In November 2014, voters in Oregon, Alaska, and Washington, DC, approved the legalization of recreational marijuana, following the lead of Colorado and Washington State. More than 20 states have enacted laws to allow the sale of marijuana for medicinal purposes, and others have moved to reduce criminal penalties for possession of small amounts. But the more marijuana legalization reaches mainstream acceptance, the more the divisions of opinion within the mental health field—presumably the professionals who have the most scientifically informed perspective on the debate—become apparent.

In the face of the rapidly turning tide of public opinion, sometimes a reefer-madness level of alarm can be seen, even among psychotherapists who might otherwise be counted on to know how important it is to refrain from generalizing. On a listserv I subscribe to, colleagues have made such sweeping statements as “marijuana is a dangerous drug” and “adults who smoke pot act like immature teenagers.” One colleague even issued a warning that members on the listserv should be careful about expressing pro-pot opinions, lest our licensing board catch wind of them and decide to haul us in for drug tests.

Meanwhile, a close look at the evidence concerning the mental health consequences of pot use doesn’t support extreme positions on either side of the legalization issue. After combing through 20 years worth of research, Wayne Hall, a professor and director of the Centre for Youth…