A Pound of Flesh or Just Proxy? Using Twin Differences to Estimate the Effect of Birth Weight on Life Chances

Recent research into the implications of low birth weight may be plagued by unobserved variable bias. It is unclear whether the later-life consequences found to be associated with low birth weight are a true effect of poundage' at birth, or whether this association results from underlying factors related to birth weight such as genetics, gestational age, pregnancy-related behavior, or prenatal environment. In this study, we employ twin comparisons to rule out such unobserved factors and to isolate more precise effects of birth weight on infant mortality. Using data from the 1995-1997 Matched Multiple Birth Database and deducing zygosity based on the sex ratio of twin births, we examine the effects of birth weight for both fraternal and identical twins on both neonatal and post-neonatal mortality. Results suggest that in the neonatal period, low birth weight may partially be acting as a proxy for underlying genetic conditions, but in the post-neonatal period birth weight per se increases the risk of mortality. Thus, it appears that after an initial weeding-out' period in which the more severe ailments associated with genetics may be behind birth weight effects, poundage' itself has a significant impact on life chances net of genes and other pregnancy-specific health or social conditions.