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 (for details on the experiment, see sidebar.) 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.

The good news is that if such people can be identified, it might be possible to provide therapy. Davidson and his colleagues note that both genes and environment contribute to the development of this circuit regulating emotional control, and so interventions involving medication and psychosocial training could improve its functioning. Such treatment could not eliminate aggressive impulses, but it might insure that people at risk for becoming violent could better regulate those urges. And nurturing control and peacemaking skills may not rid society of violence, but it would be a valuable start.

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