When dads are stressed-out about parenting, it may take a toll on their toddlers' development, a new study suggests.

The study, of more than 730 families, found that when fathers had high levels of "parenting stress," their sons tended to have poorer language skills at age 3. And both boys and girls typically scored lower on tests of cognition -- which refers to abilities such as paying attention, learning and reasoning.

Researchers said the findings add to the growing understanding of how fathers affect their children's development.

When it comes to kids' well-being, studies have traditionally focused on moms' influence, said Tamesha Harewood, one of the researchers on the new work. But more recently, studies have been digging into the unique role of fathers.

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Researchers found that involved dads can affect preschoolers' language skills and emotional development -- as well as older kids' risks of behavioral problems and depression.

"Times have changed. More fathers are staying at home with their kids, or becoming more involved in parenting," said Harewood, a postdoctoral researcher at Michigan State University.

In their study, Harewood and her colleagues zeroed in on lower-income U.S. couples, and their levels of parenting-related stress. That was measured with standard questionnaires that ask parents whether they agree with statements such as, "I feel trapped by my responsibilities as a parent," or "Sometimes I feel my child doesn't like me."

Overall, the study found, fathers' stress levels influenced their young children's language and cognitive development.

There was also evidence that kids tended to have more behavioral problems -- in early and later childhood -- when dad was chronically stressed or had depression symptoms.

That was not simply because their wives tended to be stressed, too, Harewood pointed out. "Their [fathers'] influence was independent of mothers," she said.

Although the study doesn't establish a direct cause-and-effect relationship, parents' stress could affect kids' development for a number of reasons, Harewood said. If their minds are racing, she said, they might not be completely responsive to their children's words or behavior, for instance.

Dr. Michael Yogman is chair of the American Academy of Pediatrics' Committee on Psychosocial Aspects of Child and Family Health. He recently co-wrote an AAP statement on the importance of fathers in their children's lives.

"This new study supports a wealth of research showing that fathers play an important role in their children's cognitive and behavioral development," Yogman said.

And, he added, it's not just that dad is a sidekick to mom. Fathers often make unique contributions, he said.

Research suggests that fathers typically affect young children's language development differently from mothers, Yogman said. As an example, he said that dads tend to have more trouble understanding toddlers' baby talk, and often encourage them to be more "clear with their words."

Many fathers also differ from moms during playtime. "Generally speaking," Yogman said, "fathers are more likely to encourage kids to explore and take safe risks."

To Harewood, the message for fathers is to remember that their own well-being matters to their kids. "You're important to your children, so you need to take care of yourself," she said.

That means admitting to being stressed-out or depressed, and asking for help if you need it, Harewood said. "Fathers shouldn't feel like they're 'less manly' because they need help," she added.

Yogman agreed. "The first few years of raising a child can be particularly challenging," he said. "If you need help dealing with stress or depression, don't suffer in silence."

He also pointed to the importance of fathers playing with and reading to their children -- for everyone's benefit. "Those joyful interactions can actually help mitigate any stress a father may be feeling," Yogman said.

The study findings were reported in recent issues of the journals Infant and Child Development and Early Childhood Research Quarterly.