The current article used the clustered design in the NLSY sample to explore the association between SDP and three measures of offspring externalizing problems (CP, ODP, and ADHP). The present comparisons of unrelated children were consistent with the results of previous studies (Wakschlag, Pickett, Cook, Benowitz, & Leventhal, 2002
) in several respects: (a) CP, ODP, and ADHP problems were significantly associated with SDP; (b) each association followed a dose-response relationship; (c) the number of CP demonstrated by children exposed to SDP was higher for male children; and (d) each association remained significant after statistically controlling for associated maternal characteristics. In addition to the use of statistical covariates used in previous studies, the current analyses utilized the multiple levels of the NLSY to account for unmeasured confounds. If SDP caused higher conduct and oppositional problems, the relation would have been evident both when comparing related (e.g. within mothers) and unrelated children (e.g. Rodgers, Cleveland, van den Oord, & Rowe, 2000
). However, when children were compared with their siblings who differed in their exposure to SDP, the offspring did not differ with respect to conduct and oppositional defiant problems. SDP accounted for no more than 0.1% of the within-nuclear family variance in offspring conduct problems. In contrast, SDP accounted for 8–14% of the between-nuclear family variance in offspring conduct problems. These results suggest that previous studies found a relationship between SDP and offspring conduct problems not because SDP causes increased risk for conduct or oppositional problems, but because environmental influences that vary between families confound associations between SDP and offspring externalizing. This finding is generally consistent with another CoT study of SDP and ADHD (Knopik et al., in press
), as well as with studies that have included more precise measurement of adult characteristics that may confound the relation, such as maternal and
paternal antisocial characteristics (Maughan, Taylor, Caspi, & Moffitt, 2004
) and maternal delinquency during adolescence (Silberg et al., 2003
The present findings indicate that environmental influences that are risk factors for both maternal SDP and offspring externalizing underlie the previously observed intergenerational associations between SDP and offspring conduct problems. This has important implications for future research on the causes of conduct problems. The present findings imply that the salient aspects of the familial or social environment that are robustly associated with both SDP and offspring conduct problems have not been the focus of the research on the etiology of externalizing problems. Because SDP is significantly related to offspring conduct problems after controlling for essentially all of the parent and family characteristics that have been examined as risk factors for offspring conduct problems (Wakschlag & Hans, 2002
), the present findings suggest that aspects of the family or social environment that are unrelated to maternal antisocial behavior, maternal age, and other well-studied risk factors are responsible for the relations to SDP. Hopefully, the present findings will lead to an expanded search for these environmental risk factors. The important clues offered by these findings is that they are correlated with SDP, but not correlated with the risk factors that have been controlled in this and previous studies of SDP.
The present analyses, however, suggest a minimal role of SDP with offspring ADHP problems, but the magnitude of the association was greatly reduced compared to previous observations. A recent review has documented a small association between SDP and ADHP after controlling for various confounds (Linnet et al., 2003
), and the small association, consistent with a direct causal effect of SDP on offspring ADHP, may be related to the influence of SDP on birth weight (Knopik et al., in press
The current article used two analytical approaches to explore the associations between SDP and offspring externalizing because each approach has its own advantages and limitations. HLMs were used because they enabled the inclusion of every family and offspring. Furthermore, interactions between variables (e.g. SDP and offspring sex) and the effects of measured family characteristics can be more easily and directly included. Although the HLMs indicate that the relation between SDP and offspring externalizing is not causal, it is difficult to identify whether the origin of relevant confounds are genetic or environmental within the HLM approach. Interpreting the between and within-family parameter estimates from the multiple family groups in the NLSY (cousins, half-siblings, ambiguous-siblings, full-siblings, and twins) becomes quite computationally burdensome (results not shown). In contrast, the SEM approach provides a more straightforward approach to identifying the intergenerational effects of genetic, common environmental, and non-shared environmental influences. The SEM approach, however, cannot include adult sibships that differed in genetic relatedness from other pairs within their household. Furthermore, the sibling correlations and SEM suggest no influence of genetic factors on SDP, in contrast to other reports with larger samples (D’Onofrio et al., 2003
; Knopik et al., 2005
). Accordingly, we conclude that the results suggest
that the confounds are environmental rather than genetic in origin. Analyses with larger samples will be required to more specifically elucidate the magnitude of genetic and common environmental processes responsible for the intergenerational association. Regardless of the nature of the confounds, the analyses question whether associations between SDP and offspring externalizing in young children are causal.
