Many research studies such as the one discussed in Exercise 4.82 focus on a link between marijuana use and psychotic disorders such as schizophrenia. Studies have found that people with schizophrenia are twice as likely to smoke marijuana as those without the disorder. Data also suggest that individuals who smoke marijuana are twice as likely to develop schizophrenia as those who do not use the drug. Contributing to the apparent relationship, a comprehensive review done in 2007 of the existing research reported that individuals who merely try marijuana increase their risk of developing schizophrenia by 40%. Meanwhile, the percentage of the population who has tried marijuana has increased dramatically in the United States over the past 50 years, whereas the percentage of the population affected by schizophrenia has remained constant at about 1%. What might explain this puzzling result?