Dr. Donald F. Klein , whose research into panic attacks, depression, childhood anxiety disorders and related areas reshaped how such conditions are thought about and treated, died on Aug. 7 in Manhattan. He was 90 .

His wife , Rachel Klein, said the cause was cardiopulmonary arrest.

In the 1950s Dr. Klein, a psychiatrist with a background in biology, brought a new rigor to the study of whether some psychiatric problems might have a biological basis that could be treated with drugs. Psychotherapy was the go-to tool for treating depression and other problems at the time, and the use of medications was somewhat haphazard.

“Donald Klein began his research career at a time when psychiatry was struggling to find its footing in the broader scientific and medical community,” Dr. Daniel Pine, chief of the Section on Development and Affective Neuroscience at the National Institute of Mental Health, said by email. “One of his most significant contributions was to extend approaches to psychiatry from other areas of medicine.”

Since psychiatric problems aren’t easily measured the way, for instance, a heart rate is, methods to gauge a medicine’s effects were needed. These included clinical observation, a patient’s own reports, and statistical analysis.