These notes are a summary of one (major) paper which has explored whether or not Asperger Syndrome can be seen as a unique diagnostic category and differentiable from high functioning autism.

The current conclusion is that the evidence would not support such a differentiation, although research outcomes may yet emerge to the contrary.  The need for caution is reinforced by the finding of common methodological questions and criticisms.

The advice is for the use of the term “autistic spectrum disorder” unless or until the viability of Asperger Syndrome as a diagnostic category is established.


(These notes would usefully be read in conjunction with those entitled  “Asperger Syndrome and Autism : Differentiation and Overlap” - May 2004)


M.J.Connor                                                                                                August 2004




The review of existing evidence by Macintosh and Dissanayake (2004) to determine whether Asperger Syndrome can be meaningfully differentiated from autism was undertaken given the continuing need to explore this issue because of possible implications for aetiology or diagnosis as well as for planning interventions.


The authors note that the validity of Asperger Syndrome has not yet been established; and that there is much similarity and overlap between this syndrome and autism in terms of core impairments in communication, imagination, and socialisation. However, if the two conditions are shown to be distinct in terms of aetiology and diagnostic characteristics, there may be a greater probability of early identification and even preventative measures, alongside an enhanced capacity to predict longer- term outcomes.


The existing approaches used to examine whether the two conditions are separable have fallen into two categories. 

Firstly, there has been the exploration of subtypes via cluster analysis of signs and symptoms; and, secondly, the close comparison of individuals who have been placed in one or other of these diagnostic categories.




The process of sub-type analysis involves identifying participants who have been diagnosed with one form of other of pervasive developmental disorder, and collecting data on a range of relevant areas of functioning thus to determine if sub-groups can be observed and differentiated.


Various studies are cited, such as Myhr 1998, which have examined participants covering a wide range of ages and levels of cognitive and communicative functioning, and have suggested the possible existence of between 2 and 12 sub-types.

However, a fairly consistent conclusion has been that differences between groups seem to be largely a matter of the severity of the symptoms and of the level of intellectual ability and adaptive functioning.  Few attempts have been made to examine the apparent similarity between high functioning autism and Asperger Syndrome.


Only two studies have been identified in respect of comparisons between these two categories.


Firstly, Szatmari et al (1989) completed a cluster analysis using 28 children with an Asperger diagnosis and 25 children with high functioning autism.

The children were matched on full IQ and classified into two or three clusters according to parental information about performance in socialisation, communication, and imagination. 

Where two clusters were used, the dichotomy was held to be a matter of severity of deficits in each area.  However, where three clusters could be discerned, a “mixed” group was identified, resembling the autism group in the nature of language and imagination deficits, and resembling the Asperger group in socialisation deficits.


These findings were taken as support for the spectrum concept of autistic disorders, with differences between subtypes quantitative rather than qualitative; although the conclusion should be treated with some caution in the light of limited description of the sampling methods, and of the timing of this study before there were available agreed and published criteria for Asperger Syndrome which may reduce the generalisability of the finding to children diagnosed by current classification systems.


The second study was completed by Prior et al (1998) using a sample of 135 participants identified with high functioning autism, Asperger Syndrome, or unspecified pervasive disorder.  Intellectual levels were controlled, and data were gathered from parents in respect of developmental and familial history, and the triad of core impairments.

Again, the group differences appeared attributable to variation in symptom severity rather than to actual differences in symptom profiles; and the conclusion held that Asperger Syndrome could be seen as part of the autistic spectrum.

The methodological problem here was that the Asperger group was found also to meet diagnostic criteria for autism according to DSM-IV, hence some confounding of findings due to possible sample selection problems.


In any event, Macintosh and Dissinayake summarise this area by holding that sub-type analysis have not offered any real insight into the relationship between autism and Asperger Syndrome, and it cannot be established whether individuals with high functioning autism could equally well be diagnosed with Asperger Syndrome.


Comparative Studies


Language and Communication


According to existing diagnostic systems, autism and Asperger Syndrome are differentiable in terms of language development.  Delays or disorders in language areas are required for a diagnosis of autistic disorder, while one diagnostic criterion for Asperger cases is the absence of significant language delay.


Nevertheless, there exist data from a number of investigations (eg  Eisenmajer et al 1996) which indicate that some children whose symptoms largely support an Asperger diagnosis have experienced significant language difficulties. 

Meanwhile, it may be so for the majority of cases, but not all children diagnosed with autism have a severe language delay.


One common emphasis in exploratory studies has been upon pragmatic skills and weaknesses. 

For example, Fine et al (1991) found that children with high functioning autism could be differentiated from those diagnosed with Asperger Syndrome in terms of their poor use of intonation in conversation.

Other findings have included a weakness in referencing during conversation among both groups, but a typically greater level of difficulty among the autistic group in linking current utterances to what has gone before.


