The role played by family background in educational opportunity is a central focus in the sociology of education and stratification. Scholars have long recognized the importance of families in determining their children's educational success and, thus, in reproducing status over the generations. In the United States, children from single-parent families, children with many siblings, and children with parents of lower socioeconomic status (SES) tend to have lower educational achievement and attainment because of disadvantages in the quality and quantity of household resources. Although scholarship on this topic has attained a high level of conceptual and technical sophistication, fewer researchers have examined the extent to which U.S.-based findings on family structure also apply in other societies. There is now growing interest--but still too little evidence--about systematic, cross-national variations in the association between family structure and education. In this article, using data from the Program for International Student Assessment (PISA) across 24 Western industrialized countries, the author investigates reasons for variations in the role played by the number of siblings (sibship size) in determining educational achievement. He examines the relevance of welfare-state theories to educational stratification, and he assesses the possibility that generous family welfare provisions may attenuate the strong linkage between sibship size and educational outcomes that is commonly found in the United States. Building on recent comparative research into the varied effects of single parenthood on educational performance, the author hypothesizes that sibship-size effects similarly vary across modern welfare states because distinct welfare regimes differ in their provision of family benefits and services and in their attempts to equalize opportunities across families. (Contains 6 tables and 21 footnotes.)