The cannabis or the psychosis? European researchers have challenged the commonly held belief that people may start using cannabis because they are predisposed to psychosis. They surveyed 2500 young Germans, aged 14 to 24 years, once in 1995 and again, four years later, in 1999. A predisposition to psychosis, determined from a self-report symptom checklist, did not predict future cannabis use four years later. However, exposure to cannabis did increase the risk of psychotic symptoms later in life, especially in those with a predisposition to psychosis. Moreover, there was a dose-response relationship — the greater the cannabis use at baseline, the greater the risk of developing psychotic symptoms, independent of a predisposition to psychosis.