aggression is characterized as predatory, instrumental, and “cold.” Unlike reactive aggression, proactive aggression entails a calculated and purposeful act of aggression with a specific goal, rather than an impulsive response to extinguish the source of fear or threat.

People prone to proactive aggression show reduced emotional sensitivity, less empathy for their victims, and less remorse over their actions.

Interestingly, this is how psychopaths behave. Most humans aren’t psychopaths. At least not to their own group members. To outsiders, though, humans can be astoundingly cruel.

I once heard a psychologist describe a psychopath as a person who treats their ingroup the same way that normal people treat their outgroup.

Proactive aggression is typically used against members of outgroups.

Proactive aggression is marked by the presence of a deliberate plan and an absence of emotion at the time of assault. 

There is a brilliant scene in the fifth season of The Sopranos showcasing Tony’s use of (verbal) proactive aggression.

Tony urges his sister Janice to enter anger management classes.

To everyone’s surprise, the classes actually help Janice. She and her husband Bobby invite Tony over for dinner. Tony observes the progress Janice has made. This vexes him.

Throughout his therapy sessions with Dr. Melfi, Tony repeatedly states that his depression, his panic attacks, and his temper all stem from being brought up in a dysfunctional household. He doesn’t really believe he can get better, and resents himself for not doing so.

So seeing Janice (who grew up in the same circumstances) become more jovial irritates Tony. He can’t stand the idea of his sister escaping the hell that their parents created for them so he drags her back down. Tony uses his upbringing to excuse his behavior, and Janice’s self-improvement invalidates those rationalizations.

So he decides to strategically antagonize her.

Tony finds ways to bring up the fact that she abandoned her firstborn child, until Janice finally explodes with anguish and rage.

Tony walks away, satisfied that he has stunted his sister’s personal progress. In a series where we see the protagonist cheat, steal, and murder, this is arguably the moment the audience’s perception of Tony really begins to shift.

His other transgressions can be excused as being part of the mafia code or to Tony’s impulsive and sometimes uncontrollable temper (reactive aggression).

But in this case, Tony strategically inflicts emotional pain on someone he loves in order to dispel his own feelings of inadequacy and self-resentment.

Interestingly, there is also research indicating resting heart rate is a predictor of aggression.

For instance, a meta-analysis found that low heart rate was associated with greater levels of delinquency, aggression, and violence in children and adolescents.

Other research has found that men, on average, have lower resting heart rates than women.

The book claims that this tendency to remain calm, unfazed, and in control of our behavior is one major reason for why humans are capable of kindness and cruelty.

There are individual differences in aggression, too. Although aggression has been found to be sixty-five percent heritable, environmental effects are more responsible for rule-breaking (34 percent non-shared environment; 18 percent shared environment). This suggests that while genes play a large role in how aggressive people are, their parents and peers shape how it is expressed.  

What factors were responsible for the self-domestication of humans?

Homo sapiens are thought to have arisen about 300,000 years ago.

Based on DNA analysis and findings from paleontology, Wrangham estimates that the self-domestication process began approximately 400,000 to 600,000 years ago. It started among the “Mid-Pleistocene Homo,” an early forerunner of Homo sapiens.

The book states that “the execution hypothesis” is key to the process. Capital punishment practiced in small human groups gave rise to a less aggressive psychology that uniquely defines Homo sapiens compared with other primate species.

Executing the most antisocial individuals selected against aggression in favor of greater docility and conformity.

There are physical characteristics associated with human self-domestication, including neotenous facial features, reduced sexual dimorphism, and smaller teeth. Our ancestors, before the self-domestication, had a more mature appearance, larger brows, larger teeth, and greater visible sex differences existed between men and women.

In the course of evolution, human communities selected against reactive aggression.

In other words, early humans united to inflict penalties (including death) on impulsive and domineering members of their communities.

Wrangham writes:

“For the first time, coalitions of males became effective at deliberately killing any member of their social group who was prepared to use violence on his own behalf and simply did not care what others thought about him. In the end, execution was the only way to stop such a male from being a tyrant…The killing of aggressive males is an alarmingly potent form of social control and a human universal.”

If a man repeatedly irritated his companions in the group with aggressive and selfish behavior, then gradually, a whispered consensus emerged against him.

A conspiracy formed among the other men, and the aggressor was killed.

Throughout thousands of prehistorical generations, those with a high propensity for reactive aggression were targets of execution.

Killing these individuals gradually led humans to have a calmer, less overtly hostile temperament.

Relatedly, as Christopher Boehm and other anthropologists have found, hunter-gatherer communities are largely egalitarian. At least among the adult men.

Wrangham gives an example from the Ona (Selk’nam), a nomadic people in South America:

“A certain scientist visited our part of the world and, in answer to his enquiries on this matter, I told him that the Ona had no chieftains, as we understand the word. Seeing that he did not believe me, I summoned Kankoat, who by that time spoke some Spanish. When the visitor repeated his question Kankoat, too polite to answer in the negative, said: ‘Yes, Señor, we the Ona, have many chiefs. The men are all captains and the women are sailors.”

