Cause of Mental Retardation

Lewis (929) distinguished two kinds of mental retardation (as it was then called): subcultural, i.e. the lower end of the normal distribution curve of intelligence in the population, and pathological, i.e. retardation due to specific disease processes. In a study of the 1280 mentally retarded people living in the Colchester Asylum, Penrose (938) found that most cases were due not to a single cause but to an interaction of inherited and environmental factors. Subsequent research has confirmed that learr;'ing disability has multiple causes. This is particularly true for mild learning disability which is usually due to a combination of genetic and adverse environmental factors, and which is more common in the lower social classes.

Among the severely learning disabled, physical causes are found in 55-75% of cases (Broman et al. 1987). Prenatal causes predominate, of which the most frequent are idiopathic cerebral palsy, Down's syndrome, and fragile X syndrome; the proportion with fragile X syndrome is increasing steadily, accounting for 10-15 %. Another three causes - other chromosomal anomalies, single-g(we disorders, and idiopathic epilepsy - each account for 5-10% of cases.

It should be noted that increasing success in identifying specific causes of severe learning disability does not remove the need to consider all the additional social and other factors in every case. Until recently, severe learning disability was thought to be evenly distributed in the population; now it is known to be more common in the lower socioeconomic groups, possibly because preventive measures have been less effective in this group.

Whilst the aetiology of learning disability in developed countries is predominantly due to genetic and perinatal causes, postnatal factors (hypothyroidism, infection, trauma, toxicity) are important in developing countries. For a review of aetiology see Scott (1994), Deb and Ahmed (2000), and Kaski (2000).

Social factors - one of the cause of mental retardation

Studies of the general population suggest that factors in the social environment may account for variation in IQ of as much as 20 points. The evidence comes from two kinds of enquiry. The first is epidemiological. Low IQ is related to lower social class, poverty, poor housing, and an unstable family environment. Such social factors may be the effects of low intelligence and do not necessarily exclude a genetic cause. Thus learning-disabled people might drift into an adverse social environment and bring up their children there. Follow-up studies from birth to adolescence have found marked differences in measured IQ according to psychosocial predictors (Sameroff et al. 1987).

The second source of evidence 'comes from . attempts to enrich the environment of deprived children in special residential care (O'Connor 1968) and to provide special education. In one early experiment, children from large and unsatisfactory institutions were transferred to small well staffed children's homes or given more stimulating education. Twenty years later they were found to have higher IQs than those who, as children, had remained in their original institutions (Skeels 1966). More recent studies have confirmed that well-planned and prolonged intervention can be beneficial for socially deprived children (Garber 1988).