The first longitudinal study of testosterone levels in fathers has found that the longer a man has been a father--and the more involved with the daily care of his children he is--the lower his testosterone level drops.

The study measured testosterone levels in 21-year-old men before they became fathers, and then again 5 years later.  Those who became fathers had more than double the drop in testosterone than non-fathers (all men experience a drop in testosterone as they age).  And those men who spent more than three hours caring for their children each day had the lowest level of all.