Journal of Intellectual Disability Research 372 pp –
Blackwell Science, LtdOxford, UKJIRJournal of Intellectual Disability Research-Blackwell Publishing Ltd, 5372378Original ArticleTheory of mind deficits in children with FXSK. Cornish et al.
Theory of mind deficits in children with fragile X syndrome
K. Cornish1,2,3, J. A. Burack1,3, A. Rahman1,3, F. Munir4, N. Russo1,3 & C. Grant5
1 Department of Educational Psychology, McGill University, Montreal, Canada 2 Department of Neurology and Neurosurgery, McGill University, Montreal, Canada 3 Canadian Centre for Cognitive Research in Neurodevelopmental Disorders, Montreal, Canada 4 Institute of Work, Health and Organizations, University of Nottingham, Nottingham, UK 5 School of Psychology, University of Birmingham, Birmingham, UK
Abstract revealed qualitative differences in error types (realist vs. phenomenist), suggestive of atypical development Background Given the consistent findings of theory that goes beyond general cognitive delay. of mind deficits in children with autism, it would be Conclusion The findings are discussed in terms of extremely beneficial to examine the profile of theory the teasing apart of different components of social of mind abilities in other clinical groups such as frag- cognition in order to identify syndrome-specific defi- ile X syndrome (FXS) and Down syndrome (DS). ciencies and proficiencies. Aim The aim of the present study was to assess whether boys with FXS are impaired in simple social Keywords autism, developmental psychopathology, situations that require them to understand their own Down syndrome, fragile X syndrome, social and others’ mental states – in essence: do they have cognition, theory of mind a ‘theory of mind’? Method Well-standardized tasks of theory of mind, the location change false belief task and the Introduction appearance–reality tasks were employed to examine Fragile X syndrome (FXS) is the world’s most com- whether any impairment might be specific to the mon form of hereditary intellectual disability (ID), FXS or part of a more generalized developmental with a prevalence of in male births and in deficit. female births and it is a result of silencing of a Results The results suggest that children with FXS single gene, the fragile X mental retardation- do have impairment in theory of mind that is com- (FMR) gene (de Vries et al. ; Turner et al. parable to the deficit reported in other groups with ). In recent years, it has become one of the learning disabilities such as DS. However, closer most widely researched and well-documented of inspection of the impairment between these groups genetic conditions. At a genetic level, it is now established that the FMR gene is the major con- tributor to the pathogenesis of FXS and that the key Correspondence: Kim Cornish, Canada Research Chair in Neuropsychology and Education, McGill University, McTavish issues relate to a lack of messenger RNA (mRNA) Street, Montreal, Canada (e-mail: [email protected]). and a lack or absence of the protein product of the
© Blackwell Publishing Ltd
Journal of Intellectual Disability Research 373 K. Cornish et al.•Theory of mind deficits in children with FXS
FMR gene – FMRP. The extent to which these (Fisch ; Bailey et al. ; Dykens & Volkmar discoveries explain some of the phenotypic out- ) and around –% of children with FXS meet comes in FXS are beginning to be unravelled with the diagnostic criteria for autism (Reiss & Freund the application of more finely tuned neuropsycho- ; Turk & Graham ; Dykens & Volkmar ; logical and neuropsychiatric approaches to under- Bailey et al. ). Yet, despite these commonalities, standing atypical development. In normal certain social abilities that are particularly deficient individuals there are – repeats with repeats among children with autism remain relatively intact found on the most common allele (DNA sequence in children with FXS, including recognition of emo- at FMR- gene site). Alleles with between – tion and face perception (Simon & Finucane ; repeats are called premutations and generate some Turk & Cornish ) and conversational abilities protein. When or more CGG repeats are (Sudhalter ). So while similarities between syn- present, there is hypermethylation and a subsequent dromes are present, the differences in behavioural silencing of the FMR gene. This is commonly characteristics necessitate the development of cogni- referred to as the FMR full mutation. tive and behavioural profiles of intact and impaired At a cognitive level, the syndrome presents with abilities in FXS that will facilitate comparisons mild to severe ID, severe problems of inattention and between related disorders. hyperactivity (Turk ) and uneven abilities across One way to test the association between FXS and and within cognitive domains. Relative strengths in autism would be to assess ‘theory of mind’ among language accompany relative weaknesses in visuospa- children with FXS because