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Is autism one or multiple disorders?

From the earliest description of autism in 1943 to the present day, there has been a widely held view that the behavioural anomalies associated with the disorder occur more often together than would be expected by chance, and therefore there will be a single causal pathway that explains the non-random co-occurrence of these symptoms.1 The phenotypic variability of autism has proved to be a major stumbling block for aetiological research. The heterogeneity spans the entire range of intelligence quotients (IQs) and language abilities, as well as other behavioural, communicative and social functions. While any psychiatric condition is likely to incorporate a degree of heterogeneity, the variability in the nature and severity of behaviours observed in autism is thought to exceed that of other disorders.1,2 The variety of presentations of people with autism is described in the Box.

Major advances in aetiological research have been made over this period; most notably, the discovery from twin studies of greater concordance for autism among monozygotic (70%–90%) compared with dizygotic (0–10%) twin pairs, providing clear evidence that the disorder is, at least in part, genetic in origin.3 However, after seven decades of intense investigation, the research community is yet…