One need only pick up the daily newspaper to see how serious a problem violence is in today’s society. Although the incidence of violent behavior in the US has fallen significantly in the past few years, there is still about an 80% chance that a person will be the victim of a violent crime during his or her lifetime. Even more troubling is the trend of increasing violence among the very young. After each school shooting, there is a media blitz of experts searching to explain how and why troubled teens sometimes turn to violence. Much of what they say is the result of research on the psychobiology of aggression, a field that has recently experienced many breakthroughs in identifying correlates of violent behavior. Some researchers claim that we are coming closer to predicting from a brain scan or a blood test whether a person is at risk for committing an act of violence. Ethical complications aside, a closer look at the neurobiology of aggression shows why we are unlikely to find a conclusive test for potential violent behavior. While there are many biological factors associated with aggression, their predictive value remains still quite low.

hypothalamus01The first hurdle in researching aggression is how to define it. It is an easier task with animals, who tend to display stereotyped patterns of violence such as killing to gain food or territory. With humans and non-human primates, classifying aggression becomes more difficult because there is complication of intent. Punishment, for example, represents an especially gray area. Should spanking be considered an aggressive act? What about capital punishment? Indeed, almost all acts we consider aggressive have been socially sanctioned by some cultures over the years. To simplify matters, many psychologists and ethologists find it useful to classify aggressive behavior into one of three main categories: (1) predatory aggression, which refers to stalking and killing of other species, (2) social aggression, which is unprovoked aggression that is directed an members of the same species for purposes of establishing dominance, and (3) defensive aggression, which refers to attacks delivered when an animal is cornered by a threatening aggressor. There is evidence from animal studies that suggests the different types of aggression are controlled by different subsets of brain structures within the limbic system, including the amygdala, the septum, and the hypothalamus (figure 1). For example, in the rat, lesions of the lateral septum decrease social aggression but increase predatory aggression, suggesting that neural substrates for offensive and defense aggression are intertwined but separate.

Is it in the Genes?
One of the earliest attempts to link genetics and violent behavior occurred during the 1960s, when researchers thought they had discovered a propensity for violence in men born with an extra Y chromosome. Although the studies attracted a lot of attention at the time, further examination of XYY males revealed that they did not display any particularly violent tendencies. Furthermore, XYY males are extremely rare, and thus the syndrome could not possibly explain the frequency and prevalence of violent behavior around the globe. Scientists agree that there is probably a genetic component to aggression because violent behavior tends to run in families. However, with a complex behavior like aggression, it is especially difficult to separate genetic and environmental contributions. Most likely it is possible to inherit a predisposition to violence, but psychologists also stress that modeling aggressive behavior in the home is the surest method for propagating violence.

A large body of research implicates the amygdala as a key brain structure for mediating violence. One of the first indications that the amygdala might be important for fear and aggression came from Kluver and Bucy’s 1939 descriptions of monkeys who had their temporal lobes removed. They noted that the animals were remarkably tame and showed little fear. Later research indicated that docile behavior associated with Kluver-Bucy syndrome is likely mediated by the amygdala, as selective removal of that structure produced similar effects on fear and aggression. It is also possible to increase aggression through modulation of the amygdala. In animals, electrical stimulation of the amgydala augments all types of aggressive behavior, and there is evidence for a similar reaction in humans. Sniper Charles Whitman, who killed several people from the University Tower at Texas, left a note behind that begged people to examine his brain for possible dysfunction. His autopsy revealed he had a tumor pressing into his amygdala.

“Sniper Charles Whitman, who killed several people from the University Tower at Texas, left a note behind that begged people to examine his brain for possible dysfunction. His autopsy revealed he had a tumor pressing into his amygdala.”

Hormones and Serotonin
Testosterone is another attractive candidate for mediating aggression because males in of all ages, races and cultures are more physically aggressive than their female counterparts. In animals, testosterone is linked to social aggression. Reducing testosterone in the alpha male by castrating him eliminates his dominant social status, and restoring testosterone through injection causes him to regain his social status. However, administering testosterone to males with less social status does not usually allow them to take over the alpha male position, indicating that there is not a direct relationship between testosterone and position in the dominance hierarchy. There is some evidence in humans that high testosterone males are more likely to be socially aggressive, but no evidence that they are necessarily more violent. Often they are successful in professions that thrive on competition, such as successful leading of a company, running for president, or pursuing a sports career. Also, a few psychologists have suggested that females are not necessarily less aggressive than males; rather, they display a different kind of aggression. Females are more likely to show non-violent types of aggression such as ostracizing their peers or spreading false rumors with the intent to cause pain. Thus, while there does seem to be a connection between testosterone and physical aggression, a person’s testosterone level will not necessarily be a good predictor of aggressive behavior.