the deficit in ‘theory of tial cognition (Freund & Reiss ; Cornish et al. mind’ appears to underlie many of the social and ) and executive functioning, most notably for communicative impairments that are characteristic of skills that require sequential processing, short-term autism (for a review see Yirmiya et al. ). Much memory recall, or reproduction of items in a serial or of the research about ‘theory of mind’ ability relies temporal order (Jakala et al. ; Wilding et al. on false belief (FB) tasks as indicators of whether or ; Cornish et al. a). In contrast, performance not a child has mental state understanding. Children is relatively good on skills requiring simultaneous with autism show pronounced deficits in mental state rather than sequential information processing, or on understanding or the commonly used FB tasks, in face and emotion recognition (Hodapp et al. ; which one’s understanding of the event or situation Turk & Cornish ). must be differentiated from others, or appearance– At the behavioural level, certain problems associ- reality (AR) tasks, in which one must distinguish ated with FXS appear to reflect a link with autism. between the perception of an object (its appearance) For example, children with FXS show some impair- and their knowledge of it (its real identity) (Baron- ments in social and communicative functioning Cohen ). On the AR tasks, children with autism (Reiss & Freund ; Lachiewicz ; Turk & Cor- make significantly more overall errors than children nish ), two of the three domains that comprise with Down syndrome (DS). More specifically, chil- the triad of impairments that are central to the cur- dren with autism make qualitatively different types of rent diagnostic systems of autism (APA ). Within errors (phenomenist) compared with children with these domains, both groups display language delay, DS who make an equal number of phenomenist and echolalia, and perseverative speech (Hagerman et al. realist errors (see Method section for examples of ; Cohen et al. ; Hagerman et al. ), but these error types). commonalities between the groups also extend to Given the consistent findings of theory of mind attentional difficulties (Baumgardner et al. ; deficits in children with autism, it would be extremely Turk ; Munir et al. a), poor eye contact, and beneficial to examine the profile of theory of mind stereotypic movements (Cohen et al. ; Hager- abilities in other clinical groups such as FXS. Of the man et al. ). The similarities between autism few published studies on theory of mind abilities in and FXS are consistent with evidence of the co- other syndromes, an initial study by Garner et al. occurrence of the two disorders (Cohen et al. ; () reported that more children with FXS failed Feinstein & Reiss ; Bailey et al. ), because standard FB tasks than a comparison group of chil- –% of cases of autism appear to be caused by FXS dren with ID of unknown aetiology. In the present
© Blackwell Publishing Ltd, Journal of Intellectual Disability Research , –
Journal of Intellectual Disability Research 374 K. Cornish et al.•Theory of mind deficits in children with FXS
study, we extend these findings in two main ways. tings. None of the children met the ICD- criteria Firstly, we incorporate a relatively large sample size for autism. Furthermore, none of the children in any of children with FXS. Secondly, by comparing per- of the groups had sensory impairments including formance across two distinct syndromes of known hearing deficits and decreased visual acuity. Finally, aetiology we begin to delineate the specificity of none of the children were on stimulant drugs such as theory of mind deficits across neurodevelopment methylphenidate (Ritalin) for hyperactivity which disorders. might influence cognitive performance. Our objectives were to determine whether young Verbal mental age (VMA) was assessed using the males with FXS, who were screened for autism and British picture vocabulary scale (BPVS) (Dunn et al. did not receive a diagnosis, would display a specific ). The VMA for the FXS group was years and deficit in theory of mind as measured by both FB (i.e. months (range = .–., SD = months). For Sally Anne) and AR tasks, on which the typically the DS group, the VMA was years and months developing trajectory is well-documented (Frith & (range = .