Conduct disorder represents an issue of significant social, clinical and practice concern, with evidence highlighting increasing rates of child conduct problems internationally.1, 2
Identifying risk factors and understanding mechanisms by which these risk factors influence conduct problems has important implications for future intervention and prevention efforts.
Maternal smoking during pregnancy is known to be a risk factor for offspring psychological problems, including attention deficits and conduct problems.3, 4
Plausible biological mechanisms have been proposed to explain the prenatal effect of nicotine on neurodevelopmental processes in animals;5–7
however, the underlying mechanisms specific to smoking in humans are not well understood.3, 8
It has been suggested that anorexigenic, hypoxic, vascular and placental effects of nicotine may have direct teratogenic influences on the fetus and result in adverse physiological and psychological development.9
Longitudinal epidemiological studies have reported statistical associations between the extent of maternal smoking during pregnancy and subsequent offspring conduct disorder,10–14
attention deficit hyperactivity disorder,15, 16
and criminal behaviour.17, 18
Some studies have provided evidence of a dose-response relationship between the amount of cigarettes smoked during pregnancy and the rate of subsequent conduct problems in offspring.19
However, it is important to note that the effect of maternal pregnancy smoking on offspring conduct problems can be confounded by a number of background factors, including low socioeconomic status, early age of pregnancy, race, history of maternal psychopathology, and child-rearing environment.11, 13, 20–23
For example, mothers who smoke during pregnancy are more likely to provide a child-rearing environment that promotes or at least condones externalizing behavior.21
Therefore, the postnatal environment (independent of pregnancy smoking) may influence the development of conduct problems. A number of studies have found that the association between maternal smoking during pregnancy and offspring conduct problems persists after accounting for these possible confounders, while others have failed to demonstrate the association when confounders were considered.21, 24
Another problem with correlational family-based studies is the possibility of genetic risk factors and unmeasured environmental factors confounding the relationship between maternal smoking during pregnancy and offspring conduct problems.25
Both maternal smoking during pregnancy26
and conduct problems27, 28
are influenced by genetic factors that have been shown to overlap.29
Maternal smoking during pregnancy is associated with externalizing problems and forming partnerships with antisocial males.21, 30, 31
Moreover, adults with a history of externalizing behavior tend to provide postnatal environments that foster the transmission of this behavior across generations.32
Indeed, passive genotype-environment correlation (rGE) may be a factor in this association whereby genetic factors common to both the rearing environment (e.g., harsh parenting) and the specific phenotype considered (e.g., child conduct problems) underlie any observed association.33
Thus, maternal smoking during pregnancy could be a marker of a genetic liability, rather than a direct cause of children’s later conduct problems. Therefore, the association between maternal smoking during pregnancy and offspring conduct problems may be genetically, rather than environmentally, mediated.
Recent studies using genetically-sensitive designs have attempted to overcome this limitation of prior studies. Studies using sibling designs suggest that environmental variables influencing both pregnancy smoking and offspring conduct problems account for the observed associations.34–37
The IVF study design, where children are either genetically-related or -unrelated to the mother undergoing the pregnancy,38
and the children-of-twins study design,39
also suggest that unmeasured confounders that are indexed by inherited influences contribute to the link.
Much of the existing evidence has been obtained from studying biological parents rearing their biological children, which does not allow the effects of genetic from prenatal and postnatal environmental factors to be clearly disentangled, or for the role of passive rGE to be disentangled from genetic and postnatal environmental (e.g., parenting behavior) associations.
The present study focuses on examining links between prenatal smoking and offspring conduct problems and the contribution of psychosocial and inherited factors using data from three independent studies: the Christchurch Health and Development Study (CHDS) in New Zealand, the Early Growth and Development Study (EGDS) in the United States, and the Cardiff IVF (C-IVF) Study in the United Kingdom. In these three studies, data on pregnancy smoking and the behavioral outcomes have been gathered from:
- CHDS: 1088 children reared by genetically-related mothers and 36 children reared by genetically-unrelated adoptive mothers;
- EGDS: 310 children reared by genetically-unrelated adoptive mothers;
- C-IVF: 636 children reared by genetically-related mothers and 206 children reared by genetically-unrelated mothers.
This complement of genetically-sensitive research designs offers a number of advantages that allows advance of this important research question relative to past studies (see ). First, it allows examination of associations between maternal smoking and conduct problems in children who are reared by genetically-related or genetically-unrelated mothers. Second, all the studies provide information on multiple covariates: child gender, ethnicity, maternal age at birth, maternal education, family SES, family breakdown, breast feeding, birth weight, placement age, and parenting practices. Third, results obtained from individual studies can be pooled using a meta-analytic approach to allow examination of the magnitude of common effects generated across studies. Finally, two of the studies employed allow examination of the contribution of prenatal and possible postnatal passive genotype-environment correlation influences on derived associations.
Summary of genetically-sensitive designs of three longitudinal studies: Christchurch Health and Development Study (CHDS), Early Growth and Development Study (EGDS) and Cardiff IVF Study