Aggression and the Brain

When most people think of aggression, they think of road rage, physical fights, and violent crime. However, not all aggression is bad. Aggression is adaptive, helping people and animals alike to guard their homes from intruders and protect their children from threats. Problems arise when aggression is taken too far, escalating abnormally and becoming violent. Neuroscientists are working to identify brain regions, neurotransmitters, and genes that are involved in escalated aggression and violence. This research may one day help identify individuals at risk of developing dangerous behaviors and new treatments to prevent such episodes in at-risk individuals.

Like humans and many other animals, fruit flies battle each other for control over territories and mates. Scientists are studying factors that influence levels of aggression.

Although previous studies implicated the brain chemical serotonin in all types of aggression, research suggests that it may only be involved in escalated aggression and violence. Studies also question whether changes in the serotonin system are the cause or effect of escalated aggression. Research now suggests that unchecked aggressive behavior can eventually change the brain in ways that alter serotonin levels and, perhaps, increase violent behavior.

Researchers modeled pathological aggression in wild mice and rats by permitting them to physically dominate other rodents repeatedly. With such positive reinforcement, a relatively small proportion of the animals’ initially normal aggressiveness gradually became transformed into a more violent form — similar to the kind seen in violent people.

Researchers found that levels of serotonin, particularly in the prefrontal cortex, decreased in animals perpetrating repeated victorious episodes of aggression but not as a result of performing normal, functional acts of aggression. These findings are consistent with studies in humans showing that serotonin deficiency is associated with escalated rather than more functional forms of aggression. The results suggest that regulation of the serotonin system may be beneficial for people with anger problems. In animal studies, exposure to serotonin receptor agonists, drugs that increase serotonin activity, suppressed aggressive behavior, including its escalated form.

Genetic studies in the fruit fly Drosophila melanogaster have identified other brain compounds important in aggression. Researchers selectively bred Drosophila for highly aggressive behaviors. By comparing gene expression in docile and highly aggressive Drosophila, they have identified new candidate genes and cellular mechanisms involved in aggression. Fruit flies bred for aggression showed altered expression of genes involved in pheromone-based communication. Male fruit flies use pheromones to identify and locate food and other males and to attract females.

Highly aggressive Drosophila showed increased expression of an enzyme important in processing hormones and pheromones and decreased expression of a protein important in pheromone sensation. This data suggests that abnormally aggressive fruit flies may have difficulty identifying the territories of other males, resulting in more frequent or more violent “turf wars.” In humans, imaging technologies have helped neuroscientists identify brain regions associated with inappropriate aggressive behavior. Damage to certain regions of the brain, most notably the prefrontal cortex, can result in violent behavior.

However, research also implicates brain circuits involved in moral judgments in violent behavior. The researchers found that people with antisocial, violent, or psychopathic tendencies tended to have overlapping damage in brain structures involved in making moral judgments. In normal, healthy individuals, moral decision-making activates the dorsal and ventral prefrontal cortex, the amygdala (important in emotions, fear, and stress), and the angular gyrus (involved in language and cognition). Antisocial individuals tended to show more damage in these brain regions than did control subjects. Some adolescents respond to even mild perceived threats with inappropriate aggression. Research shows that teenage boys with this reactive type of aggression show abnormal brain activity relative to their peers. In response to fear-inducing images, these boys showed more activity in the amygdala and less activity in the frontal cortex, which is involved in impulse control, than other teenagers.

Violence is harmful not only to society, but also to the health of both victim and aggressor. Being the recipient of an aggressive social encounter can cause changes in the brain that lead to depression, anxiety, and susceptibility to immune-related illnesses. Surprisingly, animal research shows that aggressors may suffer from many of these same effects. Aggressive encounters increased circulating levels of stress hormones in both dominant and submissive mice, suggesting that aggression affected both groups similarly. Chronic exposure to social stress increased sensitivity to bacterial infection in both groups, but more so in dominant than in submissive mice.

Unlike most behaviors, individual acts of escalated aggression and violence have the potential to impact society as a whole. Research from fruit flies to humans is helping to decipher the biological causes of these abnormal behaviors. This research promises to reveal new avenues of treatment and prevention in the years to come.