approximately age 21 in the Virginia Adult Twin Study of Psychiatric and Substance Use Disorders (VATSPSUD;(Kendler et al. 2011)). However, this shift occurred earlier at an average age of 17.5 in the FinnTwin12. There are differences both in the legal drinking age and cultural norms regarding alcohol use between Finland and the United States (Bloomfield et al., 2010; Helasoja et al., 2007; Mäkelä et al., 2012). In Finland, the legal drinking age is 18, and underage drinking at age 17 is socially tolerated in the Finnish society. Conversely, the legal age to drink is 21 in the US, and greater measures are taken to prohibit underage drinking. However, the observed age differences in the shift in genetic influence may be attributable to recall biases in the retrospective reporting of past drinking by men at a mean age of 40.3 years [SD=9.0] in the VATSPSUD, whereas FinnTwin12 reports were made prospectively. Additionally, the same measure of alcohol use behavior was used across development in the VATSPSUD, whereas different, but developmentally appropriate measures of problematic alcohol use behaviors were used in the present study, which could also contribute to observed differences across these studies.
