age at first use. Perhaps the most intriguing finding in this area comes from a study of 12 to 25 year-olds by Koopmans et al. (1997), who found high correlations between shared environmental influences on initiation of alcohol and cigarette use in 12 to 16 year-olds, but concluded that among 17 to 25 year-olds, common influences on initiation were attributable to genetic sources. The trend for heritability estimates for substance use initiation to increase with age has been noted in a review of twin and adoption studies of adolescent substance use (Hopfer et al., 2003) and may help to explain the relatively high estimates of genetic contributions to onset of first use in our young adult sample.
