September 12, 2011

expert reaction to fatherhood and reduced testosterone levels, as published in PNAS

New evidence suggested that fatherhood reduces testosterone production in men.

Dr Allan Pacey, Senior Lecturer in Andrology at the University of Sheffield, said:

“This is an elegant and interesting paper which documents the testosterone levels of a group of men over a five-year period as some of them develop relationships and become fathers.

“Testosterone is the key hormone that defines male physiology. We know that levels correlate with a man’s sex drive, his risk taking behaviour and social dominance. It has also been suggested that it may increase his attractiveness to women and help him find a mate.

“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.”

Prof Ashley Grossman, Society for Endocrinology spokesperson and Professor of Endocrinology at the University of Oxford, said:

“Testosterone is the major hormone produced by males and is responsible for producing the male body shape, the distribution of hair in men (and the loss of head hair!), and also sexual interest and activity. There is also reasonable evidence that the production of testosterone by the male foetus ‘masculinises’ the brain. However, little has been understood about changes in testosterone in adult men except in serious disease states, other than the very slight fall with age above the age of 25. If there is excessive physical trauma or disease, or (very) excessive exercise, then the body will turn off the reproductive axis: in a sense it is saying ‘we have more to worry about than reproduction at the moment’. However, for the normal male most endocrinologists consider that ‘enough is enough’, and once testosterone is within the normal range, changes within this range are of little importance.

“This recent study from a cohort of men followed in the Philippines adds to the small but increasing amount of evidence that these views are much too simplistic. These researchers followed a group of 624 young men and correlated their chances of finding a partner with their testosterone levels. They report that the men who became ‘partnered fathers’ were those with significantly higher levels of testosterone; testosterone predicted both having a partner and becoming a father. Equally fascinating, they also noted that following ‘fathering’ the testosterone levels fell, especially in those with very young babies and in those fathers with most child care responsibilities. These are critical studies, as other cross-sectional studies could be construed as showing that men with lower testosterone levels were more likely to be caring. These findings suggest a different cause-and-effect relationship: fathering actually appears to lower testosterone.

“What is the significance of these findings? I think they are two-fold. Firstly, higher levels of testosterone are associated with reproductive success, which from a Darwinian perspective may make sense. But in addition, the male reproductive axis may be attenuated by the presence of a child and more so as the intensity of child care increases. This shows the hormonal and behavioural trade-off between mating and parenting, one requiring a high and the other a low testosterone level. Life and biology may be much more subtle and adaptable than we had previously thought.”

Proceedings of the National Academy of Sciences (PNAS) Article 11-05403: “Longitudinal evidence that fatherhood decreases testosterone in human males,” by Lee T. Gettler, Thomas W. McDade, Alan Feranil, and Christopher W. Kuzawa.