This paper examines whether parental marriage confers educational advantages to children relative to cohabitation. We exploit a dramatic marriage boom in Sweden in late 1989 created by a reform of the Widow’s Pension System that raised the attractiveness of marriage compared to cohabitation to identify the effect of marriage. Sweden’s rich administrative data sources enable us to identify the children who were affected by parental marriage due to this marriage boom. Our analysis addresses the policy relevant question whether marginal marriages created by a policy initiative have an impact on children. Using grade point average at age 16 as the outcome variable, we first confirm the expected pattern that children with married parents do better than children with cohabiting parents. However, once we control for observable family background, or use instrumental-variables estimation to compare the outcomes for those children whose parents married due to the reform with those children whose parents remained unmarried, the differences disappeared. A supplementary sibling difference analysis also supports the conclusion that the differentials among children of married and cohabiting parents reflect selection rather than causation.