Dad bottle feeding When a man is actively engaged in the life of his child, the child is more likely to be better adjusted socially, psychologically, and academically than the children of emotionally distant fathers. Previous studies indicate a man’s fathering style is based on his level of testosterone, with distant fathers having higher levels of the male sex hormone than their more nurturing counterparts. A new study from Emory University in Georgia suggests the size of a man’s testicles is a tell-tale sign of fathering style, too.

Emory University anthropologist James Rilling found that the most nurturing fathers of toddlers had smaller testicles than other men. His study was a multi-part examination of fathering style and biology.

The Fathers

Seventy men and their families were enlisted for the study. Each man was the biological father of a child 1 or 2 years old. Each child lived in a family unit that included his or her biological father and biological mother.

The Questions

The fathers and mothers were interviewed to gauge the man’s participation in the daily life of his toddler. Each parent was asked about the father’s willingness to change diapers, bathe his child, and such. Questions included the father’s response to his sick child, too, including if he would stay home from work with a sick child and if he would take the child to the doctor.

The Brain Activity

The men underwent a functional magnetic resonance imaging (fMRI) scan to observe each father’s brain activity in a controlled situation. A standard MRI captures an image of internal organs in freeze-frame style; an fMRI is more like a video that shows movement, such as brain activity, in real time.

Each man was presented with photos of his child’s face as well as that of an unfamiliar child and an unfamiliar adult. All three faces, in multiple photos, were exhibiting expressions of happiness, sadness, and emotional neutrality.

The Measure

Testicle size was measured. Testosterone in the bloodstream can be measured but the Emory researchers wanted to measure testicle volume, too. The count and quality of sperm relates more closely to testicular volume than testosterone and the researchers wanted to see if size matters when it comes to hands-on fatherhood.

When the men with smaller testicles viewed photos of their own child, an area of their brain — the ventral tegmental area (VTA) — exhibited a higher level of activity than the brains of men with larger testicles. The VTA is part of the brain’s network that process reward and motivation.  The faces of strangers had no effect, regardless of testicle size. The study indicates men with lower levels of testosterone, and therefore smaller testes, make for more nurturing fathers.

Source: Rilling, James K, et al. “Testicular volume is inversely correlated with nurturing-related brain activity in human fathers.” PNAS. National Academy of Sciences. Sep 9, 2013. Web. Jun 8, 2014.