New dads who feel down are much more likely to spank their kids than dads in good spirits, U.S. researchers said Monday.

They found 40 percent of depressed men said they had spanked their kids in the last month, compared to only 13 percent of men without symptoms.

“This finding is particularly concerning given that children were only 1 year of age in our study,” Dr. R. Neal Davis, of Intermountain Healthcare in Murray, Utah, and colleagues write in the journal Pediatrics.

At that age, they add, spanking is more likely to cause injuries, and children probably won’t understand why they are being hurt.

The study is based on interviews with more than 1,700 new fathers from 20 large cities in the U.S.

“There has been other research showing that moms who are depressed are more likely to spank their kids, but this is the first time it has been shown for dads,” said Elizabeth Gershoff, an expert in child development and family relationships at the University of Texas at Austin.

Gershoff, who wasn’t involved in the study, added that it might make sense for mothers to keep an eye on their partner if he is depressed and offer to take care of the kids if things get hectic.

“Parents who are depressed have very short fuses,” she told Reuters Health. “They have much less tolerance when their children misbehave.”

According to a 2009 report from the Institute of Medicine, more than 15 million American kids are living with an adult who suffers from depression.

Earlier studies have hinted - but not proven - that spanking may leave a psychological mark on toddlers, prompting aggression and other problems years later.

Many psychologists recommend time-outs and other types of non-physical punishment instead, and both the American Academy of Pediatrics and the American Psychological Association discourage spanking.

Still, a 2008 U.S. survey showed that 77 percent of men and 65 percent of women agree that a child sometimes needs a “good hard spanking.”

The new study doesn’t prove that depression leads to spanking in itself, since a host of other problems could be involved.

But even ruling out some of the most obvious competing explanations - such as unemployment, poverty and education - dads who’d felt blue in the past year were four times as likely to swat their kids as those who felt fine.

They were also less likely to read stories for their children, although they played and sang songs for them just as frequently as their less troubled peers.