A persistent difficulty in identifying the etiology of ASD arises from a high level of inter-participant variability. This is evidenced by the number of studies that discuss participant sub-groups within the ASD sample, and the number of studies in which estimates of variation around the mean/median are larger in the ASD group than in the control group. In addition, although many cognitive functions appear to be impaired in those with ASD, the literature is littered with examples of non-replication, suggesting that there is no specific cognitive impairment that is consistent and universal in ASD (see also Happé et al., 2006). The data on impairments in cognitive task performance in ASD may therefore be better interpreted as arising from a pervasive and generalized impairment rather than a collection of several, different, modality, or task specific impairments. A promising candidate for such a generalized impairment is increased levels of intra-participant variability, as this would lead to increased variability between participants, and represents a parsimonious explanation for the many areas of cognition that appear to be impaired in those with ASD. Furthermore, increased