Several lines of converging evidence indicate that the neurotransmitter serotonin plays a key role in mediating aggressive and violent behavior. Mice with a selective knockout of the serotonin 1B receptor show an increase in aggression. Similarly, depleting serotonin levels in vervet monkeys increases their aggressive behavior, whereas augmenting serotonin levels reduces aggression and increases peaceable interactions like grooming. Serotonin has also been implicated in human aggression. For example, pharmacological interventions that augment serotonergic efficacy have been shown to reduce hostile sentiment and violent outbursts in aggressive psychiatric patients. Also, people with a history of impulsively violent behavior, such as arsonists, violent criminals, and people who die by violent methods of suicide show low levels of the serotonin in their cerebral spinal fluid. These findings represent an interesting correlation, but it is important to remember that the direction of effect is unclear. It may be that aggressive behavior induces low serotonin levels in the cerebral spinal fluid rather than vice versa.

Measures of brain functioning such as the EEG have long suggested that violent criminals have impaired neurological processes, but the recent advancement of neuroimaging techniques has allowed researchers to examine violent offenders’ brains in more detail. Adrian Raine and colleagues have conducted the largest and most thorough study to date, in which they used positron emission tomography (commonly called a PET scan) to compare brain activity in 41 convicted violent offenders to activity in 41 age matched control subjects. They found that the people convicted of murder had reduced activity in the prefrontal cortex and increased activity in subcortical regions such as the thalamus. This finding fits nicely with previous research showing that the damage to the prefrontal cortex impairs decision making and increasing impulsive behavior. Indeed, Raine’s work is perhaps the best evidence yet that impaired brain functioning may underlie some types of violent aggression. However, it is important to remember that his subjects lie at the extreme end of a spectrum and may not be typical of most aggressors. Also, there are plenty of examples of people with prefrontal cortex damage who do not commit violent acts, so PET scans cannot be used to ferret out potential murders.

Reducing Violent Behavior
Researchers have been successful in identifying biological factors associated with aggression but have had less luck figuring out how these factors might contribute to pathological aggression and violence. At this point, there is no neurological marker to identify a person at risk for violent behavior, and it seems unlikely that a definitive test will ever exist. As the example of high testosterone males illustrates, aggression can often be channeled into healthy and beneficial behaviors. Thus it seems the best road to reducing dangerous kinds of aggression is learning more about the factors that shape aggressive behavior. Many people point to the media as a key instigator of violence, citing statistics about the thousands of dramatized murders American children watch on television each year, and there is some evidence to support this idea. However, television cannot possibly be the sole mediator of violent behavior. Toronto receives the same television programming as Chicago but the crime rate in the Canadian city is not even a tenth of the American one. The hard truth about pathological aggression is that it does tend to propagate through families, and once started, the cycle can be very difficult to break. Research on the neurobiology of aggression has already provided some valuable clues about possible targets for biological intervention, but there is no quick fix. The good news is that scientists in the fields of psychology, sociology and biology are increasingly aware of their mutual interest in this topic. Each brings a piece of the puzzle to the table, and their unique combination offers our best hope for understanding the complex behavior of pathological aggression.

Fuller, RW, “The influence of fluoexetine on aggressive behavior.” Neuropsychopharmacology, 14: 77-81, 1996.

Mann, JJ. “Role of the serotonergic system in the pathogenesis of major depression and suicidal behavior,”Neuropsychopharmacology, 21 (2): 99S-105S, 1999.

Raine, A., Buchsbaum, M., LaCasse, L., “Brain abnormalities in murders indicated by positron emission tomography,” Biological Psychiatry, 42: 495-508, 1997.

Virkkunen, M, Rawlings, R., Tokola, R., Poland, RE, Guidotti, A. Nemoff, D. Bisette, G., Kalogeras, K., Karonen, SL, Linnoila, M. “CSF Biochemistries, glucose metabolism, and diurnal activity rhythms in alcoholic, violent offenders, fire setters, and healthy volunteers,” Arch of General Psychiatry, 51: 20-27, 1994.