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In repsonse to a reader comment, here are data on a study of all twin pairs in the State of Missouri - approximately 1,500 children overall. "Autistic traits in the general population: a twin study: Constantino & Todd. Arch Gen Psych, 2003 (full reference on slide)
None of these children was receiving services for ASD.
This is the largest non-service sample of children I've come across. When these kids were examined, the prevalence of ASD among the boys was 1.4% (one in 71), and among girls it was 0.3% (1 in 333). This is quite close to the British data among adults (1 in 56 men, and 1 in 200 women).
All it takes is money, and the will to do it, to scale this study up, and examine a randomly selected sample of children across mutliple geographical areas.
