Men are biologically wired to care for their children, a new study suggests.

The study, led by Northwestern University researchers, followed a large group of men in their 20s and found testosterone levels fell after they found partners and became fathers. Testosterone is the primary male sex hormone thought to contribute to behaviors and other traits that help men find mates. But lower testosterone levels could help men who are fathers better handle the demands of parenting and allow them to become more nurturing, the researchers...