This article addresses why the literature on Inequality of Educational Opportunity (IEO) reaches diverging results concerning the decline or persistency of IEO over time. The main argument in this article is that the diverging results may be caused by the fact that the social class variables used to capture trends in IEO act as proxies for unobserved family-background influences that are substantively different from social class. The article analyses extremely rich longitudinal data from Denmark spanning three generations within the same family lineage. It demonstrates, first, that the effect of social class on secondary schooling is overstated when other family influences, conceptualised as economic, cultural and social capital, and unobserved family influences are not taken into consideration, and second, as in the other Scandinavian countries, that IEO has declined significantly in the postwar period.