Several studies shed light on where aggression comes from and how it can be controlled.

Science may never be able to explain in full such violent acts as the shooting at Columbine High School, which claimed 15 lives 15 months ago. But various studies, some probing the evolutionary origins of aggression, and others, our conscious ability to control it, are changing the ways in which researchers regard violence.

Two papers review several recent lines of thought in the July 28 issue of Science. One intriguing perceptual shift is coming from those who regularly observe our closest kin, the chimpanzees, and other monkeys. Indeed, primatologists are now suggesting that aggressive behavior be viewed as a normal means for competing and negotiating within groups, and not as a fundamentally antisocial instinct. This shift, they say, could lead to a better understanding of how aggression ends and can be kept under control among humans.

Although it is hard to look at violence as anything but an attempt to destroy community, Frans de Waal, Professor of Primate Behavior at Emory University, makes a compelling argument for seeing it as an integral part of any social network. Were aggressions truly antisocial acts, he points out, there would be no way to explain the fact that the overwhelming majority of attacks involve people who know one another well. And it wouldn't explain the ways in which colonies of monkeys pick fights or make peace.

Primatologists first began to study aggression as a social phenomenon during the 1970s, when a curious incident was recorded at the Arnhem Zoo after a dominant male chimp attacked a female. The rest of the colony came to her aide, and then screamed and chased one another for a while. After a tense period of silence, the entire group began hooting, and during this chorus, two chimps embraced each other and kissed. When researchers reexamined the event, they realized that the two who had kissed were the very same two that had been fighting. Soon they found that most monkeys and apes make dramatic gestures of reconciliation after conflicts.

Additional research since then has shown that monkeys are actually more likely to seek contact with former opponents than with others, which indicates that they do not start a fight to alienate themselves from another individual but rather to renegotiate the terms of an ongoing relationship. And peacemaking, an important part of this negotiation, appears to be in part a learned skill. Of interest, de Waal notes, one of the best predictors of whether schoolchildren make peace is the level of positive contact they have had before a conflict erupts.

Accepting the idea that a cycle of violence and reconciliation provides a natural way of redefining the terms of relationships does not mean, of course, that it is the only way. And there can be no denying the fact that individuals have different thresholds for acting out. In an accompanying review to de Waal's essay, Richard Davidson, Katherine Putnam and Christine Larson of the University of Wisconsin put forth the theory that diminished emotions including fear, anger, distress and agitation can heavily predispose people to impulsive bursts of aggression.

The neural circuitry involved in quelling negative emotions appears to involve an inhibitory connection from part of the prefrontal cortex to the amygdala, and Davidson and his colleagues cite several lines of evidence as support. In rodents, for example, lesions to the prefrontal cortex render the animals much slower at eliminating aversive responses. Similar lesions in humans produce syndromes marked by impulsivity and aggression. And positron emission tomography (PET) scan studies have shown that murderers tend to have decreased glucose metabolism in the prefrontal cortex, as well as increased activity in parts of the amygdala.

Among 43 normal subjects, the authors tested individual variations in the circuit by first arousing negative emotion and then giving instructions to enhance, maintain or suppress that emotion. They then measured the subjects' tendencies to startle a reaction produced by the amygdala in response to negative emotion. As it turned out, subjects asked to suppress the negative emotion showed much lower "startle magnitudes" than those asked to maintain or enhance their feelings, meaning these people had successfully activated the circuit.

What's more, the neuroscientists found in subsequent work that among people asked to suppress negative emotions during the experiment, those who were best able to dampen their startle response also showed greater electrical activity in the prefrontal scalp regions, suggesting that the prefrontal cortex was very likely playing its inhibitory part. And given how well the level of prefrontal activation predicted an individual's ability to control negative emotion, it is conceivable that such testing might provide a way to screen for people at high risk for aggressive behavior.

— www.sciam.com