Chunk #14 — Basic methodology of genetic epidemiology: an overview of twin studies — Principle 3: The environment moderates the importance of genetic influences
Adolescence is a rich area for studying gene-environment interaction since there are so many shifts in relevant environmental factors across this developmental phase. As this has become a research area of increasing interest (and there have been corresponding advances in statistical modeling to test these more complex interactions; Purcell, 2002; van der Sluis et al., 2012), a number of such interactions have been detected. Twin studies have demonstrated that genetic influences on adolescent substance use are enhanced in the presence of substance-using friends (Dick et al., 2007a), and in environments with lower parental monitoring (Dick et al., 2007b). Figure 2 illustrates the dramatic shift in the relative importance of genetic effects that can take place across different environments using data from a Finnish twin project: at the extreme low end of parental monitoring, genetic effects assumed the greatest role in impacting adolescent smoking, whereas in homes with very high parental monitoring, genetic effects played little to no role, and common environmental factors were the most important influence (Dick et al., 2007b). Similar effects have been demonstrated for more general externalizing
