Answered by Irving I. Gottesman, Ph.D.:
The question requires a book-length answer to do justice to the information. I have written such a book (Schizophrenia Genesis, W.H. Freeman & Company, 1991), and it is in the recommended reading list for this American Experience program. I have co-authored an informational guide, "Schizophrenia & Genetic Risks," which is available in the primary sources section of this Web site.
It is the accumulation of facts, all converging on the conclusion that genetic factors, currently unspecified at the level of the genes themselves, and environmental factors, currently unspecifiable, combine in some fashion over time to produce this severe disorder in our species.