Dimensions of psychosis in affected sibling pairs.

Factor analytical studies of schizophrenia symptoms have consistently suggested three or more symptom dimensions, but it is not known whether any of these dimensions have a genetic basis. The purpose of this study was to investigate to what extent the dimensions show familial aggregation. Symptom ratings were made using the SAPS and SANS and the OPCRIT checklist on the members of 109 sibling pairs with DSM-IV schizophrenia or schizoaffective disorder. Factor analyses were performed on the ratings of both instruments, and correlations were made of within-pair factor scores. Analyses were also performed on the 89 pairs in which both members had a diagnosis of schizophrenia. Factor analysis of SAPS and SANS ratings resulted in positive, negative, and disorganization factors; analysis of OPCRIT ratings resulted in positive, negative, disorganization, and first-rank delusion factors. Only the disorganization dimension showed significant within-pair correlations, but these were of modest size and not significantly greater than the correlations for the other dimensions. None of the dimensions showed sufficient familial aggregation to suggest that they are close markers of genetic or common environmental factors that contribute liability to schizophrenia. They may be weakly associated with such factors and with factors that do not contribute liability to schizophrenia but do influence the form taken by the illness.