A new study out of Northwestern University showed that testosterone levels for men who had become fathers dropped between 26 percent and 34 percent. The study followed 624 young men in the Philippines for over 4.5 years, some were married and others had long-term relationships with girlfriends. The drop in testosterone was about twice as much as testosterone levels dropped in the single men who didn't become fathers during the study. What the researchers don't know is why the testosterone drops. Some researchers in this article
guess that it might be to make room for increasing bonding hormones such as oxytocin and prolactin; another guess is that it helps boost the immune system, which would prevent passing along sickness to newborns. The article also points out that the findings don't prove that fatherhood directly affects testosterone levels, and they're not sure how the hormonal systems in men detect they've become fathers in the first place. One thing for sure is that this testosterone drop seems to be to the benefit of being better fathers; and that's about as masculine as it gets.