Further, there is some converging of evidence that echolalia and pronoun reversal are more common in an autistic group compared to an Asperger group, although few differences have been reported by parents in the frequency of initiating speech, the use of repetitive language, or using/understanding non-verbal forms of communication.


Methodological problems again have to be acknowledged, both in terms of possible bias from a  reliance upon retrospective information from parents and of the timing of such studies before the development of formal diagnostic criteria for Asperger Syndrome.


However, further converging evidence has it that echolalia is less common among Asperger samples during the pre-school period; but inconsistencies are revealed when it comes to parental reports of the use of repetitive speech in  the two groups from pre-school onward.


The use by Ghaziuddin and Gerstein (1996) of taped examples of speech, evaluated by speech therapists unaware of the diagnostic label attached to the children in question, suggested that pedantic speech was more common among adolescents with Asperger Syndrome.  This finding appears to fit with other evidence that, with increasing age, fewer differences between the two groups are noted in respect of communication difficulties and a greater similarity gradually emerges.


Meanwhile, Ozonoff et al (2000) used groups established according to strict DSM-IV criteria and found that they could be differentiated on the basis of the age at which words were first uttered (according to parental reports). 

Further, the autistic group showed more severe language dysfunction during the pre-school years, including a greater probability of delayed acquisition of speech, impairments in two-way conversation, and stereotyped or repetitive language style.


However, when the children were into the primary school period, many of the earlier difficulties which had differentiated the two groups in favour of the Asperger group were no longer apparent.  The only consistently differentiating area was performance on tests of expressive language where the Asperger group produced superior results.


This kind of pattern was also identified by Gilchrist et al (2001) who found that echolalia and pronoun reversal were more common among children with autism, and that these children also showed delays or anomalies in their use of gesture.  Similarities with Asperger children were noted in regard to verbal rituals, stereotyped utterances, or inappropriate questioning.

By adolescence, the only observable difference between the two groups was that the individuals with Asperger Syndrome were more willing to engage in “social chat”. 


The conclusion drawn by Macintosh and Dissanayake is that communication impairments are evident in both groups and they provide little basis for differentiation, albeit with the caveat about relying upon retrospective parental information. 

The general findings would suggest that the extent to which language and communication difficulties can differentiate the two groups varies as a function of age.


Cognitive and Neuropsychological Profiles


There is some suggestion of differential areas of intellectual strengths and weaknesses among children with autism compared to those with Asperger Syndrome.


For example, Ehlers et al (1996) found that individuals with autism often showed relative strengths in visual and spatial areas or in perceptual reasoning skills, with weaknesses in verbally-mediated knowledge.  The opposite pattern typically applied to individuals with Asperger Syndrome. 

However, it was acknowledged that these differences may apply to groups, but there is much individual variation.


There is also evidence that a higher non-verbal IQ than verbal IQ is typical of autism, while the reverse applies to Asperger Syndrome.


Meanwhile, Iwanaga et al (2000) used DSM-IV criteria to diagnose and classify participants and found some support for the view that preschool children with high functioning autism have better visuo-motor skills and Asperger children have better verbal abilities, so that, for example, the autistic group commonly had difficulty in following directions and repeating instructions.


Nevertheless, Macintosh and Dissanayake note from their ongoing review that these differences may be common but are not wholly consistent; and there are some methodological issues such as a frequent failure to match participants on actual and mental ages alongside the somewhat broad criteria used to identify Asperger Syndrome.

They go on to argue that the decision either to retain or to reject the criterion of no significant language delay or disorder in Asperger Syndrome may well determine whether or not group differences are found.  When this criterion is used, it is more likely that individuals with autism and those with Asperger Syndrome will be differentiated on their patterns of cognitive abilities.  When this criterion is not used, few differences will be found.


Manjiviona and Prior (1999) compared cognitive profiles of samples drawn from the two groups with and without the use of the “absence of language problem” criterion for the Asperger individuals. 

They found no consistent differences between the groups on verbal or non-verbal subtests of standard intelligence tests.  There was also found to be much variability at the individual level, with no consistent areas of strength or weakness identified for either group.  The main difference between groups was the higher full-scale IQ among Asperger cases because of the better verbal performance.


The overall conclusion from such findings based upon formal diagnostic criteria is that no particular profile of cognitive abilities is characteristic of either disorder.

(The authors also conclude from what studies exist that there appear to be no differences between Asperger Syndrome and autism in “Executive Function” .. viz, the integration of various abilities required for planning complex behaviour, including managing impulses, problem solving, and mental flexibility.)


Processing of Global and Local Stimuli


The matter of interference between global and local stimuli was investigated by Rinehart et al (2000) using samples of young people with high functioning autism and Asperger Syndrome. 

