Lower birth weight babies have worse outcomes, both short-run in terms of one-year mortality rates and longer run in terms of educational attainment and earnings. However, recent research has called into question whether birth weight itself is important or whether it simply reflects other hard-to-measure characteristics. By applying within twin techniques using an unusually rich dataset from Norway, we examine the effects of birth weight on both short-run and long-run outcomes for the same cohorts. We find that birth weight does matter; despite short-run twin fixed effects estimates that are much smaller than OLS estimates, the effects on longer-run outcomes such as adult height, IQ, earnings, and education are significant and similar in magnitude to OLS estimates.
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