Anthropologist James Rilling and colleagues at Emory University in Atlanta, Georgia reported trial based evidence that more nurturing fathers have smaller testes in the Sept. 9, 2013, edition of the journal Proceedings of the National Academy of Sciences.
The objectives of the study were to find a physiological basis for nurturing behavior and to examine the understudied role of fathers in child nurturing. The experiment also tested the validity of an evolutionary concept that proposes there is a competition between mating and parenting in males.
The small study examined the size of the testes of 70 biological fathers of children between one and two years old that were living with the child’s biological mother and the brain response of the fathers when viewing photographs of their children as measured by functional magnetic resonance imaging. The children’s mothers were questioned privately about the father’s role in child care activities as a correlation. The men’s testosterone levels wee also measured.
Smaller sized testes were highly correlated with more nurturing child care behavior in men. Lower testosterone levels wee noted in more nurturing fathers. Higher brain activity in the ventral tegmental area of the brain correlated with lower size in testes and more nurturing behavior in men.
The researchers conclude that testes size may be a function of more nurturing behavior that causes testes to shrink. High testosterone levels were correlated with lower levels of nurturing behavior in fathers.
Testes size was not all there is to being a nurturing father according to the researchers.