Among some hunter-gatherer bands, leaders, headmen, and chiefs do exist. But they tend not to hold much power.  

Their main purpose is to assist with consensus-seeking when the group needs to make important decisions.

For example, the Mae Enga of Highland New Guinea will discuss their options for warfare and raiding at length. Every man has the option to contribute to the discussion. The leaders within the community maintain a low profile.

As group opinion sorts itself, and a consensus appears to emerge, a leader will exert his limited influence to help crystallize an agreement. But they tend to do this very judiciously, lest they elicit collective scorn from the other men.

Hunter-gatherer communities do not jump straight to executions to control and eliminate hostile males. They start out with softer approaches first, such as leveling mechanisms—norms to constrain people’s egos.

For instance, the book quotes anthropologist Elizabeth Cashdan:

“The proper behavior of a !Kung (Ju/’hoansi) hunter who has made a big kill is to speak of it in passing and in a deprecating manner; if an individual does not minimize or speak lightly of his own accomplishments, his friends and relatives will not hesitate to do it for him.”

Among the Ju/’hoansi hunter-gatherers, people were eight times more likely to deliver criticism than praise.

Similar observations have been made by anthropologists observing modern hunter-gatherer communities. Shaming, teasing, ridicule, and laughter are all used to cut down men who might believe themselves to be better than the others.

More drastic measures include shunning or ostracism, which are often effective because social exclusion tends to be deeply painful. As they say, there’s a reason why solitary confinement is considered by prison inmates to be the worst punishment.

Language was key to the ability to form conspiracies to take out domineering aggressors. It gave humans the ability to murmur about how much you resent some other individual, gauge the reactions of your companions, and coalesce around the decision to impose a penalty.

The capability to quietly and quickly communicate birthed many tools of social control, from ridicule to gossip, all the way up to ostracism and killing.

Wrangham illustrates how such executions work with an example from the Yanomamö hunter-gatherers. Some of the men in the group were annoyed by the arrogance and bullying by a particular male. One day, they encouraged the bully to climb a tree to extract honey. He agreed, and laid down his weapons before climbing up.

His assassins collected the bully’s weapons and merely waited for him to come down before easily killing him.  

Wrangham states that self-domestication and morality are intertwined.

Morality can only evolve among a species that is intensely sensitive to social disapproval.

Negative moral judgment is extremely unpleasant for most people.

For our ancestors, cultivating a good reputation was crucial. Troublemakers were ridiculed, ostracized, and sometimes murdered.

Today, we often feel self-conscious even in one-shot interactions with people we will never meet.

This doesn’t make rational sense, until you understand that early humans almost never interacted with anonymous strangers. For them, every social interaction held potentially life-threatening importance. We still carry this psychology with us today. We will endure minor and sometimes even extreme discomfort to avoid the negative judgment of strangers.

Of course, morality has differed throughout time and across cultures. As the book states:

“Society influences what we care about, but evolution has produced the fact that we care.”

In my view, morality is analogous to language.

Both are human universals. But the specifics of each vary by culture and change over time.

Morality and language are both governed by certain rules. Though languages differ, they all have some underlying similarities. This is also the case with moral edicts. The specifics may differ by time and place, but all languages have rules about nouns. And though the specifics differ, all moralities have rules about harm.

We also absorb both language and moral beliefs through osmosis. Who you grew up around is the best predictor of what language you speak and what your moral values are.

Some people are moral nihilists who say nothing is really right or wrong because morality differs so much by time and place and culture. Others are moral realists who think there is one true morality, and that moral values are indistinguishable from objective facts.

My view is that morality is “real” in the same way that language is real. Both can change, but still operate within certain constraints. There are rules to every language, and rules to every morality.

Saying morality isn’t real is like saying language isn’t real. And saying there is one true morality is like saying there is only one true language.

Anyway, returning to the book, people in a given culture are extremely responsive to moral accusations. They generally want the best moral reputation they can get.

In the ancestral environment, being thought of as a bad person could get you killed. And being thought of as a good person increased the likelihood of obtaining social allies, romantic partners, social status, and access to crucial resources.

Morality among hunter-gatherers grew out of the execution of dominant males. This is because early humans discovered that they could conspire to kill anyone, not just hostile and aggressive males. They could also eliminate any kind of troublemaker who broke the rules of the community.

This threat gave rise to collective obedience and docility among humans, who feared the possibility that others in the group would coordinate to ostracize them or kill them.

Evolution, then, favored individuals who avoided being viewed as social outcasts. Our ancestors had to learn which behaviors were “right” and which were “wrong.” And getting it wrong could be fatal. Early humans, who successfully navigated this perilous terrain, were the ones who got it right.

