“On Monday, the scientific journal Addiction released a sweeping new review of 20 years of research into the recreational use of marijuana. Dr. Wayne Hall, director of the Centre for Youth Substance Abuse Research at the University of Queensland and an adviser to the World Health Organization, began the review in 1993. At that time, there were “very few epidemiological studies of the health effects of cannabis,” he says. The existing scientific literature was “dominated” by “animal studies from the 1970s,” and the only human laboratory studies available were from the late ’70s and early ’80s. And even those analyzed only “the effects of sustained cannabis use over 7–35 days on the health of college students.”

What changed to make a review like this possible? Two things: First, people are smoking more weed. “During the past half-century,” the authors say, “recreational cannabis use has become almost as common as tobacco use among adolescents and young adults,” which means there are simply more weed smokers to study.” *