Men are less likely to stray after they have children because fatherhood lowers their testosterone, a study has shown.
The effect is nature's way of helping to ensure the male of the species stays at home and contributes to child care, scientists believe.
Testosterone is the male hormone that increases sex drive and drives men to compete for mates.
But once they become fathers, macho mating activity may conflict with parental responsibilities, say researchers.
Lowering testosterone is likely to make a man a better father by helping to bring out his nurturing feminine side, the study suggests.
Scientists followed a group of 624 men aged 21 to 26 for more than four years and measured their testosterone before and after they became fathers.
Fatherhood for partnered men typically reduced testosterone by 34%.
"On the contrary, the men who started with high testosterone were more likely to become fathers, but once they did, their testosterone went down substantially. Our findings suggest that this is especially true for fathers who become the most involved with childcare."
The study, conducted in the Philippines, showed that 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 engaged in childcare.
"Humans are unusual among mammals in that our offspring are dependent upon older individuals for feeding and protection for more than a decade," said study co-author Christopher Kuzawa, from Northwestern University.
"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."
The research is published today in the journal Proceedings Of The National Academy Of Sciences.
The findings may provide a clue to why single men often have poorer health than married men and fathers.
"If fathers have lower testosterone levels, this might protect them against certain chronic diseases as they age," said Dr Kuzawa.