A twin study examining the effect of parity on the prevalence of psychiatric disorder.

The study compares the prevalence of current and lifetime psychiatric illness in twin pairs where one twin is more parous than the other. The main finding was that parity did not affect the prevalence of psychiatric disorder. This adds confirmation to the study of Bebbington et al. (1991) that high prevalence rates in women with children is due to an effect of marriage rather than an effect of parity. There was a diagnostic difference between the twins with the less parous twin having significantly more cases of Research Diagnostic Criteria (RDC; Spitzer et al., 1978) major and minor depression and the more parous twin more cases of generalised anxiety disorder. The life events and difficulties in the previous 12 months were also established in the whole group and events and difficulties over the whole of the subjects life for the first 20 pairs. The total number of events in the previous 12 months was significantly higher in the more parous twin of the pair, these were child-related events and not associated with an increased likelihood of current psychiatric illness. There were no differences in the number of events and difficulties during the lifetime between the first 20 twin pairs.