They say having a family changes a man forever. But it may not just be a sense of duty that drives new dads to swap evenings in the pub for quiet nights at home。
　　A study has found that when men become fathers, their bodies suppress production of the sex hormone, making them more interested in parenting than mating。
　　The researchers measured testosterone levels in saliva from a group of 21-year-old men from the Philippines with no children. Testosterone boosts behaviors that help a male compete for a mate。
　　Over the next five years, childless men saw an average age-related testosterone decline of 14 percent, but for those who became fathers, the typical drop was 34 percent。
　　The men who reported caring for their child for between one and three hours per day saw the greatest decline, which the authors said was not accounted for by stress or sleep deprivation。
　　Co-author Lee Gettler, an anthropologist at Northwestern, said: "I think it is reasonable to predict that the men’s testosterone could remain low for a number of years if they remained heavily involved in providing care to their children."
　　This is the first to suggest that settling down and having children actually causes the fall。