Yet it turns out that producing precise, lasting definitions of extreme behavior patterns is exhausting work. It took more than a decade of observing patients before the German psychiatrist Emil Kraepelin could draw a clear line between psychotic disorders, like schizophrenia, and mood problems, like depression or bipolar disorder.

Likewise, Freud spent years formulating his theories on the origins of neurotic syndromes. And Freudian analysts were largely the ones who, in the early decades of the last century, described people with the sort of “confounded identities” that are now considered personality disorders.

Their problems were not periodic symptoms, like moodiness or panic attacks, but issues rooted in longstanding habits of thought and feeling — in who they were.

“These therapists saw people coming into treatment who looked well put-together on the surface but on the couch became very disorganized, very impaired,” said Mark F. Lenzenweger, a professor of psychology at the State University of New York at Binghamton. “They had problems that were neither psychotic nor neurotic. They represented something else altogether.”

Several prototypes soon began to emerge. “A pedantic sense of order is typical of the compulsive character,” wrote the Freudian analyst Wilhelm Reich in his 1933 book, “Character Analysis,” a groundbreaking text. “In both big and small things, he lives his life according to a preconceived, irrevocable pattern.”

Others coalesced too, most recognizable as extreme forms of everyday types: the narcissist, with his fragile, grandiose self-approval; the dependent, with her smothering clinginess; the histrionic, always in the thick of some drama, desperate to be the center of attention.

In the late 1970s, Ted Millon, scientific director of the Institute for Advanced Studies in Personology and Psychopathology, pulled together the bulk of the work on personality disorders, most of it descriptive, and turned it into a set of 10 standardized types for the American Psychiatric Association’s third diagnostic manual. Published in 1980, it is a best seller among mental health workers worldwide.