Australasian Science: Australia's authority on science since 1938

Bad dads have big balls

Men’s testes size negatively tied to parenting involvement.

In a study examining potential trade-offs between mating and parenting, researchers found that men with smaller testes were more likely than others to be involved in child care.

An evolutionary hypothesis called Life History Theory holds that mating and parenting compete for the limited energy animals expend on reproduction. Previous studies have suggested that decreases in testosterone may suppress mating efforts, potentially channeling resources toward paternal care of infants.

James Rilling of Emory University and colleagues investigated the link between testes size and parenting investment among men, given that testes volume is associated with sperm production and testosterone levels.

The authors recruited 70 biological fathers in and around Atlanta who were 21-43 years of age and had 1-2-year-old children, and used MRI to measure the men’s testes volumes and monitor brain activity in a region implicated in parental motivation, while the men viewed photographs of their own child, a stranger’s child, or an adult stranger.

The men’s partners also answered a parenting questionnaire that queried the fathers’ involvement in tasks such as taking children to health care visits and attending to children at night.

The authors report that fathers’ testes size was negatively tied to nurturing-related brain activity triggered while they viewed pictures of their own child. In particular, fathers who were more responsive than others to their child’s emotional faces tended to have smaller testes and be more involved in infant care.

According to the authors, the link between testes size and parenting-related brain activity suggests a trade-off between mating and parenting.

PNAS