Alcohol Use v Marijuana Use - US Youth and "The Displacement Hypothesis"

"Alcohol and marijuana are the two most commonly used substances by teenagers to get high, and a question that is often asked is to what extent does change in one lead to a change in the other. If the substances co-vary negatively (an increase in one is accompanied by a decrease in the other) they are said to be substitutes; if they co-vary positively, they are said to be complements. Note that there is no evidence that the 13-year decline in marijuana use observed between 1979 and 1992 led to any accompanying increase in alcohol use; in fact, through 1992 there was some parallel decline in annual, monthly, and daily alcohol use, as well as in occasions of heavy drinking among 12th graders, suggesting that the two substances are complements. Earlier, when marijuana use increased in the late 1970s, alcohol use also increased. As marijuana use increased again in the 1990s, alcohol use again increased with it, although not as sharply. In sum, there has been little evidence from MTF over the years that supports what we have termed 'the displacement hypothesis,' which asserts that an increase in marijuana use will somehow lead to a decline in alcohol use, or vice versa.8 Instead, both substances appear to move more in harmony, perhaps both reflecting changes in a more general construct, such as the tendency to use psychoactive substances, whether licit or illicit, or in the frequency with which teens party. However, with alcohol use decreasing and marijuana use increasing over the past few years, it is possible that the displacement hypothesis is gaining some support. As a number of states are changing their policies regarding marijuana, our continued monitoring will provide the needed evidence concerning whether alcohol and marijuana are substitutes or complements."

Source: 

Miech, R. A., Johnston, L. D., O’Malley, P. M., Bachman, J. G., & Schulenberg, J. E. (2015). Monitoring the Future national survey results on drug use, 1975–2014: Volume I, Secondary school students. Ann Arbor: Institute for Social Research, The University of Michigan, pp. 161-162.
http://monitoringthefuture.org...
http://monitoringthefuture.org...

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