Testing Causal Effects of Maternal Smoking During Pregnancy on Offspring's Externalizing and Internalizing Behavior

Abstract

Maternal smoking during pregnancy (SDP) is associated with increased risk of externalizing and internalizing behaviors in offspring. Two explanations (not mutually exclusive) for this association are direct causal effects of maternal SDP and the effects of genetic and environmental factors common to parents and offspring which increase smoking as well as problem behaviors. Here, we examined the associations between parental SDP and mother rated offspring externalizing and internalizing behaviors (rated by the Child Behavior Checklist/2-3) at age three in a population-based sample of Dutch twins (N = 15,228 pairs). First, as a greater effect of maternal than of paternal SDP is consistent with a causal effect of maternal SDP, we compared the effects of maternal and paternal SDP. Second, as a beneficial effect of quitting smoking before pregnancy is consistent with the causal effect, we compared the effects of SDP in mothers who quit smoking before pregnancy, and mothers who continued to smoke during pregnancy. All mothers were established smokers before their pregnancy. The results indicated a greater effect of maternal SDP, compared to paternal SDP, for externalizing, aggression, overactive and withdrawn behavior. Quitting smoking was associated with less externalizing, overactive behavior, aggression, and oppositional behavior, but had no effect on internalizing, anxious depression, or withdrawn behavior. We conclude that these results are consistent with a causal, but small, effect of smoking on externalizing problems at age 3. The results do not support a causal effect of maternal SDP on internalizing behaviors.

Keywords: Causality; Childhood behavioral problems; Parental prenatal smoking; Pleiotropic effects.

Fig. 1

Path diagram of the model. Ph1 (ph2) is the phenotype as observed in twin 1 (twin 2). The variables z_f and z_m are the standardized smoking variables in father and mother, respectively. The residuals (e1, e2) are subject to a ACE decomposition, to account for the residual covariance (in the analysis of overactiveness and anxious depression, we fitted an ADE model). As explained in the text, the parameter b₂ of main interest, as b₂ > 0 implies that maternal SDP has a greater influence than paternal SDP, i.e., consistent with a direct causal effect of SDP on the phenotype