Historical variation in drug use trajectories across the transition to adulthood: the trend toward lower intercepts and steeper, ascending slopes.

This study examines historical variation in individual trajectories of heavy drinking and marijuana use from age 18 to 22. Unlike most studies that have examined cohort differences in drug use, it focuses on differences in both level of use and rates of change (growth). Nearly 39,000 youths from the high school classes of 1976-2004 were surveyed at biennial intervals between the ages of 18 and 22 as part of the national Monitoring the Future study. Between 1976 and 2004, adolescent heavy drinking decreased substantially. However, because the age 18-22 heavy drinking growth rate increased threefold for males and sixfold for females during this period, heavy drinking among 21- to 22-year-olds remained largely stable. The growth rate for marijuana use was more stable across cohorts, and historical declines in use were sizable across the entire 18-22 age band. Generally, historical variation in use was unrelated to college status and living arrangements as well as to historical changes in the distribution of young adult social roles. Findings suggest that historical fluctuations in use were less the result of proximal young adult factors and more the result of historical variation in distal adolescent factors, the effect of which diminished with age, especially for heavy drinking.