Selection and the Effect of Prenatal Smoking
- Presenter:
Chair: Phil Cook; Discussant: TBA Mon June 5, 2006 15:30-17:00 Room 213
Author. Angela Fertig (afertig@uga.edu). College of Public Health and Carl Vinson Institute of Government, University of Georgia.
Title: Selection and the Effect of Prenatal Smoking
Rationale: A large number of studies find 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.
Objectives: The objective of this paper is to examine the importance of selection on the effect of prenatal smoking by comparing the effect sizes across groups whose selection varies.
Methodology: I use three British birth cohorts where the mothers’ knowledge about the harms of prenatal smoking varied substantially. Specifically, little was known about the harms of smoking prior to the 1964 U.S. Surgeon General’s Report, nor the particular detrimental relationship between prenatal smoking and birth outcomes until the 1969 Surgeon General’s Report. Thus, mothers who smoked during pregnancies that occurred prior to these reports were a less select group than mothers who smoked during pregnancies following them. I compare the effect sizes across three British birth cohorts - those children born in 1958, 1970, and 2000 - and expect that the effect is larger for the latter two cohorts than for the earliest cohort if selection plays an important part in the measured effect.
Results: I find 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.
Conclusions: If adverse selection is responsible for the increase in effect size, the implication is that policy interventions to discourage smoking among pregnant women may not prevent as many unhealthy births as usually reported. At the same time, the evidence presented here is further confirmation that prenatal smoking has a causal effect on birth weight conditional on gestation since I find that there is a significant effect in the absence of adverse selection.
