The great question is why? Most of the factors are similar between the two couples: living together, in a committed relationship, have children, sharing expenses, sharing wealth, etc. However, for some reason because married mom and dad have a certificate of marriage their children are at a lower risk for having negative outcomes? It is an interesting phenomenon.
I recall once having a University professor who called those who cohabit a different breed of human relationship expression compared to married couples. It could be that cohabiting couples have a different way of expressing and showing love.
Now I am not at all saying cohabiting parents are terrible and married couples are exemplary. Of course cohabiting couples can raise fantastic children, and married couples can corrupt theirs.
