For example, the genetic influences on the use of tobacco and alcohol are either muted or nonexistent among those who are raised with a strong religious upbringing with stringent norms against substance use of any kind (Koopmans et al. 1999; Timberlake et al. 2006). Second, according to the social trigger model, genetic factors differentiate between individuals only in the presence of social pressures to consume cigarettes (Perrin and Lee 2007). Therefore, genetic influences on smoking should increase when smoking begins to become widespread, social sanctions against it are removed, and social pressures to smoke emerge. For example, Boardman et al. (2008) show that genetic influences on daily smoking among adolescents are significantly higher for those who attend schools in which the most popular students also smoke the most. In this case, the pro-smoking norms serve as a trigger for genetic influences.
