Extradyadic sex and union dissolution among young adults in opposite-sex married and cohabiting unions

•Among married and cohabiting young adults, 1 in 4 report extradyadic sex (EDS).•Men self-report EDS more than women, women report partner's EDS more than men.•Partner's EDS is associated with union dissolution, respondent's own EDS is not.•Links between EDS and union dissolution do not vary for cohabiters and spouses.•Links between EDS and union dissolution do not vary by gender.

This study investigates extradyadic sex (EDS) among contemporary opposite-sex married and cohabiting young adults and examines how EDS is associated with union dissolution. By analyzing data from 8301 opposite-sex spouses and cohabiters in the National Longitudinal Study of Adolescent to Adult Health, we estimate the prevalence of self-reported EDS, reports of partners' EDS, and reports of mutual EDS (i.e., both partners' engagement in EDS). Roughly 1 in 4 respondents reported that either they, their partner or both engaged in EDS. Young men were more likely than women to self-report EDS, while young women were more likely to report partners' EDS. Relative to no EDS, partners' EDS was associated with union dissolution, but self-reported EDS and mutual EDS were not. A partner's EDS was also associated with union dissolution relative to self-reported EDS. Associations between a partner's EDS and dissolution were consistent among spouses and cohabiters and among men and women.