From the Telegraph in the UK
A study found that men’s testosterone levels fell by around a third in the days and months after their partner gave birth. The more caring side of a man’s character emerged as levels of the hormones fell, said scientists, who believe that the process is nature’s way of trying to ensure that fathers stay for the long haul of child–rearing.
They found that men with higher testosterone levels – associated with dominant and aggressive behaviour – were both more likely to secure a partner and father children. But after the birth itself testosterone levels in these men dropped.
“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 faculty fellow at the Institute for Policy Research at Northwestern University in Chicago, and a coauthor of the study, published in the journal Proceedings of the National Academy of 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.”