In President Obama's and Romney's cases, fathers -- present or absent -- are a large part of that cloth.

CNN: STORY HIGHLIGHTS

The positive spin: Most experts, myself included, agree that it's better for a child to have an absent parent than a parent who's present but neglectful -- or worse.

CNN: Why there are more walk-away moms

The twin rule of pathology states that any heritable disease will be more concordant (that is, more likely to be jointly present or absent) in identical twins than in non-identical twins and, in turn, will be more concordant in non-identical twins than in non-siblings.

ECONOMIST: Twin studies, genetics and the environment