Researchers find that a drop in testosterone levels can signal the condition in dads, and a spike may be a sign of aggression.

Postpartum depression is often associated with mothers, but a new study shows that fathers face a higher risk of experiencing it if their testosterone levels are low nine months after their children are born.

The same study revealed that a father’s low testosterone may also affect his partner - but in an unexpectedly positive way. Women whose partners had lower levels of testosterone postpartum reported fewer symptoms of depression themselves nine and 15 months after birth.

High testosterone levels had the opposite effect. Fathers whose levels were high faced a greater risk of experiencing stress due to parenting and a greater risk of acting hostile - such as showing emotional, verbal or physical aggression - toward their partners.

The study, published in the journal Hormones and Behavior, supports earlier studies that show men have biological responses to fatherhood, said Darby Saxbe, the study’s lead author and an assistant professor of psychology at the USC Dornsife College of Letters, Arts and Sciences.

“We often think of motherhood as biologically driven because many mothers have biological connections to their babies through breastfeeding and pregnancy,” Saxbe said. “We don’t usually think of fatherhood in the same biological terms. We are still figuring out the biology of what makes dads tick.

“We know that fathers contribute a lot to child-rearing and that on the whole, kids do better if they are raised in households with a father present,” she added. “So, it is important to figure out how to support fathers and what factors explain why some fathers are very involved in raising their children while some are absent.”

Saxbe worked with a team of researchers from USC, UCLA and Northwestern University.

For the study, the researchers examined data from 149 couples in the Community Child Health Research Network. The study by the National Institute for Child Health and Human Development involves sites across the country, but the data for this study came from Lake County, Ill., north of Chicago.

Mothers in the study were 18 to 40 years old; African-American, white or Latina; and low-income. They were recruited when they gave birth to their first, second or third child. Mothers could invite the baby’s father to participate in the study as well. Of the fathers who participated and provided testosterone data, 95 percent were living with the mothers.

Interviewers visited couples three times in the first two years after birth: around two months after the child was born, about nine months after birth and about 15 months after birth.

At the nine-month visit, researchers gave the fathers saliva sample kits. Dads took samples three times a day - morning, midday and evening - to monitor their testosterone levels.

Participants responded to questions about depressive symptoms based on a widely used measure, the Edinburgh Postnatal Depression. They also reported on their relationship satisfaction, parenting stress and whether they were experiencing any intimate partner aggression. Higher scores on those measures signaled greater depression, more stress, more dissatisfaction and greater aggression.Men’s testosterone levels were linked with both their own and their partners’ depressive symptoms, but in opposing directions for men and for women.

For example, lower testosterone was associated with more symptoms in dads, but fewer symptoms in moms. The link between their partners’ testosterone levels and their own depression was mediated by relationship satisfaction. If they were paired with lower-testosterone partners, women reported greater satisfaction with their relationship, which in turn helped reduce their depressive symptoms.