"Because we now have the acceptance of long-term cohabitation, people who go into marriage and stay in marriage are a more homogenous group. They are people who believe in certain things that contribute to stability. So the selection effect is really important. Yes, it's true that married couples on average stay together longer than cohabiting couples. But cohabitation is such an unhelpful word because it covers a whole ragbag of relationships, so it's not really comparable. We're better off talking about formal and informal marriages: those that have legal certificates, and those that don't. Is there any difference between a formal and informal marriage? If we really compare like with like, I'm not sure you'd see much difference." – Penny Mansfield