"The Downside of Cohabiting Before Marriage"

Surprisingly that's the title of what currently shows up on the New York Times homepage as most emailed, and second most viewed:

Researchers originally attributed the cohabitation effect to selection, or the idea that cohabitors were less conventional about marriage and thus more open to divorce. As cohabitation has become a norm, however, studies have shown that the effect is not entirely explained by individual characteristics like religion, education or politics. Research suggests that at least some of the risks may lie in cohabitation itself.