The idea that psychology and medicine are closely intertwined is anything but new. It can be traced back to the Greek physician Hippocrates and his colleagues around 400 BC. In the Middle Ages, the Persian physician and philosopher Avicenna completed the Canon of Medicine, which described several psychological disorders and elaborated the relationship of the four humors of ancient medicine to emotional, mental, moral, and behavioral aspects of life. The Canon of Medicine was a standard medical text in medieval Europe after the twelfth century and remained in use in a few universities until the seventeenth century, along with texts by Hippocrates, Aristotle, and Galen (Lutz, 2002, p. 60).

In the nineteenth century, European physicians, including Pierre Janet, Jean-Martin Charcot, and Josef Breuer, began modernizing the medical perspective on psychology. However, it was an Austrian doctor named Sigmund Freud (1856–1939) who succeeded in bringing it to the attention of the world. In the decades before World War I, Freud hitched his audacious ideas about unconscious sexuality and aggression to the rising stars of the modernizing medical perspective on psychology and of scientific medicine in general. This new constellation proved so dazzling that it captured the imaginations of psychologists, physicians, and the general public.