Common shared environmental effects also showed substantial continuity and, as hypothesized, decreased in importance across development, on both the common factor and substance-specific residuals. There was, however, a small peak of new effects during adolescence. In addition, for alcohol and illicit drug consumption, shared environmental effects were greatest at this age. This is in contrast to previous studies, which have shown a smooth decline (Koopmans & Boomsma, 1996; Koopmans et al. 1997; Viken et al. 1999). These new effects may be due to a life transition in the lives of participants during this age. For example, at the age of 16, Swedish adolescents move to ‘upper secondary school’, which is the equivalent of high school in the USA. During this time, students choose the type of studies they will pursue (e.g. vocational versus university preparation).
