The third interview included measures of retrospectively assessed peer group deviance (PGD) at five age period between 8 to 11, 12 to 14, 15 to 17, 18 to 21, and 22 to 25 years. The importance of these time periods is underscored by observations that (i) the mean onset or initiating age for most drugs use in the U.S. general population is between 12 and 20 years (45, 47, 48), (ii) cessation of drug use occurs normally by 29 years, while initiation rarely occurs after this age (49), and (iii) incidence of drug use, abuse and dependence peaks from age 15 to 25 (45, 50, 51). The PGD data were based on 10 items obtained from two validated instruments(52, 53) which assessed the proportion of the respondent's friends, at each particular epoch, who engaged in specific behaviors. Friends were defined as “…people who you would have seen regularly and spent time with in school and outside of school.” The 10 items were: (1) smoked cigarettes; (2) drunk alcohol; (3) got drunk; (4) had problems with alcohol; (5) been in trouble
