Why becoming a father for the first time saps a man's hormones

Having children is not only bad for your sleep and the family budget – it also saps testosterone levels in fathers, according to a study which suggests that men are genetically programmed for fatherhood. High testosterone levels could be linked with the chances of finding a sexual partner, but once a man becomes a father for the first time, his testosterone levels fall significantly, a study of 624 young men found.

Scientists believe this decline shows that men are biologically "hard-wired" to care for offspring, rather than paternal care being a purely a cultural phenomenon. They say that other species in which males show a similar testosterone decline are generally those in which there is innate paternal care for offspring, compared to species where there is no paternal care after mating. "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 Kuzawa, a professor of anthropology at Northwestern University in Evanston, Illinois. "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."

The study, published in the US journal Proceedings of the National Academy of Sciences, followed hundreds of young Filipino men for nearly five years, measuring their testosterone regularly before and after they first became fathers. The team found single men in the group with the highest testosterone levels were the most likely to become fathers, suggesting that the male hormone is associated with attractiveness of men in the eyes of women.

However, once the men became fathers their testosterone levels fell and the drop was greater than in anything seen among the group of single, fatherless men. Furthermore, the fathers involved in childcare had less testosterone than those who were not.

Testosterone, the male sex hormone, is associated with sex drive, aggression and development of secondary sexual characteristics such as facial hair, muscle bulk and a deep voice.

Men who are tortured, deprived of sleep or are chronically unwell usually have lower testosterone levels than normal. One possibility is that men who have naturally lower amounts of testosterone may be more likely to become fathers and be involved in childcare, compared with men with naturally high testosterone levels. However, the scientists dismissed this suggestion.

"It is not the case that men with lower testosterone are simply more likely to become fathers," said Lee Gettler, who co-wrote the study. "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 this is especially true for fathers who become the most involved with childcare."