In the task, the participants were presented with a sequence of numbers on a computer screen. The numbers were composed of small digits such as       4

                                                                                                                                                          4  4


                                                                                                                                                                       and these smaller digits were either the same as, or not the same as, those in the given number.

No differences were found between the two groups in the errors in identifying the large numbers associated with the interference from local detail. 


Such results were interpreted as further support for the view that autism and Asperger Syndrome are part of the same continuum.


Social-Cognitive Abilities


Some studies have focused upon first- and second-order Theory of Mind skills, which involve, respectively, the understanding that other people can hold inaccurate beliefs and act according to them, and that individuals may act according to their inaccurate perception about another person’s opinion or belief even if it is true.


Results from some studies cited by Macintosh and Dissinayake suggest that both types of Theory of Mind deficiencies are common to autism and Asperger Syndrome; while the findings from some other studies suggest that these deficiencies are less characteristic of Asperger Syndrome and that this difference may be one means of distinguishing between the two disorders.


One practical problem of such studies is the possibility that differences in Theory of Mind abilities may simply be a matter of a failure to match participants on verbal IQ given the strong correlation found between verbal skills and Theory of Mind skills.


Motor Skills


Evidence here is largely a matter of parental reports of motor milestones and current motor functioning. 

Szatmari et al (1989- op cit) interviewed parents about the children’s acquisition of various skills such as dressing, tying shoelaces, using pencils, or using a knife and fork. It was found that children with autism could not be differentiated from those with Asperger Syndrome in their development of such skills, with both groups showing delay in their acquisition. 

However, one problem was the lack of a clear definition of motor clumsiness along with the use of a non-standardised measure of motor abilities.


Other parental evidence cited highlighted no differences in parental reports of when children in one or other group first sat up or first crawled, although the onset of walking appeared relatively delayed among children with autism. 

However, there was again the possibility that results were confounded by inadequate diagnostic separation of the two groups in the first place.


Gillberg (1989) assessed participants on a standardised test of motor skills and found that gross motor clumsiness was more common among children with Asperger Syndrome … although this conclusion was questioned on the grounds of no clear definition of clumsiness.  

Nevertheless, Klin et al (1995) completed a review of evidence gained from children’s records, and held that Asperger children were much more likely to have a history of fine and gross motor difficulties. 

It was suggested that this difference may be explicable in terms of the diagnostic criteria adopted whereby children were only placed in an Asperger group if there was a history of motor difficulty; but the group differences in motor skill were found to persist once this criterion for selection was removed.


The work of Rinehart et al (2001) used a task involving a motor response according to given visual stimuli and found that both groups showed adequate skills in the execution of the motor response, but delays in preparing for the  response.


However, the two groups differed in the nature of the preparation deficit.  The Asperger group typically showed a deficit in the phase of organising the motor response while the autistic group typically showed a lack of anticipation … possibly reflecting between group differences in the functioning of the fronto-striatal region of the brain.


There is also a suggestion that Asperger children may be more susceptible to motor problems in the early years, but, with age, the difference in prevalence of motor dysfunction decreases between the two groups.


Aetiology and Biological Factors


The data available in this area are described as limited, but what evidence does exist

(according to the search by Macintosh and Dissinayake) has suggested that there are no between-group differences in the occurrence of pre-natal or peri-natal trauma, or in the occurrence of various medical disorders during pregnancy or during early infancy.


Meanwhile, there is converging evidence for a genetic relationship between autism and Asperger Syndrome as illustrated by the over-representation of males in both disorders, the increased incidence of autism in the families of individuals with Asperger Syndrome and vice-versa, and the increased comorbidity of psychiatric conditions like affective disorders, attentional weaknesses, obsessive-compulsive disorder, and Tourette Syndrome in both groups and their families.


It is concluded that, on present evidence, there is little by which to differentiate the two groups on aetiological or biological factors.




Based upon a limited number of population studies, it has been proposed by Gillberg and Ehlers (1998) that Asperger Syndrome may well be much more common than high-functioning autism.  Autism is variously estimated to have a prevalence rate of somewhere between 7 and 16 cases per 10,000 children, with Asperger Syndrome affecting between 3 and 7 in every 1000 children.


Szatmari et al (1989-op cit) found no significant differences (according to parental reports) between the ages at which the symptoms of autism or Asperger Syndrome were first identified, although there was a discernible trend towards earlier onset in autism.


With regard to outcomes, there seems to be a general view that individuals with Asperger Syndrome have a better prognosis than those with high functioning autism.

Support for this view emerges from the observations of higher achievement among individuals with Asperger Syndrome in terms of self help and independence skills and in social interaction, and of their lesser need for specialist educational provision.


