Because many early students of psychology were also trained in such “hard” sciences as biology and physiology, it is not surprising that these researchers turned to such physical measures in their attempts to understand mental functioning. Unlike today’s testing efforts, however, individual differences were not the focus of these studies. On the contrary, such differences were generally considered to be the result of imperfect control of experimental conditions, and every effort was made to design stud-ies in which such differences were minimized.