The associations between fertility and outcomes in the family and society have been treated as causal, but this is inaccurate if fertility is a choice coordinated by families with other life-cycle decisions, including labour supply of mothers and children, child human capital, and savings. Estimating how exogenous changes in fertility that are uncorrelated with preferences or constraints affect others depends on our specifying a valid instrumental variable for fertility. Twins have served as such an instrument and confirm that the cross-effects of fertility estimated on the basis of this instrument are smaller in absolute value than their associations.