individuals who had used at least one illegal drug, 95% used cannabis before any other illegal drug, 4% used cannabis only after using another illegal substance, and only 1% never used cannabis. Because early cannabis initiation was used as a predictor for other drug use and abuse/dependence, the present analyses included only the 6362 individuals from complete twin pairs in which neither member used another illegal substance or developed alcohol dependence prior to cannabis initiation (89.9% of complete twin pairs in the sample). At the time of interview, the respondents had a mean age of 42.0 years (SD=2.8) and most were married (77%). Based on military record data, 94% of the respondents were Caucasian, with 6% being African-American and <1% being of another ethnicity. Additional data on education (available for 96% of the respondents from a self-report questionnaire) indicated that in 1987, 3% had less than 12 years of education, 31% had 12 years of education, and 66% had some education beyond high school.
