Legalization boosts marijuana use in big way, study of twins reveals

BOULDER, Colo. — Legalizing marijuana appears to promote more use among locals, according to a new study focusing only on pairs of adult twins. Those living in a legalized state report using cannabis 20 percent more often in comparison to their twin residing in a state where recreational use is still illegal.

Similarly, when considering all participants in the study, residents in states with legalized marijuana use it 24 percent more frequently than the residents of states that prohibit marijuana use. Conducted by scientists at University of Minnesota and University of Colorado, this study of 3,400 twins is some of the most compelling work to date indicating that legalization causes increased use. On a relate note, marijuana use in general is on the rise in the United States – including among adults.

“Across America, there is a trend toward using more marijuana but we found that the change is bigger in states where it is legal,” says lead study author Stephanie Zellers, a recent University of Minnesota graduate who began the research while a PhD student at CU Boulder’s Institute for Behavioral Genetics (IBG), in a university release.

Researchers analyzed data from two large longitudinal twin studies to reach these conclusions. Those projects tracked pairs of twins, residing in either Colorado or Minnesota, from childhood. The team asked each participant how often they used marijuana both before and after 2014. That’s when Colorado became one of the first states to start selling legal recreational marijuana. Meanwhile, recreational marijuana still isn’t legal in Minnesota.

Prior to 2014, the two states showed little differences in cannabis use. However, that changed after 2014. Across all participants, those living in Colorado used cannabis 24 percent more frequently than others living in Minnesota.

Could more marijuana use have some positives?

Regarding identical twin pairs specifically, those living in a legalized state used marijuana 20 percent more often. Since twins share their genes, typically grow up in the same socioeconomic bracket, and share parental influences as well as community norms, they served as ideal research subjects, study authors explain. By studying twins, the research team was able to minimize alternative explanations for their findings.

“This is the first study to confirm that the association between legal cannabis and increased use holds within families in genetically identical individuals,” explains study co-author John Hewitt, a professor in the Department of Psychology and Neuroscience and faculty fellow at IBG. “This makes it much more likely that legalization does, in itself, result in increased use.”

Today, over 141 million Americans reside in a state with recreationally legal marijuana. Per the National Institute on Drug Abuse, cannabis use among young adults (ages 19-30) has never been higher; 43 percent report using in the past year and 29 percent in the last month.

“Typically, what we would expect to see is that people tend to increase use as adolescents and then reduce it as they transition into adult roles, family life and stable jobs,” Zellers adds. “Interestingly, we saw escalation, not reduction, in adults.”

Study authors note it is highly unlikely that legalization alone would lead to those who abstained from marijuana before to suddenly start indulging after. Additionally, early results from their broader ongoing research project indicate increased use may not necessarily be a bad thing.

“In other analyses, we are finding that this increased use is not accompanied by increased problems, may be associated with less alcohol-related problems, and otherwise does not, in general, seem to have adverse consequences,” Prof. Hewitt concludes.

The study is published in the journal Addiction.

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