Estimates of prevalence vary greatly. Recent small scale, but intensive studies give higher numbers than earlier ones, this being due to the criteria for autistic disorders having been considerably widened over the years. The highest estimates for the whole spectrum, ranges from around 40 to 90 per 10 000 births, but the true figures are still being investigated. ASD affects four times as many boys as girls. Of all the developmental disorders, ASD is the most researched and validated syndrome.

Internationally, it is estimated that ASD is approximately four times more common than Cerebral Palsy and 17 times more common than Down Syndrome.

The manifestations of Autistic Spectrum Disorder, can range from a person who is severely mentally handicapped and severely autistic, through the area of lower to middle functioning with mild to moderate autistic traits. The spectrum then moves on to those affected by Asperger Syndrome, who display a good intellectual ability, but have certain prevalent autistic traits, which most definitely hamper their daily lives.

Regardless of the presentation of autism, all people with this disability are affected by a "Triad of Impairments", which manifests in the following areas of development:-

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