There’s a hypothesis within evolution that is referred to as the ‘life history theory’. Briefly, an organism only has a finite amount of energy to expend toward its ultimate goal: reproduction. A creature that grows an enormous reproductive display may attract more mates, but it will produce smaller litters. A mother can abandon a thousand babies to the ocean currents, but she can only nurture a handful at a time. Each strategy is a trade-off.
Human males also exhibit a range of parenting and mating strategies. To put it bluntly, how much of their energy do they spend trying to get the girl and how much caring for the resulting children? The answer may lie in their hormones and in their testicles. Males (human and other species) with high testosterone have greater mating success. However, they also spend less time caring for their offspring. The authors found that there is another, independent, correlation between nurturing and testicle size.
In many species, testicle size is inversely proportional to the number of partners the females of the species are likely to mate with. That is, males from species where females are largely monogamous have small testes, whereas males from species where females have multiple partners have large testes. In case you’re wondering, among primates, humans have moderately sized testes. We’re socially monogamous but prone to cheating.
There’s a part of the brain called the ‘ventral tegmental area’ (VTA). As part of the brain’s reward circuitry, the VTA is implicated in addiction, but also in strong emotional feelings, such as when one is holding one’s child.
Mascaro JS, Hackett PD, & Rilling JK (2013). Testicular volume is inversely correlated with nurturing-related brain activity in human fathers. Proceedings of the National Academy of Sciences of the United States of America PMID: 24019499.