http://healthland.time.com/2011/09/13/why-do-dads-have-lower-levels-of-testosterone/

Researchers at Northwestern University set out to collect data over time to shed some light on the issue. They retrieved testosterone via saliva samples from 465 Filipino men when they were 21 and single, then repeated the same tests when the men were 26. The men — who gave samples first thing in the morning, when testosterone levels are highest, and again in the evening, after they’d declined — were part of a 28-year project that has followed the same cohort since they were in the womb.

Testosterone decline happens almost as twice as much in men that are active fathers.

Plunging testosterone may help a man commit to his family and reinforce his focus on caregiving, amping up his sensitivity to his children’s needs.

I do not know whether or not this data is correlated correctly, but I thought the article was interesting and would make sense to me.