Smaller Testicles and Better Fathers

What did the Researchers say?

The researchers were interested in testing an aspect of Life History Theory.  Life History Theory suggests that the key events in an organism’s life –  the development of the young organism, the advent of its reproductive ability, its attitude towards parenting, and even its death – are tuned by natural selection in such a way that its offspring receive the best possible benefit.  This means, for example, there is often a trade-off between the effort invested in mating and the subsequent effort invested in rearing young.  At the extremes, organisms could put all their effort into mating and walk away from their offspring, or only mate once and become a perfect parent.  Or, as is more common, the organism’s behaviour lies somewhere in between these two extremes.

Jennifer S. Mascaro, Patrick D. Hacket & James K. Rilling (in press). Testicular volume is inversely correlated with nurturing-related brain activity in human fathers. Proceedings of the National Academy of Science. doi: 10.1073/pnas.1305579110

Life History Theory has its critics,  but if one were to assume the theory to be true, then you can create some interesting hypotheses.  Here’s what the researchers’ hypothesis looked like (not a quote, merely a very rough paraphrasing):

If there is a trade-off between reproductive effort and parenting thanks to Life History Theory, then this trade-off might have physical consequences.  For males, the volume of the testes is a (somewhat crude) indicator of reproductive potential.  With more storage space for sperm, the animal can mate more frequently, and presumably be less invested in the offspring.  Smaller testes means less sperm to give to mates, so the animal will need to make more of an effort to make sure their genes survive.  So smaller testes and other indicators of weaker reproductive potential should be linked with more effort invested in parenting.

There is some evidence of this in primates, where testes size is linked to whether the primate is monogamous. For example, while the average monogamous male gorilla weighs about 2.5 times as much as the average man, his testes are much smaller, but polygamous male chimpanzees have testes more than twice the size of a man’s! Until this study, what wasn’t really clear was how much of this translates to humans, and human males in particular.

The authors took a sample of fathers of 1-2 year old children from Atlanta, Georgia, and measured both their testes volume and testosterone levels.  At the same time, they took questionnaires from them regarding how much time they spent on childcare tasks, and how much time they would like to spend. They then took brain scans of the fathers while viewing images of their children to assess their brain activity.

Both testes volume and testosterone levels were found to be inversely correlated with caregiving. That is, smaller testes and less testosterone were both linked with more caregiving activity and stronger brain activation when viewing their child.  ALthough other effects were also inversely correlated with caregiving, such as father’s wages and number of hours worked per week, the link between testes size and better parenting remained even when these factors were statistically controlled for.

The supplementary material shows some of the extra tests they ran to see if there were any other trends of importance.  For example, the trends they do spot seem to be independent of how old the father’s child is, as well as of the sex of the child (although the authors do note a very slightly stronger response from fathers looking at images of sad daughters, but this is not statistically significant).

What did the Media Say?

The BBC do obliquely mention Life History Theory in the article (but it is never explicitly named or linked to in any way), and they do correctly note the methods the authors used to identify the correlation.  They link to the paper, and are also careful to note factors such as the study focusing on males from Atlanta in the USA being a potential source of cultural bias, and the possibility that testosterone levels may be affected by the process of having a child.

The Independent do name-check Life History Theory, and are keen to point out a correlation between testes volume and promiscuity and divorce, which is a little poorly worded –  the researchers refer to work which shows a correlation between testosterone and divorce, and the Independent’s sentence structure makes it unclear whether testes size or testosterone level is the driver.  They use quotes from one of the researchers, James Rilling, more liberally.  He speculates on whether there is a link between the father’s own childhood and their future life choices.  Sadly, they do not present a link to the paper.

Time’s charming headline “Choose Dads with Smaller ‘Nads” reflects the playful tone of the article, but that shouldn’t convince you that it’s not worth the read.  On the contrary, it links to the paper, and quotes Rilling clearly.  They’re also quite careful to note the potential weaknesses of the experiment (such as the potential change in testosterone levels due to fatherhood).  It’s a salutary lesson in reading stories about research–style and substance are not always linked!