Sharing childcare makes for happier couples with better sex lives, US research suggests.
In a study of 487 families, parents who split childcare duties evenly reported greater satisfaction, both sexually and emotionally.
But in couples where the woman did most of the childcare, both men and women reported being less content.
The researchers said men doing a greater share of childcare did not have the same impact.
The conclusions have been drawn from a study called the 2006 Marital and Relationship Study, which was a survey into marriage and relationships among heterosexual couples.
Who does the childcare?
The data, being presented at a meeting of the American Sociological Association, shows that couples where the women performed more than 60% of childcare - specifically in terms of rule-making, praising and playing - fared the worst on scores of relationship satisfaction and conflict, as well as being less happy about the quality of their sex life.
The study leader, Dr Daniel Carlson, an assistant professor of sociology at Georgia State University, said: "One of the most important findings is that the only childcare arrangement that appears really problematic for the quality of both a couple's relationship and sex life is when the woman does most or all of the childcare."
The team found that fathers could in fact take on most or all of the childcare responsibilities without negatively affecting the quality of the couple's relationship.
The study did not look at who performed tasks such as feeding and bathing the children.
The academics are planning more research into why those couples with more equal childcare responsibilities seem to have better relationships.
"We are trying to understand what is it about sharing that couples view so positively," Dr Carlson added.
The 'new man'
Prof Sir Cary Cooper, an expert in organisational psychology and health at Manchester Business School, said the findings made sense, but they might reveal more about the kind of couples who shared their responsibilities.
"If you have a 'new' man who is happy to share childcare, he probably invests more in the relationship anyway," he said.
He added that it was becoming increasingly acceptable for men to opt for more flexible working and to take on more of the responsibility for family and domestic life.
"Increasingly there's a lot of pressure on men who wouldn't normally do that - the question is would that make a difference in the relationship. I think it could do."
The 487 couples in the study were selected at random and included low-to-moderate income couples who had children living with them and where the woman was under 45 years.
A total of 605 couples were interviewed, but the researchers only included in this study those where both partners had completed the full survey.