Local rates of inbreeding and severe mental subnormality in N. Ireland

The prevalence of severe mental subnormality in N. Ireland has increased at a relatively high rate during the last century, and the present level is higher than that in the rest of the U.K. It has been suggested that changes in the population structure have had adverse genetic consequences which, along with an increase in the level of inbreeding in certain areas, have given rise to much of the increased prevalence of subnormality. The rate of severe subnormality is not geographically uniform, and there are many examples of townlands with very high rates in close proximity to ones with rather low rates. This provokes a test of the hypothesis that variation in subnormality rates is underpinned by differences in inbreeding levels. Two pairs of townlands, one from the north and one from the south of the province, were chosen for study. One townland in each pair had a strikingly high rate of subnormality. The technique of isonymy was used to calculate the inbreeding coefficient (F) over 5 consecutive 25-yr periods from 1849 to 1973. There were consistently higher inbreeding levels in the towns which exhibited high rates of subnormality. The findings are discussed in relation to similar research in this area, and suggestions for future lines of enquiry outlined.