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Commentary on the papers by Williams et al (see page 8) and Harrison et al (see page 16)
The autistic spectrum comprises four subgroups: Asperger’s syndrome (AS),1,2 and high, medium, and low functioning autism.3 They all share the phenotype of social difficulties and obsessional interests.4 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 colleagues5 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 colleagues6 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.
BEYOND COUNTING AND PREVALENCE ESTIMATES
Now we know that ASC are common. How should we understand their causes? Harrison and colleagues6 find that the 13–15 year old age group who would have received their MMR during the data collection phase were actually less numerous than the 4–10 year old age group, suggesting this high rate cannot be due to the MMR vaccine (since both age groups were exposed to the MMR). Instead they argue that these data suggest better recognition, better recording of cases, and growth of services.
In terms of causes, the consensus is that ASC have a genetic aetiology,7 which leads to altered brain development,8,9,10,11 affecting social and communication …
Footnotes
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↵* High functioning autism (HFA) can be thought as within one standard deviation of population mean IQ (that is, IQ of 85 or above); medium functioning autism (MFA) can be thought of as between one and three standard deviations below the population mean (that is, IQ of 55–84). Low functioning autism (LFA) can be thought of below this (that is, IQ of 54 or below).
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↵† This may help to explain why videos like Thomas the Tank Engine are favourites for many children with autism: there is no agentive change and almost all the non-agentive change is mechanical and linear, with close to 100% lawfulness.
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↵†† I am indebted to Nigel Goldenfeld for suggesting this connection between hyper-systemising and IQ.
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↵‡ Reduced generalisation is seen as a consequence of hyper-systemising. Systemising presumes that one does not generalise from one system to another until one has enough information that the rules of system A are identical to those of system B. Good generalisation may be a feature of average or poor systemisers, while “reduced” generalisation can be seen as a feature of hyper-systemising.
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↵‡‡ Mind blindness in this model (see fig 1) is seen as arising from twin abnormalities: the SM being set too high, such that complex systems such as the social world are hard to predict via systemising; and atypical development of empathising mechanisms14,1516 that in the normal case make it possible to make sense of the social world via an non-SM route.
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Competing interests: none declared
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Portions of this paper are taken from elsewhere. Reproduced with kind permission from Elsevier (Neuropsychopharmacology and Biological Psychiatry, in press[46])