Cain notes that between one-third and one-half of Americans may be classified as introverts, though individuals fall at different places along an introvert-extrovert spectrum.
Cain mengatakan bahwa antara sepertiga dan setengah warga Amerika Serikat dapat dikelompokkan sebagai introver, walaupun orang-orangnya tersebar di seluruh spektrum introver-ekstrover.