The present work should be interpreted in the context of its strengths and limitations. First, we are reliant on maternal report of ADHD symptoms and twins’ self-report for conduct and alcohol problems. Although maternal report has been shown to be reliable for ADHD (Faraone et al., 1995) and other behavioral problems (e.g., Cronk et al., 2002), the addition of multiple raters (i.e., direct clinical evaluation or teacher reports) of behavior may offer additional information. Second, while not a main outcome in the study, we are dependent on retrospectively-reported broadly-defined measures of substance use during pregnancy. This could have caused us to overestimate the importance of these risk factors. While considerable research supports reliability and validity of retrospective reporting of pregnancy variables (e.g. Christensen et al., 2004; Heath et al., 2003; Reich et al., 2003), this does not preclude further investigation using more detailed assessments, including thorough timing and duration of exposure. Third, as with any cross-sectional study, the results are limited to a particular developmental period, in this case early to late adolescence. Further investigations are needed to determine whether
