Cannabis has downsides, but speculation and fear should be replaced with the best evidence available.
Are we underestimating the harms of legalizing marijuana?
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 increasein 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.
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.