Researchers Explore Roots of Aggression

Three studies, all presented at a recent conference of the Society for Neuroscience, offer new insights into the roots of aggressive behavior.

The first study, by Guido Frank and colleagues, investigated why some teenagers are more prone than others to “reactive” aggression—that is, unpremeditated aggression in response to a trigger (for instance, an accidental bump from a passerby). Says Frank, “Reactively aggressive adolescents—most commonly boys—frequently misinterpret their surroundings, feel threatened, and act inappropriately aggressive. They tend to strike back when being teased, blame others when getting into a fight, and overreact to accidents. Their behavior is emotionally ‘hot,’ defensive, and impulsive.” Research suggests that teens with this behavior are at high risk for lifelong social, career, or legal problems.

Frank and his colleagues recruited a group of male teens prone to reactive aggression and a control group without a history of this behavior, and scanned the brains of both groups while asking them to perform tasks that involved reacting to fear-inducing images. Compared to the brains of controls, the brains of the teens with high levels of reactive aggression showed greater amygdala activity and less frontal lobe activity in response to the images. The amygdala is linked to the processing of fearful or threatening stimuli, while the frontal lobes are involved in decision-making and impulse control.

In the second study, Sietse de Boer and colleagues allowed feral mice and rats to physically dominate other rodents repeatedly, causing the aggression of the dominating rodents to escalate and become pathological. During this transformation, the researchers studied chemical changes in aggression-related brain circuits involving serotonin. They report that serotonin activity decreased as pathological aggression escalated, but did not decrease in rodents exhibiting normal acts of aggression.

“Our findings support meta-analyses of serotonin activity in aggressive humans,” de Boer says. “That data showed that serotonin deficiency is most readily detected in people who engage in impulsive and violent forms of aggressive behavior rather than in individuals with more functional forms of aggression.”

In separate research, de Boer and colleagues found that the transition from normal to pathological violence in rodents is accompanied by functional, but not structural, changes in certain serotonin receptors. In animal studies, treatment with selective serotonin receptor agonists has restored the normal function of these receptors and suppressed aggressive behavior. These findings, the researchers say, may lead to better treatments for pathological aggression in humans.

In the third study, Adrian Raine and colleagues analyzed data from 47 independent brain imaging studies and found that the rule-breaking behavior common in people with antisocial, violent, and psychopathic tendencies may stem at least in part from damage to brain circuitry involved in moral decision-making. Comparing the brain images of 792 antisocial individuals and 704 controls, the researchers found that antisocial individuals tended to have overlapping damage in the dorsal and ventral prefrontal cortex, the amygdala, and the angular gyrus—all of which are involved in moral judgment.