Prenatal smoking and early childhood conduct problems: testing genetic and environmental explanations of the association.

BACKGROUND Extensive evidence now supports a statistical association between prenatal smoking and increased risk for antisocial outcomes in offspring. Though this statistical link may signal a causal association, commentators have urged caution in interpreting findings because of the likelihood of confounding. METHODS We used data from the Environmental Risk Longitudinal Twin Study, a representative British sample of 1116 twin pairs studied at ages 5 and 7 years, to assess associations between prenatal smoking and early childhood conduct problems net of the effects of both heritable and environmental risks for child antisocial outcomes. RESULTS Prenatal smoking showed a strong, dose-response relationship with child conduct problems at ages 5 and 7 years. Around half of this association was attributable to correlated genetic effects. Mothers who smoked during pregnancy differed from other mothers in a number of ways. They were more likely to be antisocial, had children with more antisocial men, were bringing up their children in more disadvantaged circumstances, and were more likely to have had depression. Controlling for antisocial behavior in both parents, depression in mothers, family disadvantage, and genetic influences, estimates for the effects of prenatal smoking were reduced by between 75% and the entire initial effects. CONCLUSIONS Observed associations between prenatal smoking and childhood conduct problems are likely to be heavily confounded with other known risks for children's behavioral development. As a result, tests of any causal influence of prenatal smoking must await findings from experimental studies.