Organisms cannot live without fear. "Fearfulness is one of the most basic physiological and behavioral responses we have. It probably supersedes everything because of its survival value," says Ned Kalin, Hedberg professor of psychiatry and psychology at the University of Wisconsin-Madison. He also is chair of the department of psychiatry and director of the university's HealthEmotions Research Institute.

But fear gone awry can debilitate animals and humans. Excessive fear can lead to psychopathology and extremely uncomfortable lives. Kalin and other investigators are probing the behavioral, hormonal, and neural components of this vital emotion to understand its "normal" basis, but also to learn what causes an individual to have an enduring fearful temperament--one marked by inhibition, withdrawal, and other indicators of a high level of stress.

In general, when an organism perceives a situation recognized as fearful, neurons running from the eye or other sense organs stimulate the amygdala, a structure...

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