Does this mean that Homer Simpson has the fathering technique down? A new study claims that men’s testosterone levels drop after they become fathers—and the levels go lower depending on how active a father a man is. The study, the first large one of its kind, found that men’s hormonal levels are affected by their behavior and how invested they are in being parents. Testosterone was measured when men were 21 and single, and then again nearly five years later. Although testosterone levels naturally decrease with age, men who became fathers showed a much greater decline. “This should be viewed as ‘Oh, great, women aren’t the only ones biologically adapted to be parents,’ ” said Lee Gettler, an anthropologist who coauthored the study.