Points of Interest
• Approximately two thirds of children with autism spectrum disorders have co-occurring intellectual disabilities, and autism is also associated with several genetic syndromes that involve intellectual disabilities.
• Persons with intellectual disabilities, including those with specific genetic etiologies, are underrepresented in autism research, often due to specific exclusionary criteria.
• Research findings from studies of individuals with autism and high IQ may not necessarily extend to lower functioning individuals with autism spectrum disorders.
• Researchers must more explicitly justify their reasoning for excluding individuals with low IQ from their research programs.
• Many research designs could be adapted to also include people with intellectual disabilities, as exemplified in phenotypic studies of persons with genetic syndromes with or without intellectual disabilities.
• Individuals with autism and low IQ or with specific genetic syndromes need to be included in research to further our understanding of the causes and mechanisms associated with autism.
This chapter presents a cautionary tale about the impact of the broader definition of autism on how investigators are conducting their research. Diagnostically, the autism spectrum has expanded to include milder types of autism that are not necessarily associated with intellectual disabilities. At the same time, the inclusionary criteria for autism studies have narrowed to primarily include persons with high IQs, to the exclusion of those with autism and low IQs or with co-occurring syndromes or neurological disorders.
In considering this broader diagnostic scope and more narrow focus on higher IQ individuals, it is important to note that intellectual disabilities (previously termed mental retardation) have long been associated with autism spectrum disorders. The DSM-IV-TR (APA, 2000) notes that up to 75% of children with ASD have co-occurring intellectual disabilities, but for many reasons, this estimate may no longer be accurate. This chapter provides a brief review of the prevalence of intellectual disabilities in ASD, and although not exhaustive, the review highlights the co-occurrence of ASD and intellectual disabilities, and factors that complicate these prevalence studies.
Beyond intellectual disabilities per se, autism is also associated with specific genetic etiologies, and many of these syndromes also involve intellectual disabilities. A growing body of literature now describes how autism spectrum disorder (ASD) is manifest in people with such diagnoses as tuberous sclerosis, Fragile X, Prader-Willi, Angelman, and other syndromes. Syndromes that involve both ASD and intellectual disabilities provide unique windows into genetic and neurobiological factors associated with autism (Dykens, Sutcliffe, & Levitt, 2004). Even so, having a known etiology is typically an exclusionary criterion for most autism studies.
Indeed, despite the robust associations between intellectual disabilities and ASD, persons with low IQ or with genetic syndromes are increasingly excluded from research in ASD. We provide data in support of this disquieting research trend, as reflected in a review we conducted of published, peer-reviewed studies. Why are children or adults with ASD and intellectual disabilities, with or without a co-occurring genetic syndrome, increasingly excluded from published ASD research?
Decades ago, studies in autism relied primarily on those with intellectual disabilities, as this reflected the conceptual framework of autism at that time. The pendulum now appears to have swung in the opposite direction, whereby persons with intellectual disabilities are routinely excluded from study protocols. The chapter discusses several possible reasons for this trend, including arguments both for and against including those with low IQs in autism research. The chapter concludes with recommendations that facilitate the inclusion of those with intellectual disabilities into future research in ASD.
Intellectual disabilities affects from 1.5% to 3% of the population, and although the term “mental retardation” was previously used in the United States, a name change to “intellectual disabilities” occurred across the major disability research and service organizations approximately 3 years ago. Beyond a change in terms, there is general agreement across several diagnostic systems that intellectual disabilities consist of: (1) an IQ score of 70 or less as determined on standardized testing; (2) co-occurring deficits in adaptive functioning (in 3 or more areas); and (3) an onset that begins in the developmental years, before age 18 (APA, 2000).
Beyond these general similarities, the major diagnostic schemas differ somewhat in their approaches. The DSM-IV-TR and ICD-10 require standardized IQ and adaptive behavior testing in order to classify persons into traditional levels of delay (mild, moderate, severe, profound). The American Association of Intellectual and Developmental Disabilities (AAIDD) rejects these classifications, as they suggest inherent deficits within people. Instead, the AAIDD adopts an approach based on the intensity of environmental supports that people need to function their best (Luckasson et al., 2002). While there is considerable merit to this perspective, the AAIDD system has not been widely adopted, and the majority of researchers continue to use traditional diagnostic systems.
Given such definitions of intellectual disabilities, how often do intellectual disabilities occur in ASDs? Based on findings to date, no one knows for certain, and estimates vary widely. Table 15-1 summarizes selected prevalence studies on ASD that included IQ data. These epidemiological studies were international in scope, based in the UK, United States, China, Australia, Japan, Portugal, Finland, and Iceland.
