During day-long in-person assessments at 11-, 14-, and 17-year-old time-points, a computerized substance use questionnaire was individually self-administered to the twins, without the presence of project staff. At intake, the dependent variable of diversity of substances used was the lifetime count of the number of different substances used by age 11. At each follow-up, it was the substance use count since the previous assessment, measured as the total number of 11 substances the twin endorsed having ever used recreationally (e.g., to get high). Questions were phrased as, “Have you ever used [substance]?” The substances asked about were (with examples given in the questionnaire shown in parentheses): tobacco (cigarettes, cigars, chewing tobacco), alcohol, marijuana/hashish, stimulants (uppers, speed, diet pills), tranquilizers (Valium, Librium, etc.), Quaaludes/downers, inhalants (gasoline, glue, aerosol sprays, peppers, rush, locker rooms), over the counter medications (NoDoz, Sominex, cold capsules, etc.), cocaine (coke, crack), PCP (angel dust, peace pills)/LSD (acid)/other psychedelics, and heroin (horse, smack)/opiates (methadone, opium, morphine, codeine, etc.). For tobacco, alcohol, and marijuana, response options were: “I have never tried [substance]”, “I have never tried [substance] but might
