It is more plausible that better-adjusted individuals are more likely to get married than the social causation hypothesis (marriage causes improvement in mental & physical health)

Huntington, C., Stanley, S. M., Doss, B. D., & Rhoades, G. K. (2022). Happy, healthy, and wedded? How the transition to marriage affects mental and physical health. Journal of Family Psychology, 36(4), 608–617. https://doi.org/10.1037/fam0000913

Abstract> Decades of research have documented the apparent health benefits of marriage, but the dynamics of how health may change across the transition to marriage are not fully understood. In two studies, we compared being unmarried or married on several indices of mental and physical health. In Study 1, we used a national sample of 1,078 individuals in different-sex relationships who completed surveys by mail. Compared with those who were cohabiting or dating, married individuals generally reported better mental and physical health than those in less committed relationships, and most differences remained when controlling for putative selection factors. Study 2 used longitudinal data from the participants in the Study 1 sample who later married (N = 168) to study changes within individuals over the transition to marriage on the same indicators. Six waves of mailed surveys spanning 20 months were employed. Findings of Study 2 indicated that although some indicators of mental and physical health were improving up until the point of marriage, these indicators then stabilized or began to decline, with women experiencing these declines more than men. Findings are more consistent with selection effects (i.e., better-adjusted individuals are more likely to get married) than social causation effects (i.e., marriage causes improvements in mental and physical health) and suggest that if marriage does have a causal effect on well-being in the short term, it may actually manifest in the lead-up to the wedding. Implications of these findings are discussed.