Researchers have found evidence - if evidence were needed - that men have less sex after becoming a father for the first time.
A study of more than 400 young men in the Philippines found that their sex lives declined significantly when they had their first child. The fall in sexual activity was associated with the men's testosterone levels, which are known to fall when men start families, but the latest research shows that the greater the fall in testosterone, the less sex men reported.
Lee Gettler, an anthropologist at the University of Notre Dame in the US state of Indiana, gathered medical and lifestyle information on the men from the ages of 21 to 26 and found there were physiological and behavioural changes as some of them married and had children.
When men got married their testosterone levels fell, and declined even further when they had their first child. That led to the question of whether falling testosterone might impact on their sex lives.
"I didn't think that testosterone would be linked to men's sexual behaviour, but when we tested it we found that as men transitioned to fatherhood, the more their testosterone declined the less frequently they reported having sex with their partner," Gettler said.
The impact on men's sex lives was not linked to the amount of time and energy they invested in childcare, he said.
Gettler described his research at a meeting of the American Association for the Advancement of Science in Chicago.
Though testosterone is often popularly linked to male libido, there is little evidence that the two are intimately entwined, at least in normal ranges of the hormone. Only extremely low levels seemed to impair the male libido.
The connection between testosterone and sexual activity will take more work to tease out, but Gettler speculates that lower levels of the hormone might help men to focus on their family commitments. The men's decision to be actively involved in fathering seems to play a role, as men who have children unknowingly do not experience falls in their testosterone.
"If their testosterone goes down, the men might be more oriented towards their family's needs and not getting into conflicts with other men or looking for new mates," Gettler said.
An alternative explanation might be that wives found their husbands less sexually appealing when their testosterone levels dropped. Levels of the hormone began to return to normal around a year after men had children.
Whatever the reasons for the physiological shift, Gettler said that men who had children underwent clear physiological changes that might help attune them into their new role as a father. "It's not just mothers. Fathers seem to have this built-in ability to respond to the demands of fatherhood," he said.