Delusional jealousy or ideas of reference also may be present. Freud once believed that paranoid delusions stemmed from repressed homosexual conflicts, but this once commonly held assumption is, in fact, highly speculative. The subtypes of schizophrenia, including those listed above as well as the undifferentiated and residual types, exhibit low diagnostic reliability, considerable overlap between types, and limited predictive validity. While the distinction between positive and negative symptoms has more promise, most schizophrenics show a mixed symptom picture. There now exists a convincing body of evidence indicating that a predisposition for schizophrenia is transmitted genetically.
Family studies reveal an increased risk for schizophrenia in relatives of schizophrenic patients.
Studies of adopted children demonstrate a high rate of schizophrenia among the biological children of schizophrenics, even when they are reared by healthy adoptive parents. Overall, it is clear that genetic factors do play a role in the etiology of schizophrenia, without explaining the whole picture. A diathesis-stress model appears to be the best fit. The theory that schizophrenia is brought on by excess activity of the neurotransmitter dopamine is based principally on the mode of action of the phenothiazines, drugs which alleviate some symptoms of schizophrenia but also produce side effects similar to Parkinson's disease, a disease caused by low levels of dopamine. The literature on amphetamine psychosis supports the dopamine activity theory.