In the days after his son was born, Rob Sandler found the thrill of becoming a new father replaced with dark feelings of dread and hopelessness. Those feelings, coupled with sleep deprivation and stress, culminated in a panic attack during his son’s bris.

As a group of old friends was saying goodbye after the ceremony, “I had this feeling that they were leaving and I was stuck in this situation that would never get any better,” said Mr. Sandler, a marketing executive in Dallas. “I just felt trapped.” What followed was months of sadness, anxiety and — perhaps most worrisome of all — a feeling of acute disappointment in his own ability to be a good parent.

In recent years, a growing body of research, and the increasing visibility of dads like Mr. Sandler, has given rise to the idea that you don’t have to give birth to develop postpartum depression. Studies suggest that the phenomenon may occur in from 7 percent to 10 percent of new fathers, compared to about 12 percent of new mothers, and that depressed dads were more likely to spank their children and less likely to read to them.

Now, a University of Southern California study has found a link between depression and sagging testosterone levels in new dads, adding physiological weight to the argument that postpartum depression isn’t just for women anymore. The study also found that while high testosterone levels in new dads helped protect against depression in fathers, it correlated with an increased risk of depression in new moms.