The results also indicate that the genetic and environmental effects on this general vulnerability are developmentally dynamic and stable, although evidence is stronger for stability. Common genetic effects at age 13–14 accounted for a majority of the common genetic effects at ages 16–17 and 19–20. In contrast to our hypothesis, we did not see common genetic effects increase with age. However, there was a general trend for substance-specific genetic effects to increase with age. This is perhaps our most intriguing finding. That is, with increasing age, genetic influences on substance consumption become more specific in their effect.
