Seeing smiling faces makes the brains of extroverted people light up more than the brains of shy people, researchers said on Thursday in a study they believe could help shed light on the biology of personality differences.

Specifically, the amygdala, a small region of the brain associated with emotional response, lights up when an outgoing person sees a picture of a smiling face, the team at the State University of New York in Stony Brook and at Stanford University in California found. More here.