Family Size, Birth Order, and IQ

Sandra Black, University of California, Los Angeles
Paul Devereux, University College Dublin
Kjell Salvanes, Norwegian School of Economics and Business Administration (NHH)

It is clear that birth order affects children’s outcomes along a number of dimensions, including education and earnings, although recent psychology literature provides mixed evidence of the effects of birth order on IQ. The evidence on the effect of family size in the economics literature is even more mixed, with inconclusive results on all outcomes. This paper uses a large dataset on the population of Norway and focuses on the effect of birth order and family size on IQ, an outcome not previously available in datasets of this magnitude. Because of the endogeneity of family size, we instrument for family size using twin births. We find a strong and significant effect of both birth order and family size on IQ.