–., SD = months). Frith ). Male children with DS (Trisomy ) matched for chronological and verbal mental age Data collection and measures were included as the comparison group because FXS and DS represent the two most common causes of The location change false belief task ID for which aetiology is known but who often dis- The procedure for the location change FB task was play contrasting patterns of neurodevelopmental pro- consistent with that utilized by Baron-Cohen et al. ficiencies and deficiencies (Hodapp et al. ; ). Each child was introduced to two doll protag- Burack et al. ; Wilding & Cornish ). How- onists, Sally and Anne. Sally placed a marble into a ever, given the behavioural similarities between basket and then left the scene. Anne then transferred autism and FXS, we expected that the performance the marble from the basket to the box and left the on theory of mind tasks by children with FXS would scene. At the time Sally returned, the experimenter reflect a similar pattern of impairment as those seen asked the child a series of questions, ‘Where will Sally among children with autism. look for her marble?’, the reality question ‘Where is the marble really?’, and the memory question ‘Where was the marble in the beginning?’. This task was then Subjects and method repeated using a new location for the marble, so that Sample now there were three locations that the participant could point to (i.e. box, basket and experimenter’s The present study involved two groups of partici- pocket). On both trials of the task, a child scored one pants: () boys with FXS (mean chronological age point for the belief question if they pointed to the years and months; range – years) recruited location at which Sally had originally left the marble. from the UK parent support group. Diagnosis of FXS was established by DNA testing which con- The appearance–reality tasks firmed the presence of FMR- full mutation, and () boys with DS (mean chronological age years The procedure for the AR tasks was consistent with and months; range = .–., SD = months) that employed by Baron-Cohen (). After a recruited from a UK parent support group. Diagnosis warm-up procedure, each child was also adminis- of DS had previously been established by cytogenetic tered a memory pretest in which an orange filter was testing which confirmed a karyotype with a free tri- placed over a white piece of paper, thus changing it’s somy . Both syndrome groups were part of an apparent colour. The child was then asked, ‘When I extensive neuropsychological study that assessed per- take this away, will the paper look white or orange?’ formance across a range of cognitive domains (e.g. This pretest indicated that the child was able to Munir et al. a,b; Wilding et al. ; Cornish remember an object’s original colour and understand et al. b). that a filter did not permanently alter the colour of All boys with FXS and DS were receiving special an object. In order to ensure that all participants education and none were living in institutional set- would understand the AR tasks, only those who
© Blackwell Publishing Ltd, Journal of Intellectual Disability Research , – Journal of Intellectual Disability Research 375 K. Cornish et al.•Theory of mind deficits in children with FXS
passed this pretest were administered the experimen- questions are presented in Table . For Trial , tal tasks. children from the FXS group and from the DS Following the pretest, four AR tasks, with two trials group passed the false belief, memory, and reality each, were presented in random order. These tasks questions. For Trial , children from FXS group consisted of the separate manipulation of an object’s and from the DS group passed the false belief, colour, size, material, and identity. In each trial, an memory and reality questions. There were no signif- appearance and a reality question was asked and once icant differences between these two groups on the the child had answered both questions, their belief questions on either Trial or (Trial : responses were coded into correct, phenomenist, Chi-square = ., d.f. = , P = .; Trial : Chi- realist or other. So, for example, on the first of two square = ., d.f. = , P = .). For Trial , of those colour trials, a bottle of milk was shown to the child, children who passed the control memory but failed who was asked to name the object and its colour. An the belief task, were FXS and were DS. How- orange filter was then placed in front of the object ever, for Trial , of the children with FXS failed and the child was asked the appearance question the control memory questions, as presented in ‘Now what colour does the milk look?’ and the reality Table . question ‘What colour is it really?’ A correct response was that the milk looked orange but was really white, The appearance–reality tasks a phenomenist response was that the milk looked Overall, the pass rates on the AR tasks were similar orange and really was orange, and a realist response for the two syndrome groups, with children with was that the milk looked white and really was white. FXS passing three or more tasks as compared with A piece of white chalk was used for the second colour of children with DS. No significant differences trial. A similar procedure was followed for the were found between these groups on the AR distinc- remaining AR tasks: size, material and identity. tion (Chi-square ., d.f. , P .). A correct answer to a pair of AR questions was = = = A task-by-task analysis (summarized in Table ) awarded point. Each child could therefore score a revealed similar performances by both groups on maximum of points ( point per trial, on each of tasks of colour (Chi-square ., d.f. , the four tasks). If three or more tasks (i.e. colour, = = P .), size (Chi-square = ., d.f. , size, material, identity) were passed, this was = = P .), and material (Chi-square ., recorded as a pass on the AR distinction overall. = = d.f. = , P = .). All three of these tasks had a pass rate of over % across both groups. The colour task Results resulted in the highest success rate (n = ; .