Incidence and prevalence of a heterogeneous group such as 'people with mental retardation', however defined, are determined by the varying frequencies of a wide range of disorders of different aetiology. Some disorders arise at conception and their causes must be sought before conception; others arise in early fetal life, around birth, and in early postnatal life. New cases become progressively less frequent, with very few after 5 years of age. But mortality of abnormal fetuses is also concentrated in early fetal life, around birth, and in early infancy. Prevalence at any age is determined by both earlier inception rates and mortality rates, and is influenced by differential migration affecting the denominator population. Known prevalence also depends upon the identification rate.
These processes give rise to variation in prevalence in different communities and at different times for all degrees of intellectual impairment. This is not surprising, given the varying social determinants of population frequency of the causes of neurological impairment. These social factors (e.g. diet in neural tube defects; patterns of fertility in Down syndrome; alcohol price and supply in fetal alcohol syndrome; consanguinity in recessive genetic disorders) are most likely to offer scope for prevention. Preventive interventions have been successful for some disorders in some communities, creating another source of variation in frequency.
For lesser degrees of intellectual impairment, the many varying social processes which determine selection and labelling as 'mentally retarded' (or whatever label) create variations in 'administrative prevalence'. Variation also arises from inconsistent definitions, classifications, and measurements.
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