Our lives are shaped as profoundly by personality as by gender or race. And the single most important aspect of personality—the “north and south of temperament,” as one scientist puts it—is where we fall on the introvert-extrovert spectrum. Our place on this continuum influences our choice of friends and mates, and how we make conversation, resolve differences, and show love. It affects the careers we choose and whether or not we succeed at them. It governs how likely we are to exercise, commit adultery, function well without sleep, learn from our mistakes, place big bets in the stock market, delay gratification, be a good leader, and ask “what if.
-- Susan Cain (Introduction paragraph 1)

Importance: The author refers to introversion and extroversion as the "north and south of temperament" because she believes that these variables are critically important in determining who we are. She cites research that shows that extroversion or introversion can affect...