This article examines the extent to which the plans of young adults concerning the choice between cohabitation and marriage are influenced by a rational evaluation of the differences between marriage and cohabitation and by the perceived opinions of significant others, and the extent to which these evaluations mediate the effects of young adults' family backgrounds and current social positions. Among a representative sample of young adults in the Netherlands, it is found that evaluations and perceived opinions are quite strong predictors of union formation intentions. However, perceived opinions of significant others are better predictors than evaluations of the inherent properties of cohabitation and marriage. Furthermore, the effects of family background and social status variables on union formation intentions are almost completely mediated by these subjective evaluations. The implications for the discussion of whether union formation choices are rational decisions or based on cultural scripts are discussed.