Period effects -- indicative of societal tolerance or support for drug use during 5-year periods between 1961 and 1990 -- declined between the early 1970's and late 1980's, while cohort effects -- indicative of early experiences of birth cohorts favoring drug use -- increased. One interpretation is that trends in incidence were determined by two opposing vectors of social forces; beginning in the 1970's, changes in social policies, values, and drug markets, as reflected in period effects, increasingly acted to reduce incidence, while changes in conditions of childhood socialization, as reflected in cohort effects, increasingly facilitated or encouraged incidence. Especially for marijuana, the increase in cohort effects was larger among females, which gives rise to gender convergence (approximately equal male and female incidence rates for both drugs) by the late 1980's. An innovative method of this study is the adjustment of incidence rates for reporting bias. 4 tables, 1 figures, a glossary, 5 notes, and 36 references