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 showed that 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.”