As the evolutionary psychiatrist Randolph Nesse has written in Good Reasons for Bad Feelings: Insights from the Frontier of Evolutionary Psychiatry:

“Social anxiety is overwhelmingly common. Natural selection shaped us to care enormously what other people think…We constantly monitor how much others value us…Low self-esteem is a signal to try harder to please others.”

We inherited social anxiety, self-consciousness, and a preoccupation with social image from successful early humans who learned the local rules and generally abided by them.

Wrangham makes good on explaining the book’s title, The Goodness Paradox.

Throughout the book, he uses the term “coalitionary proactive aggression,” which means a group of individuals who come together to deliberately attack a person or another group.

This type of violence is unique to humans.

As Wrangham puts it:

“Tribalism does not distinguish us, nor does reactive aggression. It is coalitionary proactive aggression that makes our species and societies truly unusual.”

Among humans in hunter-gatherer communities, killing a member of a different group is often considered pleasurable in itself. The aim, in some cases, isn’t to obtain food or mates or resources. Rather, killing the troublemaker or members of the outgroup is a goal in itself.

Coalitionary proactive aggression is common between groups. But it is extremely rare within groups.

In the New Guinea highlands, a man told the anthropologist Polly Wiessner how his people felt about killing in a small-scale war:

“Now I will talk about warfare…When a man was killed, the clan of the killers sang songs of bravery or victory. They would shout Auu! (‘Hurray!’ or ‘Well done!’) to announce the death of an enemy.”

Wrangham states that evolution has made the killing of outsiders pleasurable.

In the ancestral environment, an outsider was typically viewed as a member of a hostile neighboring society, and regarded as nonhuman. Each early human community viewed outsiders from other groups in this way. Eliminating such rivals would reduce their power to inflict harm, and thwart their ability to compete for the same resources.

Large-scale murder of out-groups is an example of coalitionary proactive aggression. The book states:

“We are inclined to label callously planned violence such as the Holocaust as ‘inhuman.’ But phylogenetically, of course, it is not inhuman at all. It is deeply human. No other mammal has such a deliberate approach to mass killing of its own species.”

This kind of careful planning to inflict harm on outgroups comes naturally to us, and many people find it pleasurable.

As the psychologist Jonathan Haidt has observed in the context of university campuses:

“A funny thing happens when you take young human beings, whose minds evolved for tribal warfare and us/them thinking, and you fill those minds full of binary dimensions. You tell them that one side of each binary is good and the other is bad. You turn on their ancient tribal circuits, preparing them for battle. Many students find it thrilling; it floods them with a sense of meaning and purpose.”

Inter-group conflict can take on different forms.

Today, most people in developed countries would not feel joy at the mass killing of a particular ethnic or religious group. But more than a few would experience glee at inflicting pain on their political adversaries.

Wrangham holds out the hope that wars between nations will stop, and cites a study indicating that based on current trends, the implementation of a World State will be established between 2300 C.E. and 3500 C.E. Though he also observes that this might introduce “the possibilities of tyranny.”

The book states that “while war is not inevitable, conscious effort is needed to prevent it.”

Students of Rousseau say we are naturally peaceful but corrupted by society.

Students of Hobbes say we are naturally violent but civilized by society.

The truth is, we are both. Humans are extremely kind, cooperative, and altruistic. We are also exceedingly cruel, selfish, and vicious.

Wrangham’s self-domestication hypothesis sheds light on this paradox. Early humans, using language, joined forces to eliminate bullies prone to reactive aggression. Gradually, our ancestors became good at inhibiting their tempers and learned how to plan carefully coordinated attacks on troublemakers, and then on any particular targeted individual.

The human ability to control their impulses and engage in cooperation is a double-edged sword.

As the physician and sociologist Nicholas Christakis has written in Blueprint: The Evolutionary Origins of a Good Society:

“Maybe kindness and hatred are related. Mathematical analyses of models of human evolution suggest that…neither altruism nor ethnocentrism evolved on its own, but they could arise together. In order to be kind to others, it seems, we must make distinctions between us and them.”

In the ancestral environment, cold-blooded, self-centered individualists would have been wiped out by human groups who cooperated.

However, indiscriminate cooperators would have been exploited and outcompeted by those who carefully cooperated only with their team.

Modern humans descended not from cold-blooded individualists nor from indiscriminate cooperators. Rather, we are descendants of those who distinguished between us and them. Between ingroups and outgroups.

As Wrangham concludes in his fascinating book:

“The important human quest should not be to promote cooperation. That goal is relatively simple and firmly founded on our self-domestication and moral senses. The harder challenge is reducing our capacity for organized violence.”

Humans don’t fly off the handle and react with rage at the slightest challenge or provocation the way chimpanzees do.

But chimpanzees don’t coordinate to systematically engage in the mass murder of their fellow species, the way humans do.

Chimps don’t cooperate the way humans do.

The great comparative psychologist Michael Tomasello, an expert on chimpanzee cognition, has stated, “It is inconceivable that you would ever see two chimpanzees carrying a log together.”

The ability to unite for such a simple task explains much—both good and bad—about our species.