There are a number of additional limitations of the current study. First, all of the analyses are based on self-report measures completed by the mother. The retrospective report of SDP may have resulted in underreporting or greater measurement error (Wakschlag, Pickett, Cook, Benowitz, & Leventhal, 2002
), but there is a high correlation between self-reported smoking status and serum cotinine measures (McDonald, Perkins, & Walker, 2005
). The reliability of retrospective reports is also similar to the recall of other substance use (Petitti, Friedman, & Kahn, 1981
). Furthermore, mothers who report SDP may be more likely to report higher externalizing in their children. The magnitude of intergenerational associations in the current paper, however, are consistent with studies that used more sophisticated measurement strategies (Wakschlag, Pickett, Cook, Benowitz, & Leventhal, 2002
). Second, due to complexity of the analyses we did not analyze the longitudinal responses of the offspring across the age range (4 to 10 years old); rather, we relied on the average across the years. Future analyses that explore offspring characteristics associated with SDP would benefit from using analytical strategies, such as growth curve models, that can account for individual differences in initial level and change across time (e.g. Willett, Singer, & Martin, 1998
). Third, our findings are limited to measures of externalizing in young children and may not reflect the underlying mechanisms responsible for the association between SDP and adult criminal activity (Brennan, Grekin, & Mednick, 1999
; Rasanen et al., 1999
). Fourth, no father information was included in the analyses. Limited information on the fathers is available in the NLSY, and the association between SDP and offspring externalizing could be due to higher rates of paternal delinquency (Maughan, Taylor, Caspi, & Moffitt, 2004
). Methodological work on CoT Design has highlighted its strength with characteristics of individuals (such as SDP), but inasmuch as SDP is influenced by the women’s spouses, the interpretation of the parameters may be somewhat restricted (Eaves, Silberg, & Maes, 2005
). Fifth, we only explored whether offspring gender moderated the influence of SDP, but there may be vulnerability factors (both environmental and genetic in origin) that make individuals more susceptible to the influence of SDP.
Finally, as is the case with all comparisons using different levels of analyses, the design rests on a number of assumptions about women who have multiple offspring and who vary in the levels of SDP among the pregnancies. The design assumes that women who have only one child are similar to women with multiple offspring, because the within-mother parameter is based solely on women with multiple children. Furthermore, the analyses assume that women who have multiple pregnancies and variation in SDP are similar to women who have multiple offspring but smoked at the same level in each pregnancy, including their ability to accurately report SDP. Again, the within-mother parameter can only be estimated based on women with variation in SDP among her pregnancies.3
Ultimately, the limitations of the current article underscore the necessity of replicating the analyses in other samples and exploring the consequences of SDP in animal studies, which have the advantage of random assignment.
The study highlights the importance of using research designs that can pull apart different risk mechanisms and account for non-measured confounds (Rutter, Pickles, Murray, & Eaves, 2001
). Furthermore, the results underline the limitations of solely relying on measured covariates to account for confounds. Even though the initial HLMs that compared unrelated children included a measure of maternal delinquency to control for the intergenerational transmission of externalizing problems there was still an association between SDP and offspring externalizing. If we only relied on the measured covariates we would have drawn the incorrect conclusion. Methodologically, these results support the importance of using design innovations as well as statistical controls to study risk mechanisms rather than just identifying risk factors (Moffitt, 2005
; Rutter, Pickles, Murray, & Eaves, 2001
Similar to other genetically informed studies of familial characteristics, such as parental divorce (D’Onofrio et al., 2006
), the underlying processes associated with a risk factor depend on the specific outcome measure being studied. SDP appears to have a specific environmental association with offspring birth weight, in as much as genetic factors, common environmental, and measured covariates do not confound the relation (D’Onofrio et al., 2003
However, CP and ODP in young children associated with SDP are completely due to characteristics that lead to both SDP and offspring externalizing behaviors, not the consequences of SDP. As a result, each risk factor and specific measure of adjustment needs to be studied independently with designs that can separate the risk mechanisms.
Finally, we must state that we were surprised by the results of the analyses, given the existent research on SDP (though no previous research has allowed for control over unobserved heterogeneity to the extent that ours has). We want to stress the need to replicate the current findings. Furthermore, we certainly concur with other researchers (e.g. Maughan, Taylor, Caspi, & Moffitt, 2004
) in stressing the importance of reducing SDP because of its effects on a wide range of range of developmental characteristics, particularly neurobehavioral functioning (Cnattingius, 2004
; Huizink & Mulder, 2005
). Animal research has shown that prenatal nicotine exposure has neurotoxic effects (review in Wakschlag, Pickett, Cook, Benowitz, & Leventhal, 2002
). Thus, the findings of the current analyses are limited to externalizing problems during childhood. The current article, though, emphasizes that women who smoke during pregnancy are different than mothers who do not. Although the research literature has noted a number of characteristics of mothers and families associated with SDP, the analyses in the current article suggest that unmeasured characteristics accounted for the associations between SDP and offspring externalizing problems. Perhaps there are more differences than previously imagined between mothers who smoke during pregnancy than those who do not. Therefore, addressing the myriad of risk factors associated with SDP may be a more effective approach of minimizing externalizing problems in offspring than specifically focusing on reducing SDP.