Using multiple births as an exogenous shift in family size, I investigate the impact of the number of children on child investment and child well-being. Using data from the 1980 US Census Five-Percent Public Use Micro Sample, 2SLS results demonstrate that parents facing a change in family size reallocate resources in a way consistent with Becker's Quantity & Quality model. A larger family generated by twins in a later birth reduces the likelihood that older children attend private school, reduces the mother's labor force participation, and increases the likelihood that parents divorce. The impact of family size on a measure of child outcome, such as grade retention, is less clear. The results indicate that for both measures of child investment and child well-being, the 2SLS estimates are statistically distinguishable from OLS estimates, indicating an omitted variables bias in the single equation model. (Contains 7 tables and 13 footnotes.)