Testes size correlates with men's involvement in toddler care

By Carol Clark | eScienceCommons | Sep. 12, 2013

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“Mothers definitely have more of an impact on child development, but fathers are also important and their role is understudied,” says anthropologist James Rilling.

Men with smaller testes than others are more likely to be involved in hands-on care of their toddlers, finds a new study by anthropologists at Emory University. The Proceedings of the National Academy of Sciences (PNAS) published the results of the study on Sept. 9.

Smaller testicular volumes also correlate with more nurturing-related brain activity in fathers as they are looking at photos of their own children, the study shows. "Our data suggest that the biology of human males reflects a trade-off between investments in mating versus parenting effort," says Emory anthropologist James Rilling, whose lab conducted the research.

The goal of the research is to determine why some fathers invest more energy in parenting than others. "It's an important question," Rilling says, "because previous studies have shown that children with more involved fathers have better social, psychological and educational outcomes."

Evolutionary Life History Theory posits that evolution optimizes the allocation of resources toward either mating or parenting so as to maximize fitness. "Our study is the first to investigate whether human anatomy and brain function explain this variance in parenting effort," says Jennifer Mascaro, who led the study as a post-doctoral fellow in the Rilling lab.

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