Bad to the Bone: Are Humans Naturally Aggressive?

At the recent American Association of Physical Anthropologists meetings in Portland, I sat through an interesting talk about lethal aggression in chimpanzees. The presenter, Michael Wilson from the University of Minnesota, did a good job of laying out a substantial overview of all the data we have on chimpanzee lethal aggression.

The bottom line: Chimps can be pretty violent, especially males. But exactly why they are violent is not fully understood. Importantly, if one removes the largest and most violent population from the dataset, then no single explanatory pattern emerges.

Of course, this fuzzy and interesting conclusion is not what is making the rounds in the Twitterverse. Rather, folks are saying that this is just further evidence that chimpanzees, and their closest relatives (humans), are aggressive by nature. If this is true, then domestic abuse, bullying, and warfare are pretty much to be expected: It is just the way we are.

Or not. Let's get our myth-busting caps on and think about this. What does it mean to be aggressive by nature? Even more the point, what is aggression and where does it come from?

If humans have evolved as aggressors, if using violence is a core part of our nature, then aggression needs to be a thing (a trait) that can be targeted and shaped by evolutionary processes. There also needs to be evidence that humans (and our primate relatives) regularly rely on aggression, over other types of behavior, to achieve mating and other social successes.

Okay, so what do we know?

Aggression is not a single trait, or an easily described behavioral system. It is not a thing that has evolved as a package; rather, it is a suite of behaviors that has a dynamic and complicated range of expression. Anthropologists, biologists, and psychologists note different behaviors and patterns of “aggression” when defending yourself versus when planning an attack, from mothers defending their infants, from predators chasing prey, in fear-induced aggression, in sex-related aggression, and in territorial aggression.

In humans, there are no consistent patterns of aggressive behaviors that make men have more luck with women or succeed over other men for status, even though sometimes aggression does play a role. Even when fighting, many of the most effective professionals (such as in boxing and Ultimate Fighting) are good because of their ability to strategically constrain their aggression.

Okay, but what if aggression is itself a physiological system (part of our body) that has been favored over evolutionary time?

It’s not. Unlike a femur (the long bone in your upper leg) there is no single thing or pattern that we can measure and label as “aggression.” While we know that certain parts of the brain (the prefrontal cortex, the amygdala, and the hypothalamus) interact with certain neurotransmitters (serotonin, Monoamine oxidase A (MAOA)) and that a range of steroid hormones (like testosterone and other androgens) work together to produce aggressive behavior, we also see that there is no specific physiological or neurological system designed for aggression. Everything involved in the expression of aggression is tied to other systems, and its use in behavior is highly contextual.

For example, take MAOA (called the “the warrior gene”). One version of this gene is associated with hyperaggression in males (it is little studied in females). However, expression of this gene is related to childhood stressors and life experience. We see that a slightly larger percentage of men with the “aggressive” version of this gene (compared to those without it), who live through real childhood trauma and social stress are highly violent and have trouble controlling their behavior as adults. But many of those with the “warrior” version of the gene don’t have these problems at all (me, for example). These same kinds of complexities are true for serotonin, testosterone, and the other hormones and neurotransmitters associated with aggression.

There is no consistent system or pattern in the human body or mind that we can point out as the seat or the main actor in aggressive behavior.

Well, what about the other primates and our fossil ancestors: our evolutionary comparisons? We know that common chimpanzees can be highly aggressive, but their sister species, the bonobos, rarely are; both are equally related to humans. Across the primates, you find that within-species violence resulting in death is rare, and not wide-scale. There is also no one dominant or consistent pattern of male aggression tied to mating success across primate species. While aggression is important in the social lives of monkeys and apes (as in humans), it is not the “driver” of social systems.

In the human fossil and archaeological record, there is no good evidence of intense aggression and warfare until very recently; it is associated with the advent of permanent settlements, agriculture, and social stratification. Increased social inequality and more complex political and economic systems seem to correlate with more types of aggression and violence in human societies. Interestingly, these scenarios also correlate with larger and more complex peaceful relationships among and between peoples.

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Humans can, and do, engage in a wide variety of aggression. However, aggression is not our primary “go to” behavior as successful organisms. There is insufficient evidence to argue that we have evolved a suite of specifically aggressive behaviors to succeed in the world. In fact, it is largely our abilities to get along and to negotiate complex social problems, with and without aggression, that make humans one of the most successful species on this planet (a topic for a future blog).

If you really want to think deeply about aggression, violence, abuse, warfare, and human nature, have a look at the references below; where everything I’ve stated here is discussed with details, data and explanation. Dive into the actual datasets and debates, and go bust some myths for yourself.