Participants were drawn from a twin cohort consisting of all same-sextwins born in Finland between 1940 and 1957 (N = 9562 pairs) identified throughthe Finnish national twin registry. Questionnaire-based classification identified2495 MZ twins, 5378 DZ twins, and 1689 twins of unknown zygosity.24 This cohort was screened, for 1969 to 1991, for ahistory of hospitalization, medicine prescription, and work disability dueto psychiatric indication in 3 national computerized databases: the HospitalDischarge Register, the Free Medicine Register, and the Pension Register.25 These searches identified 348 index twin pairs withat least 1 co-twin with a diagnosis of schizophrenia or schizoaffective disorderand 9214 healthy pairs with no schizophrenia diagnosis in either co-twin accordingto any of the 3 sources. After exclusion because of death or emigration, atotal of 260 twins consisting of 60 (27 MZ and 33 DZ) index pairs were chosenrandomly from the available index pairs (n = 229: 50 MZ, 121 DZ, and 58 unknownzygosity), along with 70 (34 MZ and 36 DZ) demographically balanced healthypairs. Index pairs in which, on direct interview, either the proband had adiagnosis of schizoaffective disorder, affective type, or the co-twin hada psychotic disorder diagnosis were excluded (n = 1 concordant MZ pair). Healthypairs were excluded if there was a history of psychosis-related treatmentor work disability in any of their first-degree relatives or if either co-twinwas found, on direct interview, to meet diagnostic criteria for a psychoticdisorder or schizotypal, paranoid, or schizoid personality disorder (n = 15pairs: 6 MZ and 9 DZ). The selected sample of 114 pairs (n = 228) consistedof 8 pairs concordant for schizophrenia (7 MZ and 1 DZ), 51 pairs discordantfor schizophrenia (19 MZ and 32 DZ), and 55 healthy pairs (28 MZ and 27 DZ).High-resolution magnetic resonance images were acquired on 252 of the 260twins. Two images were excluded because of technical problems with the magneticresonance imaging, and 1 was excluded because of a large frontal lobe lesion,leaving 249 images on which hippocampal volumes were measured. After exclusions,the sample (n = 219) on which hippocampal volumes were gathered comprised16 twins concordant for schizophrenia (7 MZ pairs and 1 DZ pair), 94 twinsdiscordant for schizophrenia (16 MZ and 28 DZ pairs and 2 and 4 additionalMZ co-twins and DZ probands, respectively), and 56 MZ (28 pairs) and 53 DZ(26 pairs) healthy comparison individuals.