Multiple factors have been identified, but a unifying cascade of events is still elusive

Autism is a developmental disability with onset in infancy. Its clinical presentation is characterised by impairments in reciprocal social interaction and in communication with others, and by a preference for repetitive, stereotyped behaviours. Our understanding of the clinical picture of autism has changed dramatically over the past decade thanks to a much greater appreciation of the possible range of behaviours seen at different ages and degrees of functioning. Another key change has been the appreciation that several closely related “disorders” exist that share these same essential features but differ on specific symptoms, age of onset, or natural history. These disorders, which include Asperger syndrome, atypical autism, and disintegrative disorder are often conceptualised as lying on a spectrum with autism (hence the popularity of the term “autism spectrum disorders”). Current estimates of the prevalence of autism are 16 per 10 000, but this estimated prevalence increases to 63 per 10 000 when all forms of autism spectrum disorders are included1—much higher than previously reported.

Along with these changes in taxonomy has been a greater understanding of …