Epidemiology of neurodevelopmental disorders in children.

The epidemiology of mental and behavioural disorders is considered in comparison with spina bifida, chromosomal anomalies and brain tumours. Descriptive epidemiology is important not only in assessing the frequency of neurodevelopmental disorders, thereby aiding planning of service provision, but also because variations by geographical area, over time, and by personal characteristics provide clues regarding etiology. The value of the latter application is exemplified by research on spina bifida and other neural tube defects (NTDs). The descriptive epidemiology of mental and behavioural disorders has been less investigated. The descriptive epidemiology of NTDs suggested that diet might be of etiological importance. Analytical epidemiologic investigation proceeded by testing dietary hypotheses in case-control and cohort studies. Subsequently, folate supplementation was shown to reduce recurrence risk in a randomized controlled trial. The analytical epidemiology of other neurodevelopmental disorders is less well understood. Study design issues are discussed in relation to mental and behavioural disorders.