A frequently cited study of 50,000 male Swedish military conscripts (draftees) supported the claim that marijuana causes mental illness. Originally, researchers observed a connection between the use of cannabis by age 18 and a diagnosis of schizophrenia later in life. It was suggested that men who had used marijuana 50 or more times by the age of 18 were more likely to be diagnosed as schizophrenic than those who had used marijuana less than 50 but more than 10 times. However, the interpretation of these results later proved to be flawed. All the men who received a diagnosis of schizophrenia had previously been given some sort of psychiatric diagnosis by the military at the time they were drafted. Each had at some point been in trouble at school or with the police, had come from broken homes, and had histories of psychological problems. Therefore, although heavy cannabis use may be associated with a variety of psychological and social problems, and even a later diagnosis of schizophrenia, it was a leap to suggest that the marijuana use had "caused" the schizophrenia. The claim of a "marijuana-schizophrenia connection" was further undermined when over the same period of time the number of treated cases of schizophrenia had declined while marijuana use had increased.
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