Trends in U.S., past-year marijuana use from 1985 to 2009: An age–period–cohort analysis
Introduction
Increasing rates of marijuana use among today's youth in recent years (Fig. 1, Johnston et al., 2011a; Fig. 2.6, Substance Abuse and Mental Health Services Administration, 2010b) may be specific to the younger generation or may represent a general trend present among all ages. A full and accurate consideration of this possibility requires a formal age–period–cohort analysis. Currently no such analysis exists for adult trends in past-year marijuana past the year 2000.
This study contributes to the literature in two main ways. First, we update the only other age–period–cohort analysis of adult, past-year marijuana use (Kerr et al., 2007) and extend analysis to the years 2001–2009. Second, to our knowledge this paper is the first to examine potential differences across race/ethnicity in age–period–cohort marijuana trends.
Historical trends in any outcome are a function of birth cohort, historical period, and age influences. For example, marijuana prevalence among adults age 20–25 may increase from the year 2000 to 2010. If this is a general increase that affected all cohorts then it is a “historical period” effect. In contrast, if this increase is found only among adults age 20–25 in 2010 then it would represent a “birth cohort” effect. Separating historical period and cohort effects is complicated by the fact that many outcomes vary substantially by age; in this case, past-year marijuana prevalence declines substantially with advancing age (Kerr et al., 2007). It is therefore necessary for statistical analysis to, in effect, compare birth cohorts at similar ages. A formal age–period–cohort analysis provides separate estimates for these three influences net of each other.
To date, only one formal age–period–cohort analyses has focused on the outcome of adult, past-year marijuana use (Kerr et al., 2007). It combined nationally representative data from the four survey years of 1984, 1990, 1995, and 2000 for an analysis pool consisting of 19,341 individuals (Kerr et al., 2007). An independent, positive influence of cohort on past-year marijuana use, net of historical period and age effects, was uniquely high for the baby boom cohort (defined as people born between 1946 and 1964, U.S. Census Bureau, 2006) and declined for later cohorts, at least for men. In terms of broad, historical period effects the analysis indicated that from 1984 to 2000 past-year marijuana use declined for all cohorts among men, and appeared to increase among women. Finally, for both men and women the prevalence of marijuana use declined with advancing age.
This project extends the time frame of the Kerr analysis to include the years 2001–2009. Marijuana trends during these years warrant study in light of evidence that past-year use has increased. For example, among 12th graders past-year marijuana use increased monotonically from 31.5% in 2006 to 34.8% in 2010 (Table D-9, Johnston et al., 2011b). Past-year marijuana use among adults also trends in the same direction during this period; e.g., among respondents age 26–29 the prevalence increased from 17.2% to 25.5% and for respondents age 30–34 it increased from 11.7% to 18.2% (see Tables 1.12B and 1.11B in Office of Applied Studies, 2007, Office of Applied Studies, 2010). These trends suggest the presence of a recent historical period effect that increased past-year marijuana use for all cohorts, although no formal age–period–cohort analysis has yet documented it. Information on whether an increase in past-year marijuana use is general to all birth cohorts or specific to the young can help focus research aimed at identifying the specific mechanisms at work as well as guide policy aimed at addressing the trend.
This project also expands the scope of racial/ethnic groups considered in age–period–cohort analyses of past-year marijuana use. The existing age–period–cohort analysis of adult marijuana use did not stratify by race/ethnicity, most likely because of statistical power limitations. The data set for this analysis is more than 40 times larger, and provides the opportunity for separate analyses of Hispanic, non-Hispanic black, and non-Hispanic white respondents.
Analyses of the Hispanic respondents are of particular interest. The ranking of Hispanics for past-year marijuana use in relation to other racial/ethnic groups differs dramatically by age group. While Hispanics rank highest during adolescence (Johnston et al., 2011b), they rank lowest among those age 26 and over (Table 1.27B, Substance Abuse and Mental Health Services Administration, 2010a). In this project we examine whether cohort differences drive this finding across age groups, or if Hispanics ‘age out’ of marijuana use at a faster pace.
Section snippets
Methods
Data for the analysis come from the National Survey on Drug Use and Health (NSDUH), a series of annual, nationally representative, cross-sectional surveys of the U.S. civilian, non-institutionalized population. This study uses the years 1985, 1988, and every year from 1990 to 2009 inclusive.
The NSDUH was designed to provide estimates of the prevalence of illegal and legal drugs in the household population of the United States. These surveys used a multistage probability sample, with minor
Results
Fig. 1 presents the observed prevalence of past-year marijuana use by age groups, sex, and survey year. In keeping with the other, previously published age–period–cohort analysis of adult past-year marijuana use we present the age groups of 25 or less as well as 26 or older, although we stratify the 26 and older group into age 26–34, 35–49, and 50–64.
Three main findings are apparent in Fig. 1. First, prevalence of past-year marijuana use varies substantially by age group. Rates are highest
Discussion
This study presents the first age–period–cohort analysis of adult, past-year marijuana use that examines trends in the 21st century (up to the year 2009), presents the first analysis of potential differences in age–period–cohort trends across race/ethnicity, and makes use of recent developments in age–period–cohort methodology.
The analysis documents two main findings, which we discuss in more detail below. First, the positive, independent influence of cohort membership on past-year marijuana
Conclusion
The increasing rate of past-year marijuana use among the young is more consistent with a general increase in marijuana use across all age groups than it is with a cohort-specific influence unique to the youngest cohorts. It is therefore strategic for efforts aimed at specifying the driving forces behind the recent increase in marijuana use to focus on general determinants of marijuana use that affect all age groups, and not on determinants specific to adolescence and young adulthood. Such
Role of funding source
Funding for this study was provided by NIDA Grant # DA020575 (Richard Miech, P.I.); NIDA had no further role in study design; in the collection, analysis and interpretation of data; in the writing of the report; or in the decision to submit the paper for publication.
Contributors
Author Miech conceived and ran the empirical analysis. Authors Miech and Koester managed the literature searches and summaries of previous related work and wrote the paper together. Both authors contributed to and have approved the final manuscript.
Conflict of interest
Both authors declare that they have no conflict of interest.
Acknowledgement
We wish to acknowledge the invaluable assistance of Joe Gfroerer and Division of Population Surveys at the Center for Behavioral Health Statistics and Quality.
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