“Kagan has given us painstakingly documented evidence that high reactivity is one biological basis of introversion but his findings are powerful in part because they confirm what we’ve sensed all along. Some of Kagan’s studies even venture into the realm of cultural myth. For example, he believes, based on his data, that high reactivity is associated with physical traits such as blue eyes, allergies, and hay fever, and that high-reactive men are more likely than others to have a thin body and narrow face.”