“Family structure and parental relationship status are not randomly assigned – that
is, they aren’t designed for easy study. Even if researchers take steps to statistically
account for all the things that can be observed in the data that might be confounding
factors, there is still the possibility that there is something unobserved about single
parents, something that researchers can’t see in the data, that would make them
less well-equipped to be parents even if they were married, such that their children’s
outcomes would still be inferior to those of the children of married parents.”