Epidemiological surveys of ASD: advances and remaining challenges.

Recent worldwide epidemiological surveys of autism conducted in 37 countries are reviewed; the median prevalence of autism is .97% in 26 high-income countries. Methodological advances and remaining challenges in designing and executing surveys are discussed, including the effects on prevalence of variable case definitions and nosography, of reliance on parental reports only, case ascertainment through mainstream school surveys, innovative approaches to screen school samples more efficiently, and consideration of age in interpreting surveys. Directions for the future of autism epidemiology are discussed, including the need to systematically examine cross-cultural variation in phenotypic expression and developing surveillance programs.