American Scientist

Most of us probably remember a tough bully—the boy who picked on vulnerable kids at school. Our parents might have told us that he was actually a coward, and not a tough kid at all. This wisdom may not have made sense if you were a frightened victim or onlooker, but adults generally realize that bullies are troubled children. Even so, such angry adolescents are still something of a puzzle: What could make a young boy into such an angry, aggressive young man?

Science is still a long way from answering this question, but some recent research on the relation between social stress and the brain's biochemistry does shed some light on the possible causes of aggression in young males. The new research looks at the effects of threats and attacks by adult hamsters on young male hamsters that had just reached puberty (The Journal of Neuroscience, April 1). The young hamsters were chosen as targets of the aggression to explore the idea that puberty is a crucial period for the development of social relationships, write Yvon Delville and his colleagues at the University of Massachusetts Medical Center, Worcester.

In a series of experiments the authors examined the behavior of the pubescent males once they had matured into young adulthood. The results were intriguing. Compared to normal hamsters, Delville found that the subjugated males were more likely to bite smaller hamsters, but less likely to bite hamsters of equal size to themselves. In some sense, they became classic "schoolyard bullies," attacking only those weaker than themselves.

An examination of the animals' brains showed marked changes in two neurotransmitter systems—serotonin and arginine vasopressin—in the anterior hypothalamus, a part of the brain previously known to be involved in aggressive behavior. Compared to normal hamsters, the subjugated males had a decreased level of arginine vasopressin in the anterior hypothalamus, but an increase in the number of serotonin-containing nerve fibers.

Both neurotransmitters have been implicated in the regulation of aggressive behavior in a number of species. In mammals, arginine vasopressin appears to facilitate aggression, whereas serotonin appears to inhibit aggressive behavior. Indeed, high aggression in human beings is associated with high levels of arginine vasopressin and low levels of serotonin metabolites in the cerebrospinal fluid.

In the light of the neurotransmitters' usual roles in aggression, Delville's results are consistent with the reduced aggression of the subjugated males when confronting others of their own size. The hamster bullies are indeed neurochemical cowards. But the young males' tendency to attack smaller individuals suggests a complex relation between the brain's biochemistry and the expression of aggression. The neurochemistry of a bully may be a unique thing. "It suggests that future studies on the biology of aggressive behavior must consider the social context in which the aggression is expressed," says Delville.

The results also lend credence to the idea that adolescence is a formative period for molding adult social behavior. Delville notes that "there is an old idea circulating in the behavioral literature that parents who abuse their children, were often abused themselves when they were children. Of course, it is not a one-to-one relation, but it appears to be an important risk factor." Delville's results may provide the first hints of a mechanism that explains the tragic cycle.

No one believes that there is a simple relation between the brain's biochemistry and the behavior of an animal—be it a hamster or a human being—but studies such as Delville's do make one wonder about the disposition of neurotransmitters in the brains of all the angry young men in the schoolyards.—Michael Szpir