Men who have smaller testes are more likely be better dads to their toddlers

Researchers found that fathers who had smaller testes showed increased nurturing-related brain activity in the ventral tegmental area (VTA) - an area of the brain linked to reward and parental motivation - when viewing photos of their own children. The results suggest that the biology of human males reflects a trade-off between mating effort and parenting effort, as indexed by testicular size and nurturing-related brain function, respectively.