Unexpectedly expected behaviors

Can't decide whether I'm surprised to read this recent study finding: that couples who are living together do a better job of dividing housework than those who are married.

"Marriage as an institution seems to have a traditionalizing effect on couples--even couples who see men and women as equal," says Davis.

Would like to know more about the comparability of the two pools -- i.e., perhaps those who choose to marry already have more "traditional" views that lead them in that direction than do the cohabiting couples, rather than being changed after the fact. It would take a prospective study (one that started with, say, serious dating couples, without knowing which would end up married) to tease that out, which means we're unlikely to know the answer any time soon...