%) and the task producing the greatest difficulty, for both The false belief task the FXS and the DS groups, was the identity task The number of children by syndrome group who (n = ; %). Closer inspection of performance on passed both the control memory and the FB test the identity task, revealed that of the FXS group who
Ta b le 1 Performance of the children with fragile X syndrome (FXS) and Down syndrome (DS) on the false belief (FB) task test and memory questions for Trials and
FXS (n = 28) DS (n = 26)
Performance Trial 1 Trial 2 Trial 1 Trial 2
Passed FB, passed memory + reality 13 (46.4%) 12 (42.9%) 12 (46.2%) 12 (46.2%) Passed FB, failed memory + reality 1 (4.6%) 1 (4.6%) 0 (0%) 0 (0%) Failed FB, passed memory + reality 10 (35.7%) 4 (14.3%) 5 (19.2%) 3 (11.5%) Failed FB, failed memory + reality 4 (14.3%) 11 (39.3%) 9 (35.6%) 11 (42.3%)
© Blackwell Publishing Ltd, Journal of Intellectual Disability Research , – Journal of Intellectual Disability Research 376 K. Cornish et al.•Theory of mind deficits in children with FXS
Ta b le 2 Frequency of errors across the four appearance–reality dis- accurately answering the critical question of ‘Where tinction tasks will Sally look?’ after she returns. This success rate was similar to that of boys with DS and consistent Object’s Dimension Errors FXS DS with previous reports of a % accuracy rate on the FB task in children with DS compared with only a Colour Phenomenist 0 0 reported one-third of children with autism (Yirmiya Realist 0 2 et al. ). Taken together, these findings provide Size Phenomenist 2 0 additional evidence for possible qualitative differ- Realist 4 6 ences between the phenotypic outcomes of children Material Phenomenist 4 3 Realist 2 1 with FXS and autism. However, this interpretation Identity Phenomenist 3 11 needs to be treated with some degree of caution as a Realist 11 3 result of the absence, in the present study, of a com- parison group of children with autism. FXS, fragile X syndrome; DS, Down syndrome. Consistent with findings on the FB task, the per- formance of children with FXS and DS, on the AR tasks, differed from the performance previously reported in children with autism who consistently failed to make appearance–reality distinctions (Baron-Cohen ). Overall, more than two-thirds of the FXS and DS groups passed three or more tasks indicating that the ability to understand their own mental states was relatively unimpaired in these syndrome groups. However, when errors occurred they were committed predominantly on the identity task with both groups making over % of errors. Upon closer inspection of these error types, an interesting profile of differences between the FXS and DS groups emerged. Com- pared with children with DS who made predomi- Figure 1 Percentage of realist and phenomenist errors made by the nantly phenomenist errors ( % vs. %) on this FXS and Down syndrome (DS) groups on the appearance–reality task, children with FXS made predominantly realist (AR) identity task. errors (% vs. %) This tendency, of the DS group, towards phenom- enist errors has also been previously reported among made errors, .% (n ) of their total errors were = children with autism (Baron-Cohen ) and per- realist while only .% (n ) of the total errors = haps indicates a commonality between these syn- committed by the DS group were realist. This syn- drome groups. The tendency towards realist errors drome-specific difference was highly significant (Chi- among children with FXS, however, is a novel finding square ., d.f. , P .). See Fig. . = = = and suggests that, in contrast to children with DS and children with autism, the FXS child demonstrates an inability to dissociate appearance from reality, that is, Discussion they ignore the appearance of an object and instead A primary aim of the present study was to examine rely solely on real knowledge. In contrast, phenom- performance on well-standardized tasks of theory of enist errors suggest that the perceptual information mind, the location change FB task (Baron-Cohen of an object, even if it contradicts the child’s real et al. ) and AR tasks (Baron-Cohen ), knowledge of that object, overrides all other repre- among boys with FXS as compared with a group of sentations. So, in the present study, children with DS boys with DS. In the FB task, approximately half of display an inability to dissociate reality from appear- the FXS boys performed the task successfully by ance, but contrary to children with FXS, they ignored
