Susan Nolen-Hoeksema, a psychologist and writer whose work helped explain why women are twice as prone to depression as men and why such low moods can be so hard to shake, died on Jan. 2 in New Haven. She was 53.

Her death followed heart surgery to correct a congenitally weak valve, said her husband, Richard Nolen-Hoeksema.

Dr. Nolen-Hoeksema, a professor at Yale University, began studying depression in the 1980s, a time of great excitement in psychiatry and psychology. New drugs like Prozac were entering the market; novel talking therapies were proving effective, too, particularly cognitive behavior therapy, in which people learn to defuse upsetting thoughts by questioning their basis.

Her studies, first in children and later in adults, exposed one of the most deceptively upsetting of these patterns: rumination, the natural instinct to dwell on the sources of problems rather than their possible solutions. Women were more prone to ruminate than men, the studies found, and in a landmark 1987 paper she argued that this difference accounted for the two-to-one ratio of depressed women to depressed men....