By: Lamb, Kathleen A., Lee, Gary R., and Alfred DeMaris.

These researchers study data from the National Survey of Families and Households and find after controlling for demographic and socioeconomic factors that the only relationship structure that leads to a decrease in the reported depression levels of the partners relative to those who are single is marrying without cohabiting.

Union formation and depression: Selection and relationship effects. Journal of Marriage and Family 65 (4): 953-62.