Cain explains in a note at the end of the book why she uses the terms that she does for personality types. Although psychological researchers have different definitions of introversion and extroversion, as well as the alternative spelling of “extraversion” for the latter, her book uses the cultural notion of the terms rather than a clinical definition. Cain’s framework relies on a long-standing trope of Western culture: the “dichotomy between the ‘man of action’ and the ‘man of contemplation’” (269). Examples of these types in philosophy and literature go as far back as the Bible. The Old Testament presents the story of brothers Jacob and Esau, whose qualities fall neatly into the two personality types. Likewise, thinkers like Aristotle, Hippocrates, Milton, and Schopenhauer describe people in terms of the cultural definitions we still have of introversion and extroversion.

Although Cain originally planned to create her own terms, she stuck with “introversion” and “extroversion” because these terms are so deeply ingrained in society—lay people know just what is being referred to. The field of psychology, however, determines personality traits in various ways, and concrete definitions are not set.