Smile and the world smiles with you. Better yet, smile because it may be good for your health.

Studies by University of California psychologist Paul Ekman suggest that facial expressions -- including smiles -- can cause significant physiological changes within the body. "Putting on a polite expression, going to a social gathering in which one has to smile and be polite may actually change how one feels," he reports in his recent book, "Approaches to Emotion." "Our evidence suggests that putting on such an expression will actually start to produce the physiological changes that are part and parcel of a happy or an excited experience."

Facial expressions, Ekman finds, exert key changes in heart rate, blood flow and metabolism. These bodily functions are all governed by the autonomic nervous system -- a part of the nervous system that, until recently, was thought to be solely under involuntary control.

Feel anger, for example, and your heart rate goes up and your skin gets hot. In fear, your heart rate may also increase, but your skin will get cold.

Ekman and his colleagues Robert Levenson and Wallace Frieson asked actors to produce six universal facial expressions -- happiness, anger, fear, disgust, sadness and surprise. As the actor assumed the expression, researchers measured heart rate, sweating response and skin temperature of the hands -- all indicators of autonomic nervous system change. Their results show that putting on each of these faces elicits the same nervous system changes that the emotion itself would produce. The findings have since been repeated in another study that used nonactors.

The idea that controlling your facial expressions can change the autonomic nervous system has many implications, Ekman says. Could treatment for anxiety or depression someday include facial exercises? Maybe, says Ekman, who is investigating the possibility.