In total, 39 participants (28.5%) met criteria for a savant skill. Cognitively, 23 individuals (17% of total sample) met criteria for one or more exceptional area of skill on the Wechsler tests. Combining the two, 37 per cent of the sample showed either savant skills or unusual cognitive skills or both—a far higher proportion than previously reported. Moreover, for reasons already noted, this estimate of 28.5 per cent for savant skills is likely to be an underestimate. It may be concluded that unusual talents are found in at least a third of individuals with autism. Theoretical models of autism and animal models will both have to account for the frequency of unusual talents as well as the presence of important deficits.
The subtest on which participants were most likely to meet the specified criteria for an area of unusual cognitive skill was block design followed by digit span, object assembly and arithmetic. On the basis of parental report, 24 individuals (26% of those for whom data were available) were rated as having a savant skill, mostly involving mathematical/calculating abilities. Generally, there was only modest overlap between individuals with savant skills based on parental report and those who had specific cognitive areas of ability. Thus, of 93 individuals for whom cognitive and parental questionnaire data were available, only eight (11.6%) met criteria for an outstanding skill on both measures. Nevertheless, that overlap is greater than the 4.8 per cent expected by chance and, among these eight individuals, the area of special skill identified by both cognitive testing and parental report was similar. All the eight individuals were reported by parents as having talents related to numerical skills (mathematics or calendrical calculations) and in all but two cases the additional cognitive area identified was either arithmetic or digit span (one of the eight individuals was non-verbal and thus could not attempt either of these subtests).
As indicated in earlier reviews (Miller 1998
), there was a sex difference (albeit statistically non-significant) in the prevalence of savant skills. Almost one-third (32%) of males showed some form of savant or special cognitive skill compared with 19 per cent of females.
(a) Theoretical implications
From a theoretical perspective, two rather different questions arise. First, why is the rate of savant skills among individuals with autism so high (see Heaton & Wallace 2004
)? On our findings, at least a quarter, but probably over a third, of individuals with autism show unusual skills or talents that are both above population norms and above their own overall level of cognitive functioning. Adequate studies comparing autism with other neurodevelopmental disorders have yet to be undertaken, but the rate of such unusual talents or skills seems particularly high in autism. Why? Second, within the realm of special talents, why (in autism) do these particularly involve computational skills, and is their basis the same as that underlying, say memory or artistic skills? There is plenty of speculation on both these questions but very little in the way of systematic testing of alternative mechanisms. Such research is much needed.
Contrary to suggestions that the presence of savant skills is associated with ritualistic behaviours or special interests (e.g. O'Connor & Hermelin 1991
), there was no evidence in the present study that those individuals with either exceptional cognitive or parent-rated skills had higher rates of repetitive or stereotyped behaviours/interests than those who did not. Thus, although all
participants showed abnormalities in this domain, the severity of such behaviours was not related to savant abilities.
Finally, the fact that savant skills are not unique to autism, but are reported in a number of other syndromes with very different cognitive and behavioural phenotypes (e.g. Prader–Willi, Smith–Magenis and Williams syndromes), also requires further exploration. However, to date, the very different assessment methods and varying definitions used make it impossible to draw reliable conclusions about the relative rates and types of savant skills in other disorders. This is clearly an area in which better standardized measures and ascertainment criteria are crucial if comparative studies on syndrome-specific savant skills are to progress.
Methodological differences between studies also limit conclusions concerning the overall rates of savant skills in ASD. Lack of clear diagnostic criteria, inadequate definition of savant skills, the age of the cohort studied (savant skills may be less apparent in younger children) and the sex ratio (cohorts with a higher preponderance of males may well contain greater numbers of savants) all contribute to different estimates of prevalence. Questions also remain about the most effective way of collecting valid information on savant skills. Cognitive data, although the most objective, are also the most circumscribed; parental reports, while more detailed, are frequently highly subjective. Detailed observational and experimental studies such as those of Hermelin (2001)
are feasible only for individual case studies or small case series. Moreover, definitions of what constitutes a skill that is truly exceptional in terms of population norms are also variable and highly unusual characteristics may not necessarily be equivalent to special skills. One individual in the present study, for example, had synaesthesia, seeing people, objects and numbers in colours, in a way that was often distressing for him. In our view, this did not meet criteria for a savant skill but drawing distinct boundaries between this and other savant abilities clearly presents difficulties.
Although the study is not without methodological problems—in particular, the fact that cognitive and parental data were collected at different times and parental information was available for a smaller proportion (73%) of the total sample—the findings are considered valid for a number of reasons. Sample size was relatively large; standardized diagnostic criteria for autism were met in all cases; criteria for savant skills were clearly defined; data were derived from two separate sources (cognitive assessment and parental information); the parental questionnaires used provided detailed descriptions of savant-type skills; and inter-rater reliability was high. The results suggest that the rates of savant skills in autism are substantial, particularly among males, and although these estimates are higher than reported by other researchers, in other ways our findings parallel those of previous studies (e.g. Bölte & Poustka 2004
). Thus, females seem less likely than males to develop savant skills, and in both this study and that of Bölte & Poustka (2004)
, the average non-verbal IQ of the savant group was over 80, consistently higher than that of the non-savant group. Indeed, in our study, no individual with a non-verbal IQ below 50 was judged as having any special talents. These data offer no support to claims that savant skills occur most frequently in individuals with autism who are intellectually impaired.
Despite the fact that savant abilities reported in the literature cover a relatively circumscribed range, the underlying cognitive mechanisms appear to vary widely. The ability to make rapid calendrical calculations (which is clearly not
simply a feat of memory; Heavey et al. 1999
; Iavarone et al. 2007
), to calculate prime numbers, to remember dates/places/events from many years in the past or recite a long list of digits backwards may all require very different cognitive skills. Similarly, a high level of expertise in art or music is very different from being able to score highly on tests of block design or object assembly. It is possible that an overarching explanation may exist for why certain individuals go on to develop any
area of exceptional skill, but understanding why these skills encompass such different areas and what the underlying basis of such skills is may require a rather different, research-based theoretical approach than adopted thus far.
Finally, there is the issue of how innate talents can be developed to form the basis of truly functional skills. In the present study, only five individuals with exceptional abilities (four related to maths and one related to visuospatial ability) had succeeded in using these skills to find permanent employment. For the majority, the isolated skill remained just that, leading neither to employment nor greater social integration. The practical challenge now is to determine how individuals with special skills can be assisted, from childhood onwards, to develop their talents in ways that are of direct practical value (in terms of educational and occupational achievements), thereby enhancing their opportunities for social inclusion as adults.