© Blackwell Publishing Ltd, Journal of Intellectual Disability Research , – Journal of Intellectual Disability Research 377 K. Cornish et al.•Theory of mind deficits in children with FXS
their real knowledge about an object and relied proficiencies and deficiencies unique to particular instead on the changed appearance of the object. This syndrome groups. cross syndrome dissociation in error types underlines the importance of looking beyond global outcomes of cognitive and behaviour functioning and to move References towards isolating those subtle differences in informa- American Psychiatric Association (APA) () Diagnostic tion processing that point to syndrome-specific atyp- and Statistical Manual of Mental Disorders, rd edn. Amer- ical developmental trajectories. ican Psychiatric Association, Washington DC. In summary, the findings from the present study Bailey D. B., Hatton D. D., Skinner M. & Mesibov G. add to the emerging profile of social cognitive deficits () Autistic behavior, FMR protein, and develop- in children with FXS. They are important for two mental trajectories in young males with FXS. Journal of main reasons. Firstly, we provide evidence that chil- Autism and Developmental Disorders , – . dren with FXS experience theory of mind difficulties Bailey D. B., Mesibov G. B., Hatton D. D., Clark R. D., Roberts J. E. & Mayhew L. () Autistic behavior in in their ability to understand the belief and intentions young boys with fragile X syndrome. Journal of Autism of others (as represented by the FB task) at a level and Developmental Disorders , –. comparable to children with DS. These findings Bailey A., Phillips W. & Rutter M. () Autism: towards extend those of Garner et al. () and also support an integration of clinical, genetic, neuropsychological and previous efforts to examine deficits in relation to lev- neurobiological perspectives. Journal of Child Psychology els of performance reported among atypically devel- and Psychiatry , –. oping children outside of autism (Yirmiya et al. Baron-Cohen S. () Are autistic children behaviourists? ). However, the extent of the deficit in FXS is An examination of their mental–physical and appearance– reality distinctions. Journal of Autism and Developmental clearly not as severe as that previously reported in Disorders , –. children with autism suggesting a qualitative differ- Baron-Cohen S., Leslie A. M. & Frith U. () Does the ence between the two conditions. Secondly, although autistic child have a ‘theory of mind?’. Cognition , – the FXS and DS groups appeared comparable in . their ability to make AR distinctions (at a much Baumgardner T., Reiss A. L., Freund L. S. & Abrams greater level of success than that reported in autism), M. T. () Specifications of the neurobehavioural asso- upon closer inspection of our data, important ciations in males with fragile X syndrome. Pediatrics , –. differences in error types revealed subtle processing Burack J. A., Shulman C., Katzir E., Schaap T., Iarocci G. deficits between the two syndrome groups. Therefore, & Amir P. N. () Cognitive and behavioral develop- it is possible that developmental outcomes, even ment of males with fragile X and Down syndrome. when behaviourally equivalent, may differ across International Journal of Behavioral Development , –. syndromes and among typically developing Cohen I. L., Fisch G. S., Sudhalter V., Wolf-Schein E. G., children. Hanson D., Hagerman R. & Brown W. T. () Social Finally, it is also important to recognize that theory gaze, social avoidance, and repetitive behavior in fragile of mind is just one aspect of social cognition. Other X males in a controlled study. Journal of Mental Retarda- tion , –. components such as emotion and face recognition, Cohen I. L., Vietze P. M., Sudhalter V., Jenkins E. C. & eye gaze, social anxiety, and perception are also Brown W. T. () Effects of age and communication important aspects of social and communicative func- level on eye contact in fragile X males and non-fragile X tioning that need to be explored in further detail. autistic males. American Journal of Medical Genetics , Moreover, a cross syndrome comparison design that –. incorporates a longitudinal component would further Cornish K. M., Munir F. & Cross G. () Spatial facilitate the teasing apart of important differences cognition in males with fragile-X syndrome: evidence for a neuropsychological phenotype. Cortex , – and commonalities. This would be an important step . forward towards the development of precise profiles Cornish K. M., Sudhalter V. & Turk J. (a) Attention of syndrome-specific cognitive abilities and atypical and language in fragile X. Mental Retardation and Devel- developmental pathways. These profiles would serve opmental Disabilities Research Reviews , –. a clinical and educational purpose by aiding in the Cornish K., Turk J., Wilding J., Sudhalter V., Kooy F. & design of remedial programs that address the varying Hagerman R. (b) Deconstructing the attention
