The following originally appeared on The Upshot (copyright 2019, The New York Times Company).
Are we underestimating the harms of legalizing marijuana?
Those who hold this view have been in the news recently, saying that research shows we are moving too far too fast without understanding the damage.
America is in the midst of a sea change in policies on pot, and we should all be a bit nervous about unintended consequences.
Vigilance is required. But it should be reasoned and thoughtful. To tackle recent claims, we should use the best methods and evidence as a starting point.
Does Marijuana Increase Crime?
Crime has gone up in Colorado and Washington since those states legalized marijuana. It’s reasonable to wonder about the connection, but it’s also reasonable to be skeptical about causation.
The best method to investigate this may be synthetic controls. Researchers can use a weighted combination of similar groups (states that are like Colorado and Washington in a number of ways) to create a model of how those states might have been expected to perform with respect to crime without any changes in marijuana laws. Benjamin Hansen, a professor of economics at the University of Oregon, used this methodology to create a comparison group that most closely resembled the homicide trends and levels from 2000-12.
“I picked those years because they were after the tremendous crime drop in the early ’90s and most predictive of crime today,” he said. “I ended in 2012 because that’s when Colorado and Washington voted to legalize marijuana.”
This model showed that we might have predicted more of an increase in Colorado or Washington just based on trends seen in comparable states, even without legalization. When he compared the two states with the synthetic control, Colorado and Washington actually had lower rates after legalization than you’d expect given trends.
This is not evidence that legalization lowers crime rates. But it does suggest that we shouldn’t conclude that it increases them. A number of other studies agree.
What About Car Crashes?
A potential misperception involves automobile crashes. Drunken drivers are measurably impaired when their blood alcohol level is above a certain level. We can prove this in randomized controlled trials.
The tests we use for measuring the presence of THC, though, do not measure the level of impairment. They measure whether someone has used marijuana recently. If we legalize the drug, and more people use it, more people will register its recent use even when they are not impaired. So it should be expected that more people involved in car crashes will test positive even if no one is driving while high.
Using a synthetic control approach, Mr. Hansen and colleagues showedthat marijuana-related fatality rates did not increase more after legalization than what you would expect from trends and other states.
The Concerns About Schizophrenia
Dr. Ziva Cooper is one of the authors of the National Academies of Sciences, Engineering, and Medicine’s comprehensive report on cannabis.
She says some have misinterpreted the report to state that the report’s committee concluded that cannabis causes schizophrenia. It did not.
“This was stated as an association, not causation,” she said. “We do not yet have the supporting evidence to state the direction of this association.”
Dr. Cooper, research director of the U.C.L.A. Cannabis Research Initiative, went further: “We as a committee also concluded that a history of cannabis use is associated with better cognitive outcomes in people diagnosed with psychotic disorders. The blatant omission of this conclusion exemplifies the one-sided nature of some articles. Nonetheless, the strong association between cannabis use and schizophrenia means that people with predisposing risk factors for schizophrenia should most certainly abstain from using cannabis.”
Marijuana Has Downsides
No one should be under the impression that marijuana is harmless. The potential downsides are well known, and I’ve covered them. Nor should anyone be irrationally exuberant about its upsides. It’s not a wonder drug, and the proven benefits are also minimal (as I discussed here).
