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Monday, September 16, 2013

Male parenting strategies explained

If you want a male partner who will share the parenting duties, you might want to choose someone with smaller testicles. So says a new study by Jennifer Mascaro, Patrick Hackett and James Rilling from Emory University. 

There’s a hypothesis within evolution that is referred to as the ‘life history theory’. Briefly, an organism only has a finite amount of energy to expend toward its ultimate goal: reproduction. A creature that grows an enormous reproductive display may attract more mates, but it will produce smaller litters. A mother can abandon a thousand babies to the ocean currents, but she can only nurture a handful at a time. Each strategy is a trade-off.


Human males also exhibit a range of parenting and mating strategies. To put it bluntly, how much of their energy do they spend trying to get the girl and how much caring for the resulting children? The answer may lie in their hormones and in their testicles. Males (human and other species) with high testosterone have greater mating success. However, they also spend less time caring for their offspring. The authors found that there is another, independent, correlation between nurturing and testicle size.

In many species, testicle size is inversely proportional to the number of partners the females of the species are likely to mate with. That is, males from species where females are largely monogamous have small testes, whereas males from species where females have multiple partners have large testes. In case you’re wondering, among primates, humans have moderately sized testes. We’re socially monogamous but prone to cheating.

There’s a part of the brain called the ‘ventral tegmental area’ (VTA). As part of the brain’s reward circuitry, the VTA is implicated in addiction, but also in strong emotional feelings, such as when one is holding one’s child.

Seventy fathers with small children were recruited for this study. Each man and his partner reported on his involvement in child caregiving, both actual and desired. The men had their testes volume and their testosterone levels measured. Finally, the researchers measured activity in the VTA while the men looked at pictures of their kids.

As expected, men with higher testosterone levels spent less time caring for their kids, and, perhaps more importantly, had no desire to spend more time with their kids. However, testes size also influenced how much fathers cared for their kids. Plus, according to activity recorded in the VTA, men with smaller testes felt more nurturing when viewing photos of their kids than did men with larger testes.  

Obviously, this was a very small study (though perhaps we should be grateful that as many as 70 men allowed researchers to measure their testes). Still, it does align with life history theory. Men with high testosterone and/or large testes are better at mating but worse at parenting the resulting offspring.


Mascaro JS, Hackett PD, & Rilling JK (2013). Testicular volume is inversely correlated with nurturing-related brain activity in human fathers. Proceedings of the National Academy of Sciences of the United States of America PMID: 24019499.