Abnormal psychology is concerned with causes of psychological disorders and their effects on human thought and behavior. Related theories have their roots with some of the most important figures of early Western thought like Hippocrates, Plato, Aristotle, and Galen. They surmised correctly that abnormal behavior and other mental illness were the result of natural causes like traumatic events and injuries, underlying medical conditions, and even inherited predisposition.

After the fall of Rome, however, established ideas about psychology and physiology were forgotten for centuries as the Catholic church became the dominant institution in Europe and stifled scientific inquiry. Mental disorders and abnormal behavior were attributed to non-scientific causes like demons, witchcraft, and sin, as they had been before the ancients.

The next advancement in understanding and treating mental illness wasn’t until the founding of hospitals and “asylums” dedicated to the mentally ill in sixteenth-century Europe, most notably London’s Bedlam Hospital and Paris’s La Salpêtrière. An increasingly humane view of mental illness led to gradual improvements in patient care. By the eighteenth century, doctors in England, France, and America innovated therapies based on work, recreation, hygiene, and prayer that improved patients’ outcomes and allowed some to recover and return home.

Yet a comprehensive understanding about the causes of mental disorder was still lacking. This left room for quack theories like phrenology, an early nineteenth-century pseudoscience that claimed to explain personality and behavioral characteristics by the shape and topography of a person’s skull. The modern fields of psychology and psychiatry began taking form with the founding of the American Psychological Association in 1878 and with Sigmund Freud’s personality-based theories of abnormal psychology developed in the 1880s, which still have useful therapeutic value today.