Putative environmental characteristics may also increase the relative influence of genes on the risk of substance use. According to the social expression model, latent (genetic) tendencies to use substances are most likely to differentiate between individuals within environments in which there are social pressures to use various substances; the social environment triggers genetic expression. Therefore, the genetic variance of substance use should increase with increasing prevalence of substance use in the population, decreased social sanctions against using substances, and increased expectations to use substances. Evidence for this perspective was demonstrated by Boardman and colleagues (2008), who show that the heritability of daily smoking is significantly higher in schools where the most popular students were also the heaviest smokers, compared to schools with less clear pro-smoking norms. Importantly, this same association was less apparent in schools with established anti-smoking norms. In other words, in this context, the social expression model was more relevant than the social control model for this particular outcome.
