This rate is fairly consistent despite the large variations in social and environmental factors that occur throughout the world. These data indicate that genetic factors are a critical cause because the risk for schizophrenia in monozygotic twins is two to four times greater than that for dizygotic twins and 30 to 50 times the risk in the general population. The rate of schizophrenia is higher among adopted children whose biological parents are schizophrenic than among adopted children with normal biological parents. Recent studies in molecular genetics have discovered a linkage between schizophrenia and two DAN polymorphisms on the long arm of chromosome 5. Experts in the field suggest that the locus on chromosome 5 is probably only one of several genetic loci for schizophrenia.
Schizophrenia is probably a multifactorial, polygenic disease.
In this sense, schizophrenia appears similar to diabetes and hypertension. The manifestation of these diseases requires the accumulation of several genetic defects as well as environmental factors. The fact that drugs are effective in treating schizophrenia supports the idea that this disorder results from biological causes. Drugs alter brain chemistry. Some drugs will cause hallucinations.