As jurisdictions around the country legalize marijuana for recreational and medical use, a key argument from opponents has been the possibility of increased teen use of the drug.

If teens consider marijuana less harmful, they’re more likely to use it, the theory goes. And for decades data appeared to agree.

A visually compelling chart used in a slideshow by leading legalization opponent Kevin Sabet shows what appears a remarkable correlation between 1975 and 2009 in Monitoring the Future survey data from high school seniors.

The survey results show past-year pot use plunged from about 50 percent in the late 1970s as perception of risk spiked during the “just say no” years of fried-egg ads. Beginning the year Bill Clinton was elected, the pattern reversed itself.

This chart, part of a slideshow from anti-legalization group Smart Approaches to Marijuana, shows the strong historical link between teen pot use and risk perception.

Smart Approaches to Marijuana

Then for about a decade, changes in the two data points leveled off.

Last year, however, the Monitoring the Future survey results showed marijuana use stable or slightly down among eighth-, ninth- and 12th-grade students as risk perception also dropped. That prompted the deputy director of the National Institution on Drug Abuse, Dr. Wilson Compton, to tell U.S. News it may be time to reconsider the connection.

"That's what’s been surprising to me and other researchers: We've now had five years of consistent declines in perceived harmfulness and the use rates have been reasonably steady – or dropping slightly this year," Compton said. "This is a bit of a puzzle and speaks to a different relationship of these phenomena than we've seen in the past."

The 2015 results from the annual NIDA-funded, University of Michigan-administered survey now are in, and past-year, past-month and daily teen use of marijuana again did not change significantly, even as risk perception reaches historic lows. This year, just 31.9 percent of high school seniors said regular pot use puts the user at great risk, less than half the 78.6 percent majority of 1991.

But Dr. Nora Volkow, director of NIDA, says she’s not willing to say yet that the historical association is over.

U.S. students in 2015 indicated the lowest risk perception of marijuana in a generation. NIDA

“You have to see it across the whole 35, 40 years, then you can very naturally see the trend,” she says. “I think we need two or three more years to see if, in fact, they are diverging – when you have very minimal changes these basically just get stable, you can’t observe them.”

Volkow says so far “the rate marijuana use has changed has been quite low, and the rates of the perception of risk has been a little steeper, but nothing dramatic.” She says with more certainty use does not seem to have increased among students this year as Oregon became the third state to allow recreational sales of marijuana to adults 21 and older.

“I was particularly concerned about daily use, which has also been stable. It did not change from last year,” Volkow says. “That in a way indicates as of now we have not seen a negative trend on the changes in policy as it relates to the use of marijuana in teenagers who are at school.”

Sabet, a former presidential drug policy adviser who leads the group Smart Approaches to Marijuana, says the survey may undercount marijuana use among teenagers, as it does not include school drop outs.

“It will take more time to make any long-term determinations about perceptions and use,” he says, adding he believes it’s important for future surveys to ask how students are consuming marijuana, something that may evolve as edible products, tinctures and vape-pens trickle from adults users to minors.

“We know that a Big Marijuana industry pushes the gummy bear and pot ice cream for profit,” he says.

Mason Tvert, a spokesman for the Marijuana Policy Project and co-director of the campaign that made Colorado the first state to roll out a regulated recreational pot market, says it’s reasonable that teen risk perception would drop without a corresponding increase in use.

An overwhelming majority of students continue to disapprove of smoking marijuana regularly, with a much more gradual dip from about 90 percent for all ages in the early ‘90s to the 70-80 percent range today. During the same time period, perceived risk has dropped far more rapidly.

“Many young people understand that marijuana is less harmful than alcohol and other drugs, but they also recognize that it is not OK for them to use it,” Tvert says. “For decades, teens had an artificially high perception of risk based on scare tactics and exaggerations. Now that there is more information out there and it's not limited to horror stories and propaganda, they are developing a more realistic view on it.”

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Betty Aldworth, executive director of the pro-legalization Students for Sensible Drug Policy, agrees.