And although people may learn to be outgoing as adults, the brain marker for shyness still remains.
Researchers at Harvard University have taken brain scans of a group of 22-year-olds who 20 years before had been classified as either inhibited or outgoing based on their childhood behavior.
The study found that those who had been judged as 2-year-olds to be inhibited showed on scans that the amygdala structure in their brains responded much more actively to unexpected sights than did those who had been judged as children to be more outgoing, said Jerome Kagan, a researcher in the department of psychology at Harvard.
"They are now 22 years old," Kagan said of the test subjects. "A lot of the ones who were fearful aren't fearful anymore. They have overcome it. But the questions is did they still have a very active amygdala."
Based on the brain scans, Kagan said, the answer is clearly yes. A report on the research appears this week in the journal Science.
In the study, Harvard researchers conducted the brain scan tests on 13 people who had been evaluated as shy children at age 2 and compared the results with nine people who had been evaluated as children to be outgoing and bold.
"We had assumed, but never measured, that ... the shy, inhibited group had inherited a certain chemistry" in the amygdala, said Kagan.
All the subjects were exposed to a series of pictures of faces with neutral expressions. After they had become accustomed to those pictures, new faces were introduced while the researchers measured the reaction of the amygdala structure in their brains.
The brains of the once-shy children was much more active than the other subjects.
"That is support for the notion that the reason they were shy, timid and reserved when they were 2 years old is because they had an excitable amygdala," said Kagan. This suggests that shyness is a temperament that can be inherited, but the researcher said this temperament does not necessarily determine one's eventual personality.
However, Kagan said that before any firm conclusions can be drawn there needs to be a similar research using many more subjects than the 22 in the current study.
Although some children are shy and others are outgoing, he said, these traits can change with time and life experiences.
"People overcome their shyness," Kagan said. "You can also acquire shyness."
Extreme shyness can be a precursor for serious disorders, such as social phobias and depression. Kagan said that by finding the biological basis for such shyness, it may be possible to develop drugs to treat patients whose lives are adversely effected by the condition.
Other co-authors of the study are Dr. Carl E. Schwartz, Dr. Christopher I. Wright, Lisa M. Shin and Dr. Scott L. Rauch, all of the Harvard Medical School.
By Paul Recer