In 1921, the Swiss psychotherapist Carl Jung divided humanity into now familiar categories: introverts and extroverts. To simplify a nuanced theory, extroverts believe that the world beyond them is the sum of reality, whereas introverts insert subjective meaning between themselves and external objects. Introverts tend to feel secure in one tightly circumscribed position while extroverts want to be everywhere. The world is a fine balance between these equally valuable propensities, but at times, Jung believed, one of them gained the upper hand. A century ago, Jung observed that we had entered an overly extroverted age. Material achievements, successes and productivity had...