However, one study – Tonge et al (1999) – found that adolescents and young people with Asperger Syndrome were reported by parents as having more social difficulties than those with high functioning autism …. possibly because of a lack of specificity of diagnostic criteria for Asperger Syndrome (although it may also reflect a lack of awareness of the difficulties associated with Asperger Syndrome on the part of significant others and the inappropriate expectations that may be applied).


Meanwhile, there is some evidence for a greater co-morbidity of some conditions among Asperger cases, notably anxiety or attentional weaknesses, although evidence is equivocal.  Tonge et al (op cit) attributed a high level behavioural or emotional disturbance to both groups, but the parents of the young people tended to attribute more anxiety and anti-social behaviour to those with Asperger Syndrome.


Restricted or Repetitive Behaviours and Interests


Ritualistic behaviour, impaired imagination, and restricted interests are seen as core deficits in both disorders, although Satzmari et al (op.cit) gained parental information to the effect that a greater percentage of individuals with high functioning autism than with Asperger Syndrome showed bizarre pre-occupations, and the autistc individuals were also more likely to have lacked imaginative play or to have shown resistance to change.


Meanwhile, there is some evidence that circumscribed or odd pre-occupations may only develop in late childhood and adolescence, so that the Szatmari et al conclusion may have been an artefact of the use of a sample of young adults among whom there would have been the opportunity for such symptoms to emerge.


There is a further problem in that while, for example, Gillberg (op.cit) found that most of the children within an Asperger sample were said to show intense pre-occupations, it may be that this reflects a kind of circular argument based upon the probability that the symptom of circumscribed interests is a major criterion for allocating an individual to the Asperger category in the first place.


There does seem to be agreement that individuals with high functioning autism of primary school age do show greater deficits in imaginative play and more intense resistance to change.  Among older individuals, the intense and circumscribed interests are more common in the Asperger group.


On a similar theme, Ozonoff et al (op cit) found that differences between groups in restricted and stereotyped behaviours were greater in the preschool years than at subsequent ages and stages.  This accords with other findings that, with age, stereotyped behaviours or unusual attachments discriminate less between the groups.


Social Behaviour


Again, it is noted that most or all of the data in this area are the result of parental or teacher reports.


Szatmari et al (op.cit) examined the issue of responsiveness among children between birth and five years (among children who were aged between eight and eighteen years at the time of the study), and noted that Asperger children were more likely to have been socially responsive to carers and other adults, to have shared their interests, and shown an affection towards and interest in their peers.  No differences between groups were noted in reports of the extent of social interaction as reflected in eye contact or initiation of conversation. 


 A follow up study by the same team of researchers, using samples of children identified by current and more formal diagnostic criteria, demonstrated that both groups showed deficits, compared to normally developing children, on most dimensions of social and communicative skills, and any differences were in terms of greater social interactive skills on the part of the Asperger sample.


Macintosh and Dissinayake describe one striking outcome in that the Asperger children may have been the more socially aware and responsive, but this did not result in any greater involvement in social play or peer friendships than is typical among the children with autism.  The Asperger children may show a greater interest in or desire for affiliation, but do not have a greater capacity for making and maintaining friendships.


There is also evidence that fewer differences in social competence are evident between the two groups with increasing age.




Overall, the current authors suggest that the existing evidence hardly supports Asperger Syndrome as a unique syndrome and separable from high functioning autism.  The evidence is not sufficient either to support or refute such a view, and there is the complication of methodological problems among many of the studies that have been completed.


Macintosh and Dissinayake go on to conclude that there are very few qualitative differences between high functioning autism and Asperger Syndrome, with most symptoms and associated features overlapping to some extent.


Further, what differences have been identified appear to be the more pronounced during the infant years than during middle or later childhood; and there is the possibility that some differences may be an outcome of the circularity in categorising one group on the basis of some particular symptom and then finding that this group is more marked than the other group in the prevalence of this symptom !


The view is, therefore, that the absence of any real qualitative differences reinforces the concept of a spectrum, and of the placing of Asperger Syndrome and high functioning autism on a continuum.

However, caution is expressed to the effect that a clear consensus has yet to be reached so that it would still be premature to end the debate about the sameness or separability of the two disorders.


Reference is made to the school of thought which suggests that differentiability may prove to be a matter of diverging courses … viz … in the early years, the more advanced language skills of the Asperger children place them on a different developmental course than children with marked deficits in communication which will be linked with social and other areas of need.  With time, some of this latter group develop speech and communication skills, and shift to the developmental path relevant to the former group, and the mutual skills become less differentiable.


The final advice is to regard as premature the use of Asperger Syndrome as a separate diagnostic category, and to use, instead, the broader category of “autistic spectrum disorder”.


                        *          *          *          *          *          *         *


M.J.Connor                                                                                                August 2004








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This article is reproduced by kind permission of the author.

© Mike Connor 2004.

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