Comparing results to the developmental processes of specific substances also shows similarities. For example, genetic factors increase whereas shared environmental factors decrease with age for a variety of substances (Koopmans & Boomsma, 1996; Koopmans et al. 1997; Pagan et al. 2006). Our modeling confirms this by showing, in general, a decrease in both common and residual shared environmental effects and an increase in substance-specific genetic effects across age.
