All subjects underwent extensive psychiatric assessment procedures using the Comprehensive Assessment of Symptoms and History interview,20 the Schedule for Affective Disorders and Schizophrenia: Lifetime Version,21 the Structured Interviews for DSM-III-R (Diagnostic and Statistical Manual of Mental Disorders, Revised Third Edition) and DSM-IV (Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition),22,23 the Family Interview for Genetic Studies,24 and a medical history inventory. Psychiatric diagnosis was established according to criteria of DSM-IV. Diagnostic assessments were conducted by trained and experienced psychologists and psychiatrists at the Department of Psychiatry of the University Medical Center, Utrecht, the Netherlands. Consensus was reached in presence of a senior psychiatrist. The following subtypes were diagnosed in the twins with schizophrenia: paranoid (7 MZ; 4 DZ), disorganized (3 MZ; 3 DZ), undifferentiated (2 MZ; 5 DZ), residual (2 MZ; 2 DZ), and catatonic (1 MZ). Furthermore, 3 MZ patients and 1 DZ patient had an additional diagnosis of depressive disorder not otherwise specified. Diagnoses in nonschizophrenic cotwins included paranoid personality disorder (2 MZ: 1 of which also had a schizoid personality disorder and a recurrent major depressive disorder; 1 DZ), schizotypical personality disorder (2 MZ), schizoid personality disorder (1 DZ), major depressive disorder (4 MZ: 1 recurrent and 3 single episodes [1 with psychotic symptoms and 1 with agoraphobia]; 2 DZ: single episode), avoidant personality disorder (2 DZ: 1 with social phobia and 1 with partial epilepsy), generalized anxiety disorder with a dependent personality disorder (1 MZ), and no psychiatric diagnoses (5 MZ; 8 DZ). Moreover, 5 MZ and 4 DZ patients and 1 MZ and 1 DZ cotwin had histories of substance or alcohol abuse. Healthy control twins had no psychiatric illness, no schizophrenic spectrum disorders, no first-degree relatives with a history of psychiatric illness, and no second-degree relatives with a psychotic disorder. Two patients had never been on antipsychotic medication. Six patients used atypical antipsychotic medications (mean daily dose, 458.33 chlorpromazine equivalents; SD, 206.56) and 21 patients received typical antipsychotic medications (mean daily dose, 641.67 chlorpromazine equivalents; SD, 821.71).