Mental retardation is not a single disease and the causes may be multiple. It is a condition in an individual characterized by intellectual defect, and its consequences are social inadequacy and persistent dependency. In the majority of those affected the precise causes of the mental defect are unknown. The condition is often evident at birth or at an early age and is normally of lifetime duration. The criteria of mental retardation in different countries depend to some extent on the degree of tolerance for persons showing deviation from the normal, on the complexity of the demands society makes on the individual, and on the availability of services for the retarded. The numbers of people relying upon help or special services will depend, among other things, on the economy, traditions, and culture of the society in which they live and on the services available in the country.
The problems concerning mental retardation appear to be increasing, since the severely retarded now have a higher expectation of life where the impact of advance in medical and social care are felt, and problems of community integration of the retarded are complicated by accelerated rates of urbanization and industrialization.
It is estimated that from 1 to 4.5% of the population is mentally deficient. Mental retardation shades imperceptibly into normality in the higher ranges of the intelligence quotient. Of the persons classified as mentally retarded, about 75-85% are only mildly retarded. Those classed as moderately, severely or profoundly mentally retarded comprise about 4 per 1,000 of the age-group 10-14 years.
Mental retardation is 5 times more common in poor countries compared to rich ones.