None of these studies is perfect, but the results have consistently suggested that introversion and extroversion, like other major personality traits such as agreeableness and conscientiousness, are about 40 to 50 percent heritable.

The amygdala serves as the brain’s emotional switchboard, receiving information from the senses and then signaling the rest of the brain and nervous system how to respond. One of its functions is to instantly detect new or threatening things in the environment—from an airborne Frisbee to a hissing serpent—and send rapid-fire signals through the bod

The amygdala is located deep in the limbic system, an ancient brain network found even in primitive animals like mice and rats. This network—sometimes called the “emotional brain”—underlies many of the basic instincts we share with these animals, such as appetite, sex drive, and fear.

Some say that temperament is the foundation, and personality is the building.

Psychologists often discuss the difference between “temperament” and “personality.” Temperament refers to inborn, biologically based behavioral and emotional patterns that are observable in infancy and early childhood; personality is the complex brew that emerges after cultural influence and personal experience are thrown into the mix.

most effective teams are composed of a healthy mix of introverts and extroverts, studies show, and so are many leadership structures.

Some people are more certain of everything than I am of anything. —ROBERT RUBIN, In an Uncertain World

The

ages of nineteen and forty-one. The volunteers played a game in which each group member was shown two different three-dimensional objects on a computer screen and asked to decide whether the first object could be rotated to match the second. The experimenters used an fMRI scanner to take snapshots of the volunteers’ brains as they conformed to or b