A study at the University Hospital of Psychiatry in Zurich examined 23 controlled experimental studies to find variables that predict the human response to psilocybin. The studies covered 409 administrations of psilocybin to 261 healthy volunteers. The data showed the dosage was the greatest predictor of behavior. Beyond that expected result, the researchers found scoring high in the “absorption” personality trait, being in an emotionally excitable and active state before taking the drug, and having experienced few psychological problems during the previous week were most often associated with positive outcomes and a pleasant, mystical-type experience. Participants had unpleasant and/or anxious reactions to psilocybin if they had high emotional excitability, were young, and/or were in an experimental setting using positron emission tomography (PET). Some pleasant and unpleasant effects occurred in the same setting.