Men with smaller testicles are better fathers, researchers say

Results revealed that both testosterone levels and testes size were inversely correlated with the reported amount of direct paternal care.

Anthropologists at Emory University found that men with smaller testes were more likely to be directly involved in the care of their toddlers. Smaller testes also correlated with more nurturing-related brain activity.

Anthropologist James Rilling suggests that the data is indicative of a trade-off between mating investments versus parenting effort. The goal of the research was to determine the underlying reason behind fathers’ energy investment in parenting. An important question, given that previous studies have demonstrated the social, psychological and educational benefits of children with more involved fathers.

According to the Evolutionary Life History Theory, evolution maximizes fitness by allocating resources towards either mating or parenting. This is the first study to investigate whether human anatomy and brain function can explain variances in parenting effort.

There are likely many economic, social and cultural factors that also influence the level of care-giving, but the researchers wanted to focus on possible biological links. The research was based on the correlations between lower testosterone levels and greater paternal involvement, as well as higher testosterone levels and divorce or polygamy.

In addition to producing testosterone, the testes also produce sperm, of which volume correlates highly with sperm count and quality.

Seventy biological fathers were included in the study. All of the fathers had children between the ages of 1 and 2, and were living with the child and its biological mother.

Both mothers and fathers were separately interviewed regarding the father’s involvement in childcare, which included such tasks as changing diapers and taking the child to the doctor.

The men were then measured for testosterone levels, and underwent functional magnetic resonance imaging (fMRI) to measure their brain activity as they viewed photos of their own child with different expressions, as well as similar photos of an unknown child and an unknown adult. Structural MRI was then used to measure testicular volume.

Results revealed that both testosterone levels and testes size were inversely correlated with the reported amount of direct paternal care.

Testes volume was found to correlate with activity in the part of the brain system associated with reward and parental motivation. Looking at photos of their own child, men with smaller testes experienced a greater activation in this region of the brain.

Although statistically significant, the correlation between testes size and caregiving was not perfect.

According to post-doctoral fellow Jennifer Mascaro, who led the study, the fact that the team found this variance suggests personal choice. Key questions that remain to be answered are the direction of casualty and whether childhood environment can affect testes size.