Symptoms of depression are more common among the single and divorced than among married couples, says American sociologist Patrick McKenry, who analyzed surveys from 5991 people aged between 19 and 80. Tying the knot provides a "protection effect." And men and women living together but unmarried score the same on a depression scale as single people.

Why doesn't cohabitation grant the same benefits that marriage vows do? McKenry speculates that unmarried couples may lack the institutional and familial support, and the commitment that gives married couples a greater sense of well-being.