Severe mental handicap in Northern Ireland.

A retrospective study was made of all individuals with severe mental handicap (IQ less than 50) in Northern Ireland who are resident in institutions and in the community. Some 4,701 individuals were ascertained and the annual prevalence rate based on the period 1950-69 was 3.67 per 1,000 live births. Classification of individuals by cause into genetic, environmental and unknown groups shows an increase in the genetic group and a decline in the unknown group. This is thought to be due to increases in medical knowledge rather than to any real biological change. The annual prevalence rates for the years 1950-69 exhibit no definite decline. The most common causes of severe mental handicap are Down syndrome, conditions associated with pregnancy and delivery and the recurrence of severe mental handicap in children born into families who already have one affected person with an IQ less than 40 of unknown aetiology.