Is aggression genetic?

We've been conditioned to believe that some people were born violent -- but the science shows that's just not true

Published May 28, 2012 10:00PM (EDT)

     (<a href=''>stefa</a> via <a href=''>Shutterstock</a>)
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This article is an adapted excerpt from the new book "Race, Monogamy, and Other Lies They Told You", from University of California Press.

In his story of Dr. Jekyll and Mr. Hyde, Robert Louis Stevenson famously shows the dark side of humanity. The respectable and kind Dr. Jekyll devises a potion that enables him to bring to the surface his evil core. In Mr. Hyde, with his vile appearance and violent behavior, Jekyll sees that this alter ego “bore the stamp of lower elements in my soul.”

The concept that humanity has a violent and evil core is widespread; it is one of the oldest and most resilient myths about human nature. From historical and philosophical beliefs to current popular and scientific beliefs, the view that a savage and aggressive beast is a central part of our nature permeates public and academic perceptions. Given this view, it is a common assumption that if you strip away the veneer of civilization, the restraints of society and culture, you reveal the primeval state of humanity characterized by aggression and violence.

While there are many reasons for the resilience of this myth, the most powerful one is the simple fact that humans today can and do engage in extreme levels of violence and aggression. If you read the newspaper, visit online news sites or turn on the television, you are guaranteed to come across some evidence of humans behaving violently toward other humans. While many animals aggressively hunt, capture, and eat prey, it is relatively rare for most animals to engage in intense, lethal aggression with members of their own species.

Many social mammals display some intraspecific (within the same species) aggression and violence, sometimes resulting in death. A male lion might seriously injure another male lion in a fight over access to a pride of females, two rams might butt heads until one of them staggers away seriously hurt, or a male baboon might repeatedly attack a female in his group, wounding her and injuring her infant. However, these events, while aggressive and violent, are not the main ways in which the individuals in these species interact. For the most part, death of opponents in these cases is neither the premeditated goal nor the outcome of the behavior. So, while intraspecific violence occurs, most species do not exhibit extreme aggression regularly and methodically. Humans are the only species that practice premeditated homicide and full-out war. That humans can, and do, participate in aggression and violence in ways that most other animals do not (and cannot) has led many to theorize that this aggression, this inner beast or demon, our Mr. Hyde, is part of human evolutionary heritage.

The myth of human aggression holds that we are indeed evolved to be killers, or at least aggressors who use the threat of violence as a major evolutionary tool. The mark of this evolved tendency toward aggression can therefore be found in our bodies and minds, especially those of men:

When we look at humans’ bodies and brains, we find more direct signs of design for aggression. The larger size, strength, and upper-body mass of men is a zoological giveaway of an evolutionary history of violent male-male competition. . . . (Stephen Pinker, psychologist)

There is the notion that aggression, the capacity for immense violence, evolved specifically because of the benefits it gave males, including an edge in competition with one another and between groups of males. Some make the argument for indicators of aggressive cores in our closest primate relatives and suggest that aggression and violence result in evolutionary benefits:

Thus, both the patterns of deadly violence in nature and the ethnographic record of simple hunter-gatherers clearly suggest that intraspecific human violence and the threat of it, while obviously undergoing transformations and varying in form through human history, are on the whole as old as humanity itself, indeed as old as nature. . . . (Azar Gat, political scientist)

In short, the myth of human aggression is that humans (especially males) have a specific and distinct tendency toward aggression and violence and that this is patterned in our bodies and minds and arose due to evolutionary pressures of competition between men and between groups. If this were true, then aggression and violence must be a core part of who we are as humans because over evolutionary time those with the more aggressive behavioral patterns or traits must have defeated opponents and mated more successfully than those who were more pacific.

It is obvious that human aggression is an amazingly complicated thing. There is variation in conflict styles and aggression across individuals, sexes, genders, societies, and time frames. Aggression is an important part of being human, but it is not who we are at our core. We now know that aggression itself is not a uniform or consistent discrete trait, so aggression per se cannot be favored by evolutionary pressures to form the basis of the human experience.

