Does testicle size play a role in parental ability?

Wednesday September 11 2013

“Want to know if your partner will be a good dad? Measure his testicles: Men with small glands are more involved in parenting,” was the Daily Mail’s take on research into what makes some men more involved in parenting than others.

Researchers asked small groups of mothers and fathers of infants about the ways and amount the dads were involved in raising their child. The researchers also assessed men’s “desired” level of taking part in these “caregiving” tasks.

The size of the dads’ testes, and their testosterone levels were measured. The dads’ brains were also scanned to see how they responded to pictures of their own (and other) children, to see what might be influencing their caregiving tendencies.

The researchers found that larger testes and higher testosterone levels were associated with less caregiving activity from the father, and less caregiving desire.

However, this kind of research can’t say whether testosterone levels directly affect caregiving. The study also did not account for the many, potentially significant factors that could influence dads’ parenting behaviour, such as social expectations about the role of a father.

So, this study does not prove that men with larger testes or higher testosterone levels will be bad dads. Nor does it prove that men with smaller testes make better dads – as some of the headlines implied. Parenting is complex and while this study suggests a possible link between caregiving and what’s in dad’s pants, it is likely that many other factors play a significant role.

Where did the story come from?

The study was carried out by researchers from Emory University, Atlanta (US) and was funded by a Positives Neuroscience Award from the John Templeton Foundation.

The study was published in the peer reviewed journal Proceedings of the National Academy of Sciences.

Some of the media reporting went too far by suggesting “men with small testicles are better parents” (Mail Online) which is an over-extrapolation and over-generalisation of the research findings.

What kind of research was this?

This was an observational study looking to uncover why some men may be more involved with the care of their children than others.

The researchers turned to an evolutionary theory called “Life History Theory”. This suggests there is a trade-off between the amount of effort dedicated to mating and the amount of effort dedicated to parenting. The theory is usually used to explain differences between species (such as humans and gorillas – humans have surprisingly small testicles for their body size compared to gorillas and other primates). But some researchers believe it may explain some of the observed difference in different human fathers’ parenting behaviour.

The study tested this theory by measuring aspects of reproductive biology related to mating effort (testicular size, testosterone levels) in human men, as well as aspects of paternal nurturing behaviour and the brain activity related to it.

Past research, the authors report, has shown that lower levels of testosterone are associated with reduced libido while high levels predict mating success. In contrast, a decrease in testosterone accompanies fatherhood in several species, including humans, and higher levels of testosterone are related to less time invested in paternal involvement. Experimentally changing the levels of testosterone in birds has suggested that high testosterone directly causes both increased mating effort and decreased parenting effort.

Nonetheless, there have been few human studies looking at this in theory, so the researchers aimed to fill this research gap.

What did the research involve?

The study recruited 70 biological fathers (aged between 21 and 55) who had between one and four children, at least one of which was aged one or two years.

Fathers’ actual caregiving, and their desired levels of caregiving, were measured through mother and father-reported questionnaires. This asked both parents to assess who had primary responsibility for 24 tasks using a five point scale ranging from “mother almost always” to “father almost always”. These tasks included things such as “take the baby to preventative health care clinic,” “bathe baby,” and “attend to baby during night waking.” The scores were totalled to give a parental caregiving score. For each item, the parent was also asked “How would you like it to be?” These items were combined to acquire a desired caregiving score.

The researchers assessed testes volume (size) using MRI scans, blood testosterone levels, and the fathers’ brain activity while they viewed pictures of their own children, as well as unknown children.

Testes size was found to be related to the man’s height. The researchers statistically adjusted for this in their analysis, to isolate the effect of differences in testes that were related to testosterone, rather than differences in testes that were due to a man’s body size.

What were the basic results?

Completed caregiving scores were acquired from 66 fathers and 67 mothers. There was very high agreement between fathers’ and mothers’ ratings of the fathers’ actual caregiving. The researchers only used the mothers’ ratings of actual caregiving in the analysis as it was thought to be less subjective than the men’s. Testes volume was measured for 55 men, and testosterone levels measured for 66 men.

The researchers reported many results, but they all told a similar story – that higher testosterone levels and testes size were associated with less actual caregiving and less desired caregiving.

Testosterone and testes size

  • Higher testosterone levels were associated with lower levels of caregiving.
  • Higher testosterone levels were also associated with a lower caregiving desire amongst the dads. A strong association was also found between larger testes volume and a lower caregiving desire.
  • Testes volume showed a moderate positive correlation with testosterone levels – this means that the larger the testes the higher a man’s testosterone levels tended to be.

The authors noted that it could be the case that fathers who provide less practical support for their children (like changing nappies) are investing in them in other ways. For example, they might work longer hours to provide greater financial investment. This was tested and they found that:

  • Neither testes size nor testosterone was associated with the number of hours fathers worked per week, or with fathers’ earnings.
  • Testosterone levels and testes size emerged as the most important factors accounting for differences in caregiving of those tested (hours worked and income were also tested), with testosterone explaining more of the variation than testes size.

This led to the conclusion that higher testicular volume and testosterone levels were related to less paternal caregiving and desired paternal caregiving and unrelated to occupational or financial investment in the infant.

Brain activity

Activity in the parts of the brain related to caregiving (as fathers viewed pictures of their own children) was found to be linked to smaller or larger testes volume. However, no link was found for testosterone levels.

How did the researchers interpret the results?

The researchers’ reported that, “collectively, these data provide the most direct support to date that the biology of human males reflects a trade-off between mating and parenting effort. Fathers’ testicular volume and testosterone levels were inversely related to parental investment and testes volume was inversely correlated with nurturing-related brain activity when viewing pictures of their own child.”


This research highlighted a possible link between both testes size and testosterone levels and caregiving tendencies among a small number of fathers of children under two years old.

One of the main limitations of the study was that it was cross-sectional and therefore cannot determine whether variations in testosterone levels could be directly responsible for the differences in caregiving seen.

The study also did not account for a range of factors which could also affect paternal caregiving – for example, their own father’s involvement, and the influences of cultural and societal expectations on the role of the father. The effects of these other factors could potentially play a larger role than any biological effects, but this was not explored.

Parental caregiving desire and practice could be heavily influenced by culture, religion, social status, geographical location, or life circumstances at the time. So the study represents an attempt to isolate the biological influence on parental caregiving away from these other complex societal factors. However, by not taking into account these influences, the results may have been significantly biased.

Future researchers should factor in societal, ethical and cultural influences to assess the dual influence of biology and circumstance together, to get an idea of the relative influences and how they interact.

The researchers acknowledged that external factors, such as stress or socioeconomic status, could be related to testosterone and may also interfere with fathers’ ability to take a more active parental role, despite a desire to be active in their child’s life.

The study did not assess parenting involvement past the age of two years so could not report on whether the father became more or less involved over time.

The researchers also assessed some of the indirect attempts of fathers to support their children, through working longer hours and increasing household income, rather than changing nappies. However, this was a relatively narrow and short-term assessment of the possible indirect ways a father might be attempting to support the family. This means indirect caregiving activities might not have been properly factored into the results and conclusions.

Overall, this study does not prove that men with larger testes or higher testosterone levels will naturally be less involved in child caregiving than those with smaller testes or lower testosterone levels, or vice versa. It did, however, tentatively suggest there may be link between the two that may relate to evolutionary pressures. The relative effect of this in relation to societal and cultural expectations of fatherhood was not assessed and may be significant.

Analysis by Bazian
Edited by NHS Choices