In this paper, we use linked census data from England and Wales to investigate whether having a large number of siblings leads to lower educational attainment. There is a large literature suggesting that with large sibship size, parental resources will be diluted and this, in turn, will lead to lower educational attainment. Using twin births and the sex composition of the sibling group as instrumental variables, we find that the evidence of a family size effect on educational attainment is rather uncertain. Similar results are obtained when we use occupational attainment as the dependent variable. We also demonstrate the confounding of birth order and family size effects, and show that an adjusted birth order index proposed by Booth and Kee provides an effective solution to this estimation problem.
Keywords: educational attainment; family size; instrumental variables; linked census; twin study.