Dr. John Gunderson, whose studies of people with suicidal urges, fears of abandonment and squalls of emotion helped establish borderline personality disorder as a stand-alone diagnosis, providing a foundation for research and the first effective treatments, died on Jan. 11 at his home in Weston, Mass. He was 76.

His wife, Susan, said the cause was prostate cancer.

Dr. Gunderson, who was trained in Freudian analysis, was evaluating the effects of psychotherapy on people identified as schizophrenic in the early 1970s when he discovered that many of the participants in his study had received the wrong diagnosis. They did not have recurring psychoses, the signature symptom of schizophrenia, but rather a poorly understood syndrome described by the German-American psychiatrist Adolph Stern in 1938: Their mental state was on the “border” between garden-variety neurosis and full-blown psychosis.

Dr. Gunderson plunged into the literature on borderline patients and distilled their defining features. These included intense fears of being alone, floods of emotional turmoil and impulsive self-harming, like cutting. He collaborated with Margaret T. Singer on an influential 1975 paper, “Defining Borderline Patients: An Overview,” turning a collection of descriptive accounts into a firm diagnosis. He also developed a diagnostic interview that set the standard for identifying the disorder, bringing focus to what had been a baffling condition.

“The way these patients were described before was as treatment failures, as if it was somehow their fault,” Dr. Lois Choi-Kain, director of the adult borderline center at the Harvard-affiliated McLean Hospital, in Belmont, Mass., where Dr. Gunderson spent much of his career, said in a phone interview. “John came along and said no, these are symptoms of the disorder, not failures.”