The other primates show us that we do not have specific, evolved patterns of heightened aggression, especially in males. Looking at the chimpanzee species demonstrates the potential for variability in the expression of aggressive and nonaggressive behaviors in our shared ancestors. War is common in the human experience today, but it is not a central part of our evolutionary heritage. We know that males and females differ in some facets of aggression, but a lot of those differences have to do with physical size and the social and experiential contexts in which they find themselves. We know that more aggressive, more violent, or more warlike males do not necessarily do better, either in humans or in our closest relatives.

Human aggression, especially in males, is not an evolutionary adaptation: we are not aggressive, big-brained apes. We know the regions of the brain and body that influence normal aggression. While our genes do not control or determine the normative expression of aggression, abnormal biological function can influence particular patterns of aggressive behavior. The nature of human aggression is not found in our genes, but understanding the function and variation in our biology can help us better understand the pathways and patterns of aggressive behavior. As a species we do not rely on aggression and violence more than cooperation; there is no pattern of evidence to support a notion that humanity is aggressive and selfish by nature. The myth of a human nature characterized by an intrinsic aggressiveness is simply not true.

And yet the popular press and much of the public (and some academics) hold the belief that there is a specific biology or a genetic basis for aggression, especially in males. Identifying the genetic key to aggression is not possible, because it does not exist.

It is pretty clear that in humans two parts of the brain, called the prefrontal cortex and the dorsal anterior cingulated cortex, are centrally involved with the expression of behavior, especially aggression. The prefrontal cortex is linked to other behaviorally important brain structures called the amygdala and the hypothalamus. In general, these parts of the brain receive a variety of inputs from other areas of the brain (vision, smell, touch, pain, sound, memory, language, etc.) and then interact in a sort of feedback loop to stimulate other bodily systems (such as hormones, neurotransmitters, and muscles) into action.

The prefrontal cortex does a bunch of other things as well, including playing central roles in introspection, recognition of emotions, regulation of emotions, and detection of conflict situations, and it acts as a trigger to initiate a variety of other neurological systems in regard to social interactions. The dorsal anterior cingulated cortex seems to be involved in the regulation of responses to anger, pain, and social rejection. From brain imaging studies, we know that individuals who are particularly aggressive often show lowered neuronal activity, reduced glucose metabolism, or even reduced density of gray matter in the prefrontal cortex than those who are not as aggressive. Studies of individuals who have received brain damage to the prefrontal cortex and amygdala reveal that they demonstrate more impulsive and antisocial aggressive behavior or have lowered abilities to control the expression of aggression.

Additionally, shock therapy in the 1950s and 1960s directed at the amygdala and prefrontal cortex resulted in lowered overall arousal rates and severely reduced aggression. In short, there are multiple studies which all point to the action of the prefrontal cortex, the dorsal anterior cingulated cortex, and at least the amygdala, as important areas for understanding the biological infrastructure of aggression.

There are a suite of molecules (called neurotransmitters) produced by the body which directly interact with these regions of the brain and are involved in the expression of aggressive behavior (among other things). They are the 5-hydroxytryptamine receptors (5-HT for short and involved with the neurotransmitter serotonin), the neurotransmitter and neurohormone dopamine, the metabolic enzyme monoamine oxidase A (MAOA), and a variety of steroid hormones such as testosterone, other androgens, and estrogen. None of these are a smoking gun for the origin and expression of aggression in general, but some of these are implicated, to some extent, as playing a role in the emergence of particular types or patterns of aggression.

Genes are basically just stretches of DNA that contain the code for either the production of a protein molecule (or parts of a molecule) or the regulation of other genes or of themselves. Genes come in multiple forms (alleles). While genes contain codes for proteins and their regulation, the relationship between genes and complex molecules like neurotransmitters and hormones is very complicated.

