By the numbers: About 1 in 10 dads experience anxiety or depression in their child's first year — and they're far less likely to be screened for it than moms.

What they're saying: The more dads take on, the more they're "kind of shouldering those same risks" mothers have long faced, Saxbe says — a "double-edged sword" where the involvement that deepens the bond also carries the cost.

The intrigue: Couples' stress hormones and moods tend to move in tandem, Saxbe's research finds. So when one parent is depressed, the other's risk climbs. "If we care about Mom's mental health, we actually need to care about Dad's, too."

• Some risk factors overlap: disrupted sleep, an identity reckoning and a strained partnership.

The bottom line: Dads do best with hands-on time and a network around them. "We were not designed to parent in isolation," Saxbe says.