http://en.wikipedia.org/wiki/Schizophrenia_and_smoking
Studies across 20 countries show a strong association between schizophrenia and smoking.[1] For example, in the United States, 80% or more of schizophrenics smoke, compared to 20% of the general population in 2006.[2]
Though it is well established that smoking is more prevalent among schizophrenics than the general population as well as those with other psychiatric diagnoses, there is currently no definitive explanation for this difference.[3] Many social, psychological, and biological explanations have been proposed, but today research focuses on neurobiology.