Norway that allows us to precisely measure birth order effects on IQ using both cross-sectional and within-family methods … explains about 3% of the within-family variance of IQ. When we control for birth endowments, the estimated birth order effects … birth order effects occur because later-born children are more affected by family breakdown. …

Norway and focuses on one family characteristic: the effect of family size on IQ. Because of the endogeneity of family size …, we instrument for family size using twin births and sex composition. IV estimates using sex composition as an instrument … show no negative effect of family size; however, IV estimates using twins imply that family size has a negative effect on …

Norway that allows us to precisely measure birth order effects on IQ using both cross-sectional and within-family methods … explains about 3% of the within-family variance of IQ. When we control for birth endowments, the estimated birth order effects … birth order effects occur because later-born children are more affected by family breakdown. …