Once Shy, Always Shy?

Twenty years ago, researchers identified personality differences in toddlers based on their reactions to new people or toys. Some children cried when confronted by a metallic toy robot, while others approached it. Current research suggests such differences between timid and outgoing personalities may be related to underlying brain differences and persist from infancy to adulthood.

The study, published in the journal Science, involved 13 adults previously classified as inhibited and nine classified as uninhibited at age 2 based on their reactions in the robot experiment. Participants were shown photographs of faces, some of which they had seen before, and their brains were scanned with a functional MRI, which detects increased blood flow in active regions of the brain.

When viewing a novel face, inhibited participants showed more activity in their amygdala, a brain region associated with fear and emotional stimuli, than they did when viewing a familiar face. Uninhibited people showed no such change and had lower amygdalar activity overall.

Carl E. Schwartz, assistant professor of psychiatry at Harvard Medical School and lead researcher of the study, described what he called a footprint of the infant temperament in the adult.

Schwartz says the amygdala plays a role in vigilant behavior -- a sort of "what's going on?" approach to the environment.

Still, it may be impossible to say for sure whether amygdalar activity causes inhibition. "In our brains and in our minds, things are always reverberating," Schwartz says. "I think there is always a dynamic circle between brain function, brain structure and the environment."