New research provides compelling evidence that human males are biologically wired to care for their offspring, showing for the first time that fatherhood lowers a man’s testosterone levels.
US researchers led a study which followed a group of 624 young men in the Philippines, correlating their chances of finding a partner with their testosterone levels. They report that having higher testosterone levels increased a man’s chance of becoming both a partner and a father.
They also found that testosterone levels fell when the men became fathers, suggesting that becoming a father causes a drop in testosterone levels. As the authors conclude,
“Our finding… supports the hypothesis that father-child interaction likely contributes to suppressed paternal testosterone among fathers.”
An embargoed copy of the study, to be published in Proceedings of the National Academy of Sciences, and a press release can be found in the SMC Resource Library.
The Science Media Centre contacted New Zealand experts for further comment on the study.
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Dr Valerie Grant, Evolutionary Psychologist and Honorary Senior Lecturer, University of Auckland, said,
“Contemporary scientists seem to have reached a consensus on the “nature-nurture” question. They have decided both are important, and furthermore, each influences the other. This new understanding has paved the way for a resurgence of work on the once neglected “nature” side of the equation, so that studies which a couple of decades ago would have been neither funded nor published are now making a come-back.
“Evolutionary psychologists have long argued there must be physiological basis for those behaviours which make a major contribution to survival and reproduction.
“Right across the animal world, high testosterone males have an early advantage. Whether the animal is a cockroach, a bird or a mammal high testosterone ensures they are more dominant than their lower testosterone age-mates and thus more likely to be glossier, more muscular, pushier and take more risks. And across the spectrum these attributes appeal to females and enable the high testosterone males to be successful when it comes to mating.
“But then endocrinologists found that fathers had low testosterone, so people sceptical about evolutionary theory wondered if researchers had been putting too much weight on the attractiveness and success of high testosterone, or alpha, males. Ah, they said, maybe it’s the lower testosterone males who get there in the end. Testosterone may be advantageous in mating, but after the baby is born high testosterone is less of an advantage. Babies with risk-taking fathers do not survive as well as babies with fathers who moderate their risk-taking behaviour.
“Now a team of biological anthropologists has published a comprehensive longitudinal study showing that the high testosterone, high dominant male is indeed more successful at mating, but after the baby is born there is a significant drop in his testosterone levels which ensures he is then more suited to helping to raise the baby. The researchers found that the younger the baby and the more the father was involved in its care, the lower his testosterone levels fell.
“Evolutionary psychologists would argue that this is exactly what would be expected, because this would be maximally adaptive. In the competition for the best mates, the alpha male has an advantage. But such an advantage would quickly disappear if he did not also contribute to the care and protection of the infant. Hence males whose testosterone levels could change according to their circumstances – high when seeking a mate, low when parenting – were likely to do better over evolutionary time.
“In most parenting males, falls in testosterone level are likely to be temporary. It is of interest that this study ran for four and a half years. Some evolutionary researchers believe that after about four years, the pair-bond weakens.
“It would be of interest if the young males in this study from the Philippines were to be measured again for testosterone in about five years’ time, and levels compared with whether the original pair bond had been maintained, as well as whether or not further children had been born.”
Our colleagues at the UK SMC collected also the expert responses below.
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.”
Professor 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.”