Selection and the Effect of Prenatal Smoking

Angela R. Fertig, Indiana University

A large number of studies finds that smoking during pregnancy is associated with poor infant health outcomes. However, there is an on-going debate about the extent to which this association is causal. It is difficult to determine whether these poor health outcomes are the result of prenatal smoking or are also attributable to characteristics of the mother which are correlated with prenatal smoking. I examine the importance of selection on the effect of prenatal smoking by comparing the effect sizes across groups whose selection varies. Specifically, I use three British birth cohorts where the mothers’ knowledge about the harms of prenatal smoking varied substantially. Preliminary evidence indicates that the effect of prenatal smoking on low birth weight for gestational age among children born in 2000 is twice that of children born in 1958, implying that selection plays an important part in the association between smoking and birth outcomes.