It probably goes without dispute at the present time that the precise content of a schizophrenic's thought, like that of anyone else, is derived from experience. Even the remarkable mélange of Schreber's memoirs could in large part be traced to his childhood experiences (Niederland, 1959). There is generally little doubt that the exact form and the course may be determined by age and experience; the effects of genetically determined intellectual endowment are uncertain (Elsässer, 1952; Rosenthal, 1963). The very occurrence of schizophrenia as a clinical phenomenon challenges the investigator, and there are many who question whether experience alone could result in schizophrenia in any and all human beings. They ask rather to what extent and by what mechanisms is the occurrence of schizophrenia determined by genetic factors.