Do Twin Studies Underestimate the Cost of Low Birth Weight?

Previous studies have used twin designs to estimate the effect of birth weight on infant health. Twin designs allow for plausibly causal estimates that are not confounded by maternal characteristics, such as smoking behavior or socioeconomic status. It is not clear that the costs of low birth weight for twins are the same as for singletons, since the optimal birth weight for twins might be lower. I provide causal estimates of the effect of birth weight using data from the universe of twins, triplets, and quadruplets born between 1995-2000 in the United States. Infants from higher-order multiple births have lower birth weights, but experience smaller reductions in infant health when they are low birth weight. A statistical value of life calculation suggests that twin estimates underestimate the benefits from increasing birth weights by $8 billion per year.