Main causes of major mental handicap: prevalence and epidemiology.

The prevalence of educational subnormality of a severe form (between 3 and 3.6 per thousand children of school age) and the prevalence of cerebral palsy (between 2 and 2.4 per thousand) have been fairly stable up to recent years. This stability has also applied to the relative proportions of the different major causes contributing to the handicaps. Where the ascertainment of such conditions is good, their prevalence monitored and the life expectancy of affected individuals estimated, any changes in prevalence can be used to measure the effectiveness of new forms of prevention, or alternatively to indicate the existence of new environmental hazards. Only a multi-pronged campaign against many of the recognized causes will have a substantial impact on prevalence.