Results indicated that the timing of first alcohol use was attributable in part (36%) to heritable factors but that shared environment also played a role, accounting for 15% of individual differences in timing of first alcohol use. Earlier investigations have also found that initiation of alcohol use (both “ever use” and timing of first use) is influenced by a combination of genetic and shared environmental factors, but in contrast to the current study, the relative contributions of shared environmental factors have been somewhat greater than those from heritable factors20,23,24. Distinctions between our findings, based on 24 to 36 year-olds, and earlier work in this area, which has been conducted primarily with adolescent samples, may reflect the tendency of heritability estimates for substance use initiation to increase with age of the sample35,36. Koopmans and Boomsma21, for example, reported that shared environment accounted for 58-88% of variance in initiation in a sample of 15-16 year olds, whereas in their sample of 17-22 year-olds, 43% was attributable to genetic factors and only 37% to shared environmental influences. (A possible explanation for this phenomenon
