Why are some fathers willing to change diapers, prepare meals, and give the kids a bath, while others skip out on these essential parenting duties?

In a new study, researchers from Emory University in Atlanta sought to understand the variation that occurs among fathers.

“We’re interested in trying to identify the variables that determine whether or not men choose to be involved as fathers, because that’s so important for child development,” says associate professor James Rilling, co-author of the study.

According to the U.S. Census Bureau, there has a been a sharp increase in the number of single-parent families (most often headed by mothers), reaching almost 30 percent of all households with children in 2008. This shift in family structure has been linked by the  (CDC) to negative social, psychological, and educational consequences for children with uninvolved (or absentee) fathers.

Testes Size and Testosterone Linked to Paternal Caregiving

In the study, published today in the journal Proceedings of the National Academy of Sciences, researchers recruited 70 biological fathers of children ages 1 to 2.

Using blood tests, MRI scans of the testes, and feedback from the children's mothers, researchers found that “men with smaller testes, and men with lower testosterone levels, were more involved in the day-to-day caregiving of the child,” says Rilling.

These results are in line with a branch of evolutionary theory called the "Life History Theory". The theory posits that when it comes to reproduction, organisms have a finite amount of energy to invest in either mating or parenting strategies.

Because it's difficult to gather accurate information on a person’s sexual behavior, researchers instead measured “testicular volume as a proxy for investment in sperm production,” says Jennifer Mascaro, Ph.D., the lead author of the study.

Previous studies have also looked at the connection between parenting behavior and levels of testosterone—a hormone that influences potential mating traits such as muscle mass, body hair, and deepness of the voice.

By measuring testes size, however, Rilling and his colleagues were able to tease apart the effects of testosterone and sperm production on parenting. Their results showed that both traits were independently associated with a father’s level of caregiving.

Brain Imaging Supports Link Between Testes Size and Parenting

Another novel aspect of the study was a brain imaging test, which showed connections between brain activity, testes size, and parenting effort.

“Men with smaller testes had a stronger brain response to viewing pictures of their own children within an area of the brain that is involved in both reward and parental motivation,” says Rilling.

This midbrain area—called the ventral tegmental area (VTA)—is connected to the ventral striatum, an integral part of the brain’s reward system.

“We think that when these men [with smaller testes] look at their children, they find them appealing and rewarding,” says Rilling, “and that is what motivates them to interact with them and care for them.”

Testes Size and Parenting May Influence Each Other

Based on the research done to date, it’s not possible to tell whether having smaller testes is driving an increase in paternal caregiving, or vice versa.

Other studies have shown that testosterone levels drop after men have children, so it could be that men’s testes shrink as they become more involved fathers.

“We’d really like to do a follow-up study where we measure men’s testes before and after they have children,” says Rilling, “and see if there is a change, and if that change is related to how involved they become as caregivers.”

Testes Size Not the Only Factor

Mascaro emphasized that, while the study found a connection between testicle size and parenting, other social, historical, and cultural factors are also important.

“There are men who are able to overcome [biological] predispositions,” says Mascaro, “so understanding neural systems and other factors that might moderate this effect will be really important.”

The researchers are already planning a study with oxytocin—a hormone that plays a role in bonding and trust—to see if it can enhance the response of the brain’s reward system when men look at pictures of their own children.

Using oxytocin “might be a reasonable treatment,” says Rilling, “to try to enhance the quality of the bond that [fathers] form with the child, and increase their motivation to be an involved caregiver.”

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