Men appear to be biologically wired to care for their babies, say researchers who have discovered levels of testosterone go down after fatherhood.
This drop in the male hormone presumably makes the dad more family-oriented and less likely to stray, say the Northwestern University team.
Testosterone increases a man's sex drive and helps him compete for a mate.
The Proceedings of the National Academy of Sciences work followed 624 young men before and after they became fathers.
This revealed that as soon as a man had a baby, his testosterone levels dropped substantially.
Men with newborn babies less than a month old had especially reduced levels of testosterone.
Larger falls were also seen in those who were more involved in childcare.
The lead investigator of the work carried out in the Philippines, Christopher Kuzawa, said: "Raising human offspring is such an effort that it is co-operative by necessity, and our study shows that human fathers are biologically wired to help with the job.
"Fatherhood and the demands of having a newborn baby require many emotional, psychological and physical adjustments. Our study indicates that a man's biology can change substantially to help meet those demands."
And the researchers believe lower testosterone levels might protect against certain chronic diseases, which could, in part, explain why married men and fathers often enjoy better health than single men of the same age.
Prof Ashley Grossman, spokesman for the Society for Endocrinology, said life and biology may be "much more subtle and adaptable than we had previously thought.
"This shows the hormonal and behavioural trade-off between mating and parenting, one requiring a high and the other a low testosterone level."
Dr Allan Pacey, senior lecturer in andrology at the University of Sheffield, said the findings were fascinating:
"Testosterone levels in men generally don't change that much. They can slowly decline as men get older and change in response to some medical conditions and treatment. But to see dramatic changes in response to family life is intriguing.
"The observations could make some evolutionary sense if we accept the idea that men with lower testosterone levels are more likely to be monogamous with their partner and care for children. However, it would be important to check that link between testosterone levels and behaviour before we could be certain."