Men with smaller testicles are more likely to be involved with the care of their toddlers, according to a recent study conducted by Emory anthropologists.

The study was authored by Postdoctoral Fellow Jennifer Mascaro, Project Coordinator Patrick Hackett and Laboratory for Darwinian Neuroscience Director James Rilling of the University’s Laboratory for Darwinian Neuroscience.

The report added that the data the researchers collected indicated a correlation between larger testes size and decreased paternal involvement in direct caregiving.

The findings support a hypothesis derived from a branch of evolutionary theory known as Life History Theory, which says that the more a male has sex, the less investment is made in parenting, Rilling wrote in an email to the Wheel.

“We use testes size as an indirect measure of mating effort for three reasons,” Rilling wrote. “It is difficult to get accurate information by simply asking them, men with larger testes produce more sperm and data shows those males tend to mate more.”

Rilling added that previous studies had shown that children with more involved fathers have better social, psychological and educational outcomes.

The study is not the first of its kind but is unique because it is “the first to investigative whether anatomy and brain function explain parent involvement,” Rilling wrote.

Rilling explained the study included 70 biological fathers who had a child between the ages of one and three and who lived with both the child and the biological mother.

During the course of several sessions, the men participated in the study in many ways, including interviews of both parents and providing information about hands-on tasks regarding child care such as changing diapers, feeding and bathing a child, staying home to care for a sick child and taking the child to the doctor.

Researchers took multiple measurements after the interviews, including testosterone levels, functional magnetic resonance imaging (fMRI) and structural MRI to measure testicular volume, according to Rilling.

The study does acknowledge that many factors – such as economic, social and cultural status – tie into fatherhood as well as their findings regarding testes.

“It is important to emphasize that although testes size explains significant variation in paternal caregiving, the correlation is far from perfect,” Rilling wrote. “Some men with large testes are highly-involved parents, and this is because there are many other influences on paternal caregiving.”

Rilling is concerned about this potential misrepresentation in press coverage of the study and said that the lab does not endorse certain statements, such as “men with large testes are bad fathers,” that he has seen as summaries of the study.

For those worried about sizing up to the competition, “Manhood should not be measured by the size of one’s testes,” Rilling wrote. “It is not something to feel bad about.”

Rilling and the Darwinian Neuroscience Lab remain curious about other connections to parenting behavior, he wrote.

“My suspicion is that by actually measuring fathers’ parenting behavior, rather than using questionnaires as we did, we might identify stronger correlations between testes size and parental behaviors,” Rilling wrote. “We must ask questions to identify variables that help explain why some men are more involved fathers than others.”

College senior Ben Sollenberger said he was unimpressed with the study.

“[This study] is not reinventing the wheel by any means,” Sollenberger said, adding that less testosterone marks a shift toward maternal instincts.

– By Stephen Fowler