Paulson said that although animal studies have shown males may also go through hormonal changes during their partner's pregnancy and after birth -- testosterone goes down, and other stress and bonding hormones go up -- there's really no research that speaks very clearly to cause, he said.
"If I had to make my best educated guess -- it probably lies in the psychosocial domain." said Paulson.
Yet many point to simple sleep deprivation could also be a root cause.
"Probably one of the biggest risk factors for postpartum depression in women and men is sleep deprivation," said William Courtenay, a researcher and psychotherapist in Oakland, Calif., and founder of saddaddy.com.
"We know when normal healthy adults go without [good] sleep for more than a month, they show signs of depression," he said.
But unlike women, Courtenay said men may often show signs of depression through anger and irritability.
"A man who's depressed can look like someone who's stressed, angry, irritable and getting into conflict with others, or being withdrawn or drinking," said Courtenay, who in a joint project with McLean Hospital and Harvard Medical School is studying postpartum depression in thousands of men.
"We can also see classic signs of depression, a sense of worthlessness and helplessness and sad mood," said Courtenay.
Because postpartum depression in men has so many hallmarks of any other form of depression, some doctors question whether there should be such studies.
"There is not necessarily anything different about risk or magnitude of depression after pregnancy than after any other big event. Calling it postpartum depression suggests there is something different about manifestations or mechanisms, and this study does nothing to clarify those issues," said Dr. Thomas Schwenk, chairman of family medicine at the University of Michigan in Ann Arbor.
"I don't think knowing about this should change clinical behavior from what should already happen, which is to be sensitive to depression during any time of great change, loss or events," said Schwenk.
Yet Paulson and Courtenay said it's worthy of studies, and they point to further research that shows how a father's postpartum depression could harm children.
"If you account for mom's depression, if dad was depressed when the child was about a month old, the child continued to have problems at 3½ -- even if the depression went away," said Paulson, referring to a study by Paul Ramchandani at Oxford University.
"At 7 years, these kids of depressed fathers tend to have high rates of psychological problems -- behavior problems," he said.
Luckily, Schwartzberg said he overcame his postpartum depression with time and by bonding to his children apart from his wife.
"There was one moment when my son was crying, and all I could do was sit on the floor and ball together. I thought at the time I felt like my life had fallen out from under me," said Schwartzberg, who went on to write "The 40-Year-Old Version." "In retrospect, it was my first bonding experience."
As his children grew, Schwartzberg said he noticed that "suddenly they start to smile, suddenly they have preferences.
"I thought here I am not just parenting this completely needy blob, but now I recognize here I am being a father to a very unique being I help create," he said.