According to a threesome at Emory University in Atlanta, it may very well have to do with the size of the father's testicles. And it appears that when it comes to nurturing or caring for children, and the family jewels, less is more. (More proof that good things come in small packages?)
How did the intrepid researchers figure this out? In a nutshell, they scanned "the brains and berries" of 70 male volunteers while showing them pictures of their child. And the researchers found that the part or parts of the brain believed to be responsible for nurturing were more active in the fathers with less cojones.
[Quick aside: Anyone else amused and amazed by the number of synonyms for testicles?]
While the findings are not conclusive, they are consistent with observations of other primates, specifically chimpanzees (larger testicles, not good caregivers) and gorillas (smaller testicles, attentive fathers).
So ladies, when sizing up potential candidates to be the father of your child, "choose dads with smaller 'nads." (Lower testosterone levels also make for a more nurturing parent.)
Btw, you can read the study, titled "Testicular volume is inversely correlated with nurturing-related brain activity in human fathers," here.