The (mostly) male hormone testosterone drops after a man becomes a parent, probably so that he will care for his infant, rather than try to kill him or push him out of the nest, as some other creatures do.
In the September 13th edition of the New York Times, Pam Belluck quotes anthropologist Peter Gray as saying, "The descent of a man’s testosterone may even be welcomed by some, perhaps his progeny." She quotes anthropologist Carol Worthman as saying, "If guys are worried about basically, ‘Am I going to remain a guy?,’ we’re not talking about changes that are going to take testosterone outside the range of having hairy chests, deep voices and big muscles and sperm counts. These are more subtle effects."
The lowering of their testosterone did not prevent the men in the study from having more children. Belluck quotes anthropologist Christopher Kuzawa as saying, "You don’t need a lot of testosterone to have libido." She quotes evolutionary biologist Peter Ellison as saying, "The real take-home message is that "male parental care is important. It’s important enough that it’s actually shaped the physiology of men."
In other words, babies need dads, as well as moms. He says, "My hope would be that this kind of research has an impact on the American male. It would make them realize that we’re meant to be active fathers and participate in the care of our offspring."
One thing that becomes especially important when you have kids is a good calendar, so you can keep track of play dates and school schedules, and you can’t get a better one than our beautiful crop circle calendar!