Marijuana legalization is not the cause of increased marijuana use nationwide, a new study finds. Instead, it's the other way around: Marijuana legalization reflects increased acceptance of marijuana.

In the study, published this month in the journal Addiction, researchers from the Public Health Institute's Alcohol Research Group examined 30 years' worth of data from National Alcohol Surveys, which also include questions on marijuana use, and compared that data to changes in state laws.

What they found is not that pot policy drives behavior, but vice versa.

"Medical and recreational marijuana policies did not have any significant association with increased marijuana use," the authors concluded. "Marijuana policy liberalization over the past 20 years has certainly been associated with increased marijuana use; however, policy changes appear to have occurred in response to changing attitudes within states and to have effects on attitudes and behaviors more generally in the U.S."

Increasing marijuana use is "primarily explained by period effects," or social factors that impact populations across age and generational groups, and not by policy changes, the authors insist.

"The steep rise in marijuana use in the United States since 2005 occurred across the population and is attributable to general period effects not specifically linked to the liberalization of marijuana policies in some states," the paper concluded.

Those effects could include declining disapproval of marijuana among the overall population caused by increasing familiarity with the plant, as well as a tendency in surveys from earlier years for respondents to understate their actual marijuana usage.

The notion that policy does not drive drug use levels is not new. Academic researchers Peter Cohen and Craig Reinarman reported similar findings back in 2004. But the implications of such research are important: If drug policy has little impact on drug use levels, why have punitive drug policies?