Behavioral problems brought on by uncontrolled aggressive urges most often occur among teenagers and young adults, and experts have long suspected abnormal brain functions to be at least partly responsible for their seeming inability to restrain themselves. Research performed in 2007 focused this hypothesis on the amygdala, the brain structure most directly responsible for “gut” behaviors and defensive responses to outside stimuli.

Researchers gathered a control group and a selection of adolescent boys whose overly aggressive behavior had led to disciplinary problems in the past. The teens in this study were not necessarily psychotic or antisocial. They fell under the label, "reactively aggressive," frequently overreacting to perceived offenses, assaulting people or striking inanimate objects in response to what were revealed to be simple misunderstandings of others’ behavior. Most importantly, all showed remorse for actions that, beyond the heat of the moment, they clearly recognized as inappropriate.

When subjects viewed images of models making threatening faces, the amygdala of the problem group lit up with a veritable storm of activity, seeming to override the better judgement of the reasoning prefrontal cortex where activity slowed in moments of distress. These patients may very well have lashed out verbally or physically if approached by these faces in real-world situations, and the unusual relationship between those two sections of the brain may be responsible. Researchers have previously noted smaller-than-average prefrontal cortices in the brains of murderers and antisocial criminals, and while the boys in the study had thankfully not reached such extremes, many of them will experience future behavioral problems. How do the abnormalities come about? While genetics plays an obvious role, environmental conditioning has also been proposed as a root cause: physical abuse and neglect, as well as early drug and alcohol use, have been shown to alter the brain in potentially profound ways.