Geographical variation in rate of schizophrenia in rural Ireland by place at birth vs place at onset.

This study examined geographical variation in rate of occurrence of schizophrenia by place at birth vs place at onset, among a rural Irish catchment area population of unusual stability and socioeconomic homogeneity. Within a catchment area of 21 520 persons, all cases of schizophrenia were sought using current inpatient and outpatient records and key informants active in the community. Suspected cases were interviewed personally and diagnosed using DSM-III-R criteria. Place at birth and place at onset of psychosis were specified among the 32 District Electoral Divisions constituting the study region. For the 72 cases ascertained, an unremarkable overall prevalence rate/morbid risk obscured substantial and significant geographical variations therein between District Electoral Divisions. Particularly after controlling for high-density families, men demonstrated prominent geographical variation both by place at birth and by place at onset, with most men remaining unmarried and becoming ill at their place of birth; conversely, women demonstrated prominent variation by place at birth but more limited variation by place at onset, despite more frequent transitions from the parental home to the marital home before onset. Even when cases changed their location before the onset of psychosis, geographical variation in rate of occurrence of schizophrenia remained associated more strongly with factors related to the place of their birth.