Many marijuana prohibitionists insist one reason to keep marijuana illegal is that it causes mental illnesses, despite many scientific studies that say otherwise. A new study conducted through the Harvard Medical School adds to that body of research and shows that marijuana use is unlikely to be a cause of schizophrenia. The researchers studied four groups: people with no history of psychosis and no history of marijuana use, people with no history of psychosis who were heavy marijuana users as adolescents,…