The items used to measure initiation of using tobacco, cannabis, cocaine, and inhalants (such as glue or solvents) were worded as “How old were you when you… for the first time?”. First use of alcohol was measured with the item “Think about the first time you had a drink of beer, wine, or liquor when you were not with your parents or other adults in your family. How old were you then?”, and first use of other substances was measured with the item “How old were you when you first tried any other type of illegal drug, such as LSD, PCP, ecstasy, mushrooms, speed, ice, heroin, or pills, without a doctor’s prescription?” (see http://www.cpc.unc.edu/projects/addhealth/codebooks/wave1). The prevalence of the five phenotypes (reported age of first use vs. reported zero use) was tobacco = 0.46, alcohol = 0.43, cannabis = 0.26, inhalants = 0.06, and other substances = 0.09. Since the distribution of age of onset was skewed, medians are more informative than means. Median age of first use was tobacco = 13, alcohol = 14, cannabis = 15, inhalants = 13,
