About 70 per cent of mild cases belong to the 'non-specific' or 'subcultural' group, in which handicap results from a poor genetic intellectual endowment combined with a physically, educationally, and/or emotionally deprived upbringing. Mild LD is associated with poverty, overcrowding, large family size, and family disruption. Child abuse is an important but often unrecognized factor.

About 30 per cent of mild cases, and nearly all severe cases, result from a specific organic pathology. About 35 per cent of severe cases have a gene or chromosome abnormality, Down's syndrome being most frequent, and about 65 per cent have acquired brain damage. Specific causes can be classified as follows:

• chromosome or gene abnormalities, as in Down's syndrome and phenylketonuria

• primary maldevelopments of the brain of obscure aetiology, such as spina bifida with hydrocephalus, and microgyria

• prenatal factors affecting the mother, such as rubella, cytomegalovirus, toxoplas-mosis, syphilis, radiation, malnutrition, alcoholism, and teratogenic drugs

• perinatal damage, as caused by birth trauma, anoxia, rhesus incompatibility, or extreme prematurity

• post-natal damage, as caused by encephalitis, meningitis, lead poisoning, or head injury.

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