Familial-genetic and reproductive epidemiology of schizophrenia in rural Ireland: age at onset, familial morbid risk and parental fertility.

Among all ascertainable cases of DSM IIIR schizophrenia within an unusually homogeneous region of rural Ireland, family history information was sought from multiple sources. Morbid risk for schizophrenia among probands' first degree relatives was 6.1% and did not differ between male (6.5%) and female (5.5%) probands; risk among probands' siblings (8.3%) exceeded that among their parents (1.4%), with only 2% of male and 31% of female probands being themselves married. Both age at onset <25 and having >7 siblings were associated with elevated morbid risk, particularly among relatives of male probands (11.9% vs. 2.2% and 11.8% vs. 3.7%, respectively). Increased fertility particularly among parents of male patients with high familial-genetic loading may contribute to perpetuation of the disorder in the face of those patients' own extremely low fecundity.