Scientists have discovered fatherhood can halve testosterone levels in men, making them more interested in caring for their offspring than going astray.
Testosterones are sex hormones that boost behaviours and other traits that help a male compete for a mate.
"Humans are unusual among mammals in that our offspring are dependent upon older individuals for feeding and protection for more than a decade," said Christopher W. Kuzawa, co-author of the study and associate professor of anthropology in the Weinberg College of Arts and Sciences.
"Raising human offspring is such an effort that it is cooperative by necessity, and our study shows that human fathers are biologically wired to help with the job," he added.
The new study's findings also suggest that fathers may experience an especially large, but temporary, decline in testosterone when they first bring home a newborn baby.
"Fatherhood and the demands of having a newborn baby require many emotional, psychological and physical adjustments," said Lee Gettler, a doctoral candidate in anthropology at Northwestern and co-author of the study.
"Our study indicates that a man's biology can change substantially to help meet those demands."
The study was recently published in the Proceedings of the National Academy of Sciences.