Just like new mums, new fathers can be depressed, and a study found a surprising number of sad dads smacked their one year olds.
About 40pc of depressed fathers in a survey said they had smacked kids that age, versus just 13pc of fathers who weren't depressed. Most dads also had had recent contact with their child's doctor -- a missed opportunity to get help, authors of the study said.
Many child development experts warn against smacking children of any age.
Other studies have shown that children who are smacked are at risk of being physically abused and becoming aggressive themselves.
The researchers said smacking is especially troubling in children who are only one, because they could get injured and they "are unlikely to understand the connection between their behaviour and subsequent punishment".
The study was released online today in the journal Pediatrics.
The authors analysed data on 1,746 fathers from a nationally representative survey in 16 large US cities, conducted in 1999-2000.
Lead author Dr Neal Davis said that was the most recent comprehensive data on the subject, and he believes it is relevant today. Depression among fathers is strongly tied to unemployment rates, which are much higher now than a decade ago, he said.
The men were questioned about depression symptoms, smacking and interactions with their one-year-olds, but weren't asked why they smacked or whether it resulted in physical harm.
Overall, 7pc of dads had experienced recent major depression. Some likely had a history of depression, but in others it was probably tied to their children's birth, similar to postpartum depression in women, Davis said.
Postpartum depression is more common in women; by some estimates as many as 25pc develop it shortly after childbirth.
Less is known about depression in new dads and the study raises awareness about an under-recognised problem, said Dr Craig Garfield, an assistant pediatrics professor at Northwestern University and co-author of a Pediatrics editorial.