Scores on the Wechsler Intelligence Scale for Children were analyzed in a sample of 7-year-old twins from the National Collaborative Perinatal Project. A substantial proportion of the twins were raised in families living near or below the poverty level. Biometric analyses were conducted using models allowing for components attributable to the additive effects of genotype, shared environment, and nonshared environment to interact with socioeconomic status (SES) measured as a continuous variable. Results demonstrate that the proportions of IQ variance attributable to genes and environment vary nonlinearly with SES. The models suggest that in impoverished families, 60% of the variance in IQ is accounted for by the shared environment, and the contribution of genes is close to zero; in affluent families, the result is almost exactly the reverse.

Researchers have identified environmental risks that predict subsequent psychological and medical problems. Based on these correlational findings, researchers have developed and tested complex developmental models and have examined biological moderating factors (e.g., gene-environment interactions). In this context, we stress the critical need for researchers to use family-based, quasi-experimental designs when trying to integrate genetic and social science research involving environmental variables because these designs rigorously examine causal inferences by rigorously testing competing hypotheses. We argue that sibling-comparison, offspring of twins/siblings, in vitro fertilization designs, and other genetically-informed approaches play a unique role in bridging gaps between basic biological and social science research. We exemplify these principles using studies on maternal smoking during pregnancy.

Previous studies have documented that smoking during pregnancy (SDP) is associated with offspring externalizing problems, even when measured covariates were used to control for possible confounds. However, the association may be due to non-measured environmental and genetic factors that increase risk for offspring externalizing problems. The current project used the National Longitudinal Survey of Youth and their children, ages 4-10 years, to explore the relations between SDP and offspring conduct problems (CP), oppositional-defiant problems (ODP), and attention deficit hyperactivity problems (ADHP) using methodological and statistical controls for confounds. When offspring were compared to their own siblings who differed in their exposure to prenatal nicotine, there was no effect of SDP on offspring CP and ODP. This suggests that SDP does not have a causal effect on offspring CP and ODP. There was a small association between SDP and ADHP, consistent with a causal effect of SDP, but the magnitude of the association was greatly reduced by methodological and statistical controls. Genetically informed analyses suggest that unmeasured environmental variables influencing both SDP and offspring externalizing behaviors account for the previously observed associations. That is, the current analyses imply that important unidentified environmental factors account for the association between SDP and offspring externalizing problems, not teratogenic effects of SDP. Keywordssmoking during pregnancy; conduct problems; externalizing problems; ADHD; children of twins; causation Smoking during pregnancy (SDP) has been consistently linked with externalizing problems in offspring, particularly in males (reviews in Cnattingius, 2004;Huizink & Mulder, 2005;Wakschlag & Hans, 2002;Wakschlag, Pickett, Cook, Benowitz, & Leventhal, 2002). SDP has been associated with parent-reported conduct problems (Ernst, 2001), arrest history from national crime registries (Brennan, Grekin, & Mednick, 1999;Rasanen et al., 1999), contact 2 All correspondence should be sent to: Brian D'Onofrio, PhD, Department of Psychological and Brain Sciences, Indiana University, 1101 E. 10th St., Bloomington, IN 47405, bmdonofr@indiana.edu. NIH Public Access Author ManuscriptDev Psychopathol. Author manuscript; available in PMC 2013 August 08. NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscriptwith police obtained by city police records (Gibson, Piquero, & Tibbets, 2000), Oppositional Defiant Disorder (Wakschlag & Keenan, 2001), Conduct Disorder (Fergusson, Woodward, & Horwood, 1998;Wakschlag & Hans, 2002;Wakschlag & Keenan, 2001;Wakschlag et al., 1997;Weissman, Warner, Wickramaratne, & Kandel, 1999) and Attention-Deficit/ Hyperactivity Disorder (Mick, Biederman, Faraone, Sayer, & Kleinman, 2002;Rodriguez & Bohlin, 2005).Reviews of the literature note that the association is consistent with a causal connection because the association is specific to externalizing problems, has been found across diverse samples and measures, demonstr...

An assessment of the strengths and limitations of the Children of Twins design and a comparison with other research strategies suggest that the design plays a unique role in the study of developmental psychology and psychopathology. Finally, the authors describe how methodological advances and future applications of the design will provide additional insight into the causal processes underlying children's adjustment to environmental stimuli.

Psychologists in both basic and applied fields are keenly interested in the environmental influences that shape our lives. Therefore, researchers test causal hypotheses to construct models of environmental influences that can withstand attempts at refutation. Randomized experiments provide the strongest tests of causal hypotheses, but are not always feasible and their assumptions cannot always be met. In such cases, a number of quasi-experimental research designs can be used to substantially reduce confounding in tests of causal hypotheses. Sibling-comparison designs provide robust quasi-experimental tests of causal environmental hypotheses, but they are underused in psychology in spite of their power, feasibility, and convenience.

The factor mixture model (FMM) uses a hybrid of both categorical and continuous latent variables. The FMM is a good model for the underlying structure of psychopathology because the use of both categorical and continuous latent variables allows the structure to be simultaneously categorical and dimensional. This is useful because both diagnostic class membership and the range of severity within and across diagnostic classes can be modeled concurrently. While the conceptualization of the FMM has been explained in the literature, the use of the FMM is still not prevalent. One reason is that there is little research about how such models should be applied in practice and, once a well fitting model is obtained, how it should be interpreted. In this paper, the FMM will be explored by studying a real data example on conduct disorder. By exploring this example, this paper aims to explain the different formulations of the FMM, the various steps in building a FMM, as well as how to decide between a FMM and alternative models.

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