© Blackwell Publishing Ltd, Journal of Intellectual Disability Research , – Journal of Intellectual Disability Research 378 K. Cornish et al.•Theory of mind deficits in children with FXS
deficit in fragile X sydnrome: a developmental neuro- Munir F., Cornish K. M. & Wilding J. (a) Nature of psychological approach. Journal of Child Psychology and the working memory deficit in fragile-X syndrome. Brain Psychiatry , –. and Cognition , –. Dunn L., Dunn P., Whetton C. & Pintilie D. () British Munir F., Cornish K. M. & Wilding J. (b) A neurop- Picture Vocabulary Scale. NFER-Nelson, Windsor. sychological profile of attention deficits in young males with fragile-X syndrome. Neuropsychologia , –. Dykens E. M. & Volkmar F. R. () Medical conditions associated with autism. Handbook of Autism and Pervasive Reiss A. L. & Freund L. () FXS. Biology Psychiatry , Developmental Disorders, nd edn (eds D. J. Cohen & –. F. R. Volkmar), pp. –. Wiley, New York. Simon E. W. & Finucane B. M. () Facial emotion Feinstein C. & Reiss A. L. () Autism: the point of view identification in males with fragile X syndrome. American from fragile X studies. Journal of Autism and Developmen- Journal of Medical Genetics , – . tal Disorders , –. Sudhalter V. () Areas of similarity and difference between fragile X and autism. Paper presented at the Fisch G. S. () Is autism associated with the fragile X Fifth International Fragile X Conference, Portland, OR. syndrome? American Journal of Medical Genetics , – . Turk J. () Fragile-X syndrome and attentional deficits. Journal of Applied Research in Intellectual Disabilities , Freund L. S. & Reiss A. L. () Cognitive profiles asso- –. ciated with the fragile X syndrome in males and females. American Journal of Medical Genetics , –. Turk J. & Cornish K. M. ( ) Face recognition and emotion perception in boys with FXS. Journal of Intellec- Frith U. & Frith C. D. ( ) Development and neurophys- tual Disability Research , –. iology of mentalizing. Philosophical Transactions of the Turk J. & Graham P. () Fragile X syndrome, autism Royal Society of London, Series B, Biological Sciences , and autistic features. Autism , –. –. Turner G., Webb T., Wake S. & Robinson H. () Prev- Garner C., Callias M. & Turk J. () Executive function alence of fragile X syndrome. American Journal of Medical and theory of mind performance of boys with fragile-X Genetics , –. syndrome. Journal of Intellectual Disability Research , –. de Vries B. B., van den Ouweland A. M., Mohkamsing S., Duivenvoorden H. J., Mol E., Gelsema K., van Rijn M., Hagerman R. J., Jackson A. W., Levitas A., Rimland B. & Halley D. J., Sandkuijl L. A., Oostra B. A., Tibben A. & Braden M. ( ) An analysis of autism in fifty males with Niermeijer M. F. () Screening and diagnosis for the the fragile X syndrome. American Journal of Medical fragile X syndrome among the mentally retarded: an epi- , Genetics – . demiological and psychological survey. Collaborative Hagerman R. J., Murphy M. A. & Wittenberger M. D. fragile X study group. American Journal of Human Genet- () A controlled trial of stimulant medication in chil- ics , –. dren with the fragile X syndrome. American Journal of Wilding J. & Cornish K. () Attention and executive Medical Genetics , – . functions in children with Down syndrome. In: Focus on Hodapp R. M., Dykens E. M., Ort S. I., Zelinsky D. G. & Down syndrome research (ed. J. A. Malard). Nova Publish- Leckman J. F. () Changing patterns of intellectual ing, Hauppauge, NY. strengths and weaknesses in males with FXS. Journal of Wilding J., Cornish K. & Munir F. () Further delinea- Autism and Developmental Disorders , –. tion of the executive deficit in males with fragile-X syn- Hodapp R. M., Leckman J. F., Dykens E. M., Sparrow drome. Neuropsychologia , –. S. S., Zelinsky D. G. & Ort S. I. () K-ABC profiles Yirmiya N., Erel O., Shaked M. & Solomonical-Levi D. in children with fragile X syndrome, Down syndrome, () Meta-analyses comparing theory of mind abilities and nonspecific mental retardation. American Journal of of individuals with autism individuals with mental retar- Mental Retardation , –. dation, and normally developing individuals. Psychological Jakala P., Hanninen T., Ryynanen M., Laakso M., Partanen Bulletin , –. K. & Mannermaa A. &. Soininen H. () Fragile-X: Yirmiya N., Solomonica-Levi D. & Shulman C. () The neuropsychological test performance, CGG triplet repeat ability to manipulate behaviour and to understand manip- lengths, and hippocampal volumes. Journal of Clinical ulation of beliefs: a comparison of individuals with autism Investigation , –. mental retardation, and normal development. Develop- Lachiewicz A. M. () Abnormal behaviors of young girls mental Psychology , – . with fragile X syndrome. American Journal of Medical Genetics , –. Accepted August
© Blackwell Publishing Ltd, Journal of Intellectual Disability Research , –