Thursday, March 12, 2015

Primordial soup to nuts: are some men naturally selected to be better dads?

Children are the future. So “why do some men choose not to invest in their children?” This was the question that Dr. James Rilling set out to answer over the last few years. Dr. Rilling is the head of the Laboratory for Darwinian Neuroscience in the Anthropology Department at Emory University and states one of the lab’s aims is “exploring the neural basis of human social cognition and behavior, particularly those aspects that have been under strong evolutionary selection pressure.” But are absent fathers the result of natural selection?

In the last half-century, the basic structure of American families has been changing. Within two-parent households, fathers are spending more time with their children than they used to as more mothers work outside the home. However, there are also many more single mothers raising children without any paternal help and roughly half of all American children are raised by a single parent at some point during childhood [1].

These changes have occurred far too rapidly to be the result of natural selection, but this trend compels additional study into the factors underlying paternal commitment. It is not known if there may in fact be an evolutionary explanation for why some men are more committed fathers than others. More to the point, do biological differences between men influence behavioral variation? Life History Theory posits that natural selection shapes the allocation of finite resources toward aspects of growth, survivorship, and reproduction. Within reproduction, there is arguably a trade-off in this zero-sum game between parenting and mating activities, and natural selection shapes phenotypes to support optimal strategies.

In the non-human world, there is evidence for a connection between mating strategy and testicular size in males. For example, gorillas, who typically live in harem-like situations with little competition from other males have rather small testes for their body weight whereas chimpanzees, who typically face much more mating rivalry are much better-endowed [2]. Other studies have found a relationship between testicular volume and mating success within species [3,4].

Dr. Rilling’s group reasoned that perhaps testicular volume would also be indicative of mating investment within a species (ours) and tested whether testicular size or testosterone levels correlated with measures of paternal involvement. To take it a step further, they also used fMRI to examine the activation of reward pathways in fathers’ brains while viewing photographs of their children. The researchers recruited fathers of children (age 1-2) in Atlanta, GA who were co-habitating with their partner and child. They then asked the fathers – and also the mothers – to come into the lab and answer a series of questions designed to gauge the fathers’ involvement and parenting responsibilities, and also their desired level of involvement.

They then asked the fathers – and also the mothers – to come into the lab and answer a series of questions designed to gauge the fathers’ involvement and parenting responsibilities, and also their desired level of involvement. The survey assessed involvement by asking who is responsible for the child on a 1 – 5 scale (1 being mother almost always to 5 father almost always) in 24 different tasks and situations like bathing the baby or getting up at night to attend to waking. The mothers’ ratings served to corroborate the fathers’ self-reported involvement and supplemental information showed that there was in fact a very high level of agreement. The fathers then provided blood samples for hormonal measures and received structural and functional MRI scans. In the fMRI scanner, they were shown pictures of their own child, an unknown child, and an unknown adult with neutral, happy, or sad facial expressions. Finally, testicular volume was measured by structural MRI.

They hypothesized that the more invested fathers would have lower testosterone levels and smaller testes – reflecting more of an energetic investment in parenting than in mating behavior. Furthermore, they reasoned that this behavior may be related to activity in the ventral tegmental area – a part of brain’s reward system – because more involved fathers may find interaction with their children more rewarding. Indeed, this is what they found.

Mascaro and colleagues published the results of their study entitled “Testicular volume is inversely correlated with nurturing-related brain activity in human fathers” in PNAS [5]. As one might expect, headline writers (this one included) made the most of this opportunity: “Men with smaller gonads are better dads”, “Have you got the balls to be a good dad?”, “How your big balls lead to bad parenting”, etc., etc.

From The Onion

Some Internet commenters and science bloggers who, admittedly, are not by-and-large known for their levelheadedness and foresight, found the study pointless or a waste of money. These remarks may reflect a public attitude that scientific research reported by the mainstream press needs to have some impact on our daily life. They also reflect a lack of understanding of the way science funding works, as this finding was a small piece of a larger study that examined the more general biological influences on paternal behavior and which has resulted in multiple non-testes related publications. This quick-trigger dismissal of science as a waste of money is worrisome given the current state of science funding.

One couldn’t be faulted for asking, “what am I supposed to do with this information?” Does this mean that deadbeat dads should be excused for their irresponsibility so long as they are well-endowed? Not so fast. The authors were careful to note that this study does not establish causality: “it remains unclear whether greater testes volume is a cause or a consequence of male life-history strategies.” In other words, it might also be the case that fathers who choose to be more invested in child rearing experience a drop in testosterone and mating drive (but this doesn’t mean that changing diapers will shrink your manhood!). One limitation of this study is that it confined its analysis to the heteronormative parenting model. An interesting next step could be to determine if these correlations also hold in same-sex male couples.

This study, presented by lead author Dr. Jenny Mascaro at the Emory Neuroethics Program’s monthly Neuroscience, Ethics, and the News Journal Club, is exactly the type of article that this forum was created to discuss. It was published in a high-impact journal and distributed to a wide audience in many fields. The press immediately pounced on it and broadly disseminated the main findings, which is understandable due to how the topic is relatable and digestible (not to mention provocative). In addition, these findings raise interesting questions about biological reductionism and what it means for individual responsibility.

It is difficult to communicate findings such as these – which shed light on our nature and tendencies but do not necessarily explain any single individual’s behavior – to the general public without misinterpretation. Still, it is relatable, fascinating studies like this one that can keep the public engaged with scientific research and help all of us understand a little more about the complexity of human nature.


1. Cabrera, Natasha J., Catherine S. Tamis-LeMonda, Robert H. Bradley, Sandra Hofferth, and Michael E. Lamb. “Fatherhood in the Twenty-First Century.” Child Development 71, no. 1 (January 1, 2000): 127–36.

2. Harcourt, A.H., P.H. Harvey, S.G. Larson, and R.V. Short. “Testis Weight, Body Weight and Breeding System in Primates.” Nature 293, no. 3 (September 3, 1981): 55–57.

3. Schulte-Hostedde, Albrecht I., and John S. Millar. “Intraspecific Variation of Testis Size and Sperm Length in the Yellow-Pine Chipmunk (Tamias Amoenus): Implications for Sperm Competition and Reproductive Success.” Behavioral Ecology and Sociobiology 55, no. 3 (October 31, 2003): 272–77.

4. Preston, B. T., I. R. Stevenson, J. M. Pemberton, D. W. Coltman, and K. Wilson. “Overt and Covert Competition in a Promiscuous Mammal: The Importance of Weaponry and Testes Size to Male Reproductive Success.” Proceedings of the Royal Society of London B: Biological Sciences 270, no. 1515 (March 22, 2003): 633–40.

5. Mascaro, Jennifer S., Patrick D. Hackett, and James K. Rilling. “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 110, no. 39 (September 24, 2013): 15746–51.

Want to cite this post?

Purcell, R. (2015). Primordial soup to nuts: are some men naturally selected to be better dads? Retrieved on , from

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