Conversely, a related mechanism for gene-environment interaction is that of social expression or social triggering. Some environments allow greater opportunity to express genetic predispositions, and in these environments genetic influences become more potent. For example, genetic influences on smoking are highest in schools in which the most popular students smoke19. In Sweden, rates of tobacco use were very low in women born early in the twentieth century, rising over the next 50 years to rates comparable to those in men. The magnitude of genetic influences on tobacco use in women also rose markedly during that time period20. As smoking became more socially acceptable among women, genetic influences had greater opportunity to affect smoking patterns20. Social triggering, the effects of which can be cleanly separated from those of genetic influences, may act by increasing access to the substance or by creating social settings that are more accepting of substance use, both of which would allow enhanced opportunity to express genetic predispositions.
