Part 2 looks at the roots of introversion in the context of the age-old “nature versus nurture” debate. Chapter 4 examines the nature side of the equation. Cain interviews Harvard professor Jerome Kagan, who conducted some of the most important research on the link between biology and temperament. Kagan’s work involved a longitudinal study that followed its subjects from infancy into adolescence, showing that babies who reacted more strongly to stimuli often grew up to be introverts. Along with other studies, this indicated that “introversion and extroversion, like other major personality traits such as agreeableness and conscientiousness, are about 40 to 50 percent heritable” (105). Kagan’s work was all the more influential because he began his career firmly in the nurture camp and only over time, based on the data, did he come to see the impact that nature has.

The debate about the origin of personality traits, however, can never be fully resolved by stating that nature or nurture is the definitive cause. Kagan himself noted the strong influence that one’s environment must play. One theory that takes into account this interplay is the

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