Dubbed the “warrior gene” in the press, the gene that codes for monoamine oxidase A (MAOA) has recently been a central player in the study of genetic influences on aggressive behavior. This gene is found on the X chromosome (the one you get from your mother), so that males have only one copy of it and females have two (males are XY and females are XX). The gene product, the enzyme monoamine oxidase A, interacts with the neurotransmitters serotonin, dopamine, and norephedrine, regulating their release and breakdown so that once they do what they are supposed to do they don’t build up or interact with other receptors, causing problems for communication between parts of the brain.

It turns out that there are at least four different common alleles for this gene that have the effect of increasing or decreasing the amount of MAOA produced. Lowered amounts of MAOA in the brain in some mice, rhesus monkeys, and humans, under certain conditions, is associated with increased aggression and reduced ability to control impulsive behavior. A noticeable number of mice, monkeys, and human males who had the low-MAOA-production alleles and who experienced severe social and/or physical trauma or abuse during early childhood development were more likely to express heightened aggressive and antisocial behavior as adults. Some low-MAOA humans also score higher on self reports of aggressive and violent behavior. However, in some cases the high-producing alleles are correlated with aggressive behavior in male children. In a famous case of a Dutch family who have a very rare allele where no MAOA is produced at all, three males exhibited extreme aggressive and antisocial behavior. Now, not all individuals with the low-expression alleles exhibit this kind of aggression, not even all of those with the low-expression alleles who had traumatic or abusive childhoods. In addition some of the high-expression allele carriers exhibited high aggression.

All of these studies were conducted on males because it is much easier to discover which alleles are acting as the males only have one copy of the gene. In females it is more difficult to identify the actual action of the gene because they may have two copies but only one is actually active; determining which one that is can be very difficult. Thus, while this gene is often invoked as an example of a male biological basis of aggression, there have been no in-depth studies of this gene in females so we do not know if it functions the same way. We should note that the enzyme MAOA operates exactly the same in the females’ brains as it does in males.

This research focuses on the variation in allele frequencies for MAOA and the relationship that its expression has to early social and physiological experiences during development and its variation in functional outcomes in different social contexts. In other words, this is an underlying genetic element that plays an interesting role in affecting the brain structures that are associated with the expression of aggression. But the behavioral outcomes of gene variation are totally dependent on the patterns in early life experience and the social context in which some carriers of the low-production allele find themselves. The bottom line is that if you have a low-expression allele and you undergo severe childhood trauma or abuse, then the likelihood of your having problems in the neurological infrastructure of aggression that result in higher aggression is higher than if you had the regular-production allele.

Most people have heard of serotonin, but most do not know that it is tied to the neurotransmitter 5-hydroxytryptamine (5-HT for short). Of all the well-known neurotransmitters this is the one best recognized for affecting behavior, especially aggression. The main way in which 5-HT relates to aggression has been determined from studies of the variation in the receptors that interact with 5-HT. There are at least thirteen types of 5-HT receptors and multiple molecules that interact with, and regulate, 5-HT in the brain. In general, it appears that serotonin concentrations in the brain, and the way they interact with receptors, can modulate aggression and violent behavior in mammals, including humans (although the vast majority of the research has been done with rodents).

In general, low 5-HT levels are associated with higher levels of aggressive or impulsive behavior and high 5-HT levels and/or manipulation of the 5-HT receptors in different parts of the brain can act to reduce aggression. Genetic evidence for these impacts comes almost completely from studies of rodents: mice or rats missing specific genes that affect 5-HT concentrations or the production of 5-HT in certain brain regions are more aggressive. But this is not true for all 5-HT, as manipulation of certain receptors and of concentrations of 5-HT in different parts of the brain have different types of impacts on aggression, anxiety-related behavior, and impulsivity. In the few studies of humans that track 5-HT relating to aggression there is a negative relationship between the ability of 5-HT receptors to bind to neurotransmitters and aggression, which supports the notion that this molecule impacts the expression of aggressive behavior.

