Two new theories of autism: hyper-systemising and assortative mating.

T he autistic spectrum comprises four subgroups: Asperger’s syndrome (AS), 2 and high, medium, and low functioning autism. They all share the phenotype of social difficulties and obsessional interests. In AS, the individual has normal or above average IQ and no language delay. In the three autism subgroups there is invariably some degree of language delay, and the level of functioning is indexed by overall IQ.* These four subgroups are known as autism spectrum conditions (ASC). Williams and colleagues searched electronic databases and bibliographies to carry out a meta-analysis of 42 studies of prevalence of autism spectrum conditions (ASC). From this, their most generous estimate was 20 per 10 000, or 0.2%. Harrison and colleagues used the ‘‘capture–recapture’’ technique in Lothian, Scotland, and their prevalence estimate was 44.2 per 10 000, or 0.44%. This corresponds to 1 child in 225. These estimates are clearly much higher than was the case in the past, where prevalence was traditionally estimated to be 4 in 10 000.