Temperament at Birth? Jerome Kagan, developmental psychologist at Harvard, studied introversion and extroversion in children. One experiment was particularly telling. Kagan exposed a group of 4-month-old children to a tape recording of noises—voices, popping balloons, strong scents, bright colors—a lot of sensory distraction. About 40% did not react, but 20% reacted, cried, and flailed their arms and legs. Kagan hypothesized counterintuitively that the flailers would grow up to be introverts and the calm-reaction infants would become extroverts. Surprisingly, he was right! The brain’s amygdala is more active in introverts, thus making them more reactive to stimuli. As they grow up, introverts become hypervigilant kids, are more cautious, and get overwhelmed by stimuli. Extroverts take more chances—sometimes ill-considered—but push the envelope and become entrepreneurs and business leaders.