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 Guardian reports  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 Indiana, gathered medical and lifestyle information on the men from the ages of 21 to 26 years old 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.

"Does this mean that men who care for children have low testosterone and no sex? No, it has nothing to do with childcare." 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 the annual 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, Gettler said.

Levels of the hormone began to return to normal around a year after men had children.