• cohabitation;
  • cross-national;
  • life events and transitions;
  • marriage;
  • multilevel models;
  • well-being

The study aims to assess, first, whether there is a gap in well-being between unmarried cohabitants and the married, second, if selection factors can explain this so-called cohabitation gap, and third, if the size of the cohabitation gap differs across countries and how this can be explained. We use pooled data from young adults (18–44) in 3 rounds of the European Social Survey (N ≅ 31, 500). Multilevel regression analyses show that there is a moderate cohabitation gap that can be partly explained with the selection factors material resources and religiosity. Country differences were clear and could partly be explained with the level of institutionalization: In countries where cohabitation is more accepted and more prevalent, the cohabitation gap is smaller.