One of the most interesting findings from the 5-HT studies is that, in rodents, different types of 5-HT receptors have different impacts on expressed aggression, depending on whether the rodents were exhibiting “normal” territorial aggression or impulsive pathological aggression stimulated by drugs or electric shock. This suggests that the various genes that code for the different 5-HT types are involved in different systems in aggression and that they might in fact have multiple, even mutually negating, roles in the production and expression of aggressive behavior depending on the social context and the type of aggression expressed. So while 5-HT is definitely involved in the expression, and probably the modulation, of the level of aggression, there is no evidence that this is where aggression comes from.

There is ALSO a strong popular assumption that testosterone stimulates or enhances aggression, especially in males. First off, it is important to note that testosterone courses through both male and female bodies, but that on average, males have higher circulating levels than females. Testosterone is a steroid hormone closely related to estrogen and a suite of hormones called androgens. Little is known about the underlying genetic structures that influence testosterone, but there is no doubt that some genetic variation influences the production and regulation of testosterone in human bodies.

The concept that testosterone produced by males facilitates and increases aggression is an oversimplification and there are very weak or inconsistent correlations between testosterone levels and aggression in adult humans. Even when external sources of testosterone are administered to adults their aggression does not tend to increase, nor is there an increase in aggression at puberty when human males undergo a significant increase in the production of testosterone and the development of male secondary sexual characteristics.

There is evidence that in competitive or acute stress situations humans can respond by increasing the production of testosterone but there is no strong or consistent evidence that these increases result in increased aggressive behavior. The increase does appear to enhance muscle activity and efficiency and might also result in lower sensitivity to pain or punishment in both men and women. This might make individuals more likely to participate in aggressive competition, but it does not increase aggression per se. In some experiments the levels of circulating testosterone increase after dominance interactions and social competition, but again this is not necessarily tied to overly aggressive behavior. Exposure to sexual situations and to communal competitive events (like team sports) also appears to increase testosterone in males. Interestingly, males who are fathers and or long-term married partners show lower levels of testosterone than do nonfathers or unmarried males.

Overall, testosterone seems to be associated with the efficiency and activity of a variety of muscular and other physical systems, some of which are implicated in the expression of aggression. But contrary to popular misconceptions, testosterone itself is not associated in any causal way with increased aggressive behavior or in the patterns of the exhibition of aggression.

Despite popular notions that certain genes or genetic elements control or regulate the appearance and intensity of aggressive behaviors, there is no evidence for any one-to-one genetic controls, nor is there evidence for certain molecules or systems in the body that predetermine aggressive outcomes. There is no gene or system in the body that can be identified as “for aggression.” While it appears clear that genetic variation in neurotransmitters and hormones can be involved in the ways in which we express aggressive behavior, there is no direct or casual link. Our genes cannot make us aggressive.

As the anthropologist Ashley Montagu sagely cautioned, “It is essential that we not base our image of ourselves on false foundations. What is involved here is not simply the understanding of the nature of humanity, but also the image of humanity that grows out of that understanding.” Humans are not naturally aggressive, but they do have a great potential for aggression and violence. If we believe we are aggressive at our base, that males stripped of social constraints will resort to a brutish nature, then we will expect and accept certain types of violence as inevitable. This means that instead of really trying to understand and rectify the horrific and complex realities of rape, genocide, civil war, and torture, we will chalk at least a part of these events up to human nature. This is a dangerous state of mind that traps us in a vicious cycle of inaction and futility when it comes to moving forward as societies invested in understanding and managing violence.

Sure, certain things spur aggressive actions, but the common notions about our inner, natural aggressive tendencies (especially in males) ignores the complexity of human biology, psychology, history, and society. It downplays the myriad ways in which aggression is initiated and maintained, and oversimplifies what we can mean by, and understand about, human aggressive behavior. And, most dangerously, it enables a kind of inevitability in our communal sense of aggression and society, especially as it relates to males. This need not be the case.

By Agustin Fuentes

Agustin Fuentes is Professor of Anthropology at the University of Notre Dame. He is the author of "Evolution of Human Behavior," "Biological Anthropology: Concepts and Connections and Core Concepts in Biological Anthropology."

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