This study was an attempt to do better than correlation by following the same people over time (“longitudinal” rather than “cross-sectional”). In studying marriage, it is not possible to do the kind of study that is the gold standard for assessing causation: randomly assigning people to different marital statuses. http://www.belladepaulo.com/2018/07/whats-wrong-studies-claims-supposed-benefits-marriage/#more-3658