Table 15–1. ASD prevalence studies from 2000 to 2008 that included DSM-IV or ICD-10 ASD diagnostic criteria and IQ data
N with ASD
% ASD with ID
Baird et al., 2000
Baird et al., 2006
Chakrabarti & Fombonne, 2001
Chakrabarti & Fombonne, 2005
Kielinen et al., 2000
Magnússon et al., 2001
Note: ID = Intellectual disabilities (formerly mental retardation).
As Table 15-1 reveals, the prevalence estimates of intellectual disabilities in ASD range from a low of 34% to a high of 84%, with a median of 65% across the 14 studies. This estimate of intellectual disabilities in ASD is lower than the 75% noted in DSM-IV-TR, but remains impressively high. Indeed, based on these studies, the majority of children with ASD have co-occurring intellectual disabilities.
Even so, several caveats are in order, as the same challenges for epidemiological studies of ASD in general hold true for studies that report intellectual disabilities as a co-occurring or secondary condition of ASD. These methodological concerns include: variable sample sizes and age ranges of participants across studies, with low sample sizes having less reliable estimates; and different diagnostic criteria for ASD across studies and over time.
Diagnostic criteria for autism ranges from rating scales or checklists, typically seen in earlier work, to more rigorous and contemporary diagnoses based on DSM or ICD criteria, or such diagnostic interviews as the ADOS and ADI-R. Comparing rates over time is also complicated by the broadening of the autism diagnostic criteria to include milder forms of ASD, as reflected in the DSM-IV-TR (APA, 2000). An additional source of variance relates to different methods used to identify mental retardation or intellectual disabilities across studies. For these reasons, Table 15-1 only includes studies that: were published after 2000; used well-accepted diagnostic criteria for autism; and included participants’ IQ scores as opposed to their levels of delay (e.g., mild intellectual disabilities).
IQ levels do, however, differ substantially across diagnoses included under the umbrella of autism spectrum disorders. By definition, those with Asperger’s disorder do not have clinically significant delays in language or cognitive development, but share the qualitative impairments in social interaction and restricted stereotypes or repetitive interests and activities that characterize autism in general. In pervasive developmental disorder not-otherwise-specified (PDD-NOS), intellectual disabilities may or may not be present, as this term is used to describe children with social impairments along with the presence of either restricted interests or impairment in cognitive or communicative skills. PDD-NOS is often a default diagnosis used to describe children who have mild or absent symptoms in one of the three core domains of autism. Children with autistic disorder have impairments in all three core domains of autism, and are more likely to have co-occurring intellectual disabilities. Even so, the term “high-functioning autism” is often used to describe persons with relatively high IQs, or those in the average range or higher.
Examining subtypes of ASD, Witwer and Lecavalier (2008) reviewed 22 studies that compared clinical differences between two of more groups of children with reliably-established diagnoses of autism, Asperger’s disorder, or PDD-NOS. For studies in this review that reported IQ scores, we calculated the mean IQ for each subtype of ASD. As expected, the average IQ of persons with Asperger’s disorder was 103.1, based on 12 studies that included a group with this disorder. Eleven studies in the Witwer and LeCavalier (2008) review included a group with PDD-NOS, and their average IQ was 86.1. Surprisingly, however, the average IQ of children with autistic disorder was 82.3, with just two of the 13 studies that included a group with autism reporting an average IQ of less than 70. Many of the studies stipulated low IQ as an exclusionary criteria, and we return to this trend later in the chapter.
Genetic Syndromes Associated with ASD and Intellectual Disabilities
A second, related issue concerns genetic syndromes. Recent estimates suggest that approximately 10% or less of the population of persons with ASD have a known co-occurring genetic syndrome (Herman et al., 2007; Muhle, Trentascoste, & Rapin, 2004; see also the chapters by State, Lamb, and Geschwind in this book). So called “syndromic autism” varies in prevalence and core autism features across different syndromes, but collectively are not believed to represent a major proportion of the population of persons with ASD. Table 15-2 summarizes the estimated occurrence rates of ASD in selected genetic syndromes. While this review is not exhaustive, it aptly demonstrates that autism is reasonably prevalent in certain genetic syndromes.
Table 15–2. Selected genetic intellectual disability syndromes associated with Asd
% Estimate ASD
Rett, MCEP2 mutations
Cornelia de Lange syndrome
42% (primarily class I deletions)
Fragile X syndrome
25% (38% UPD; 18% deletions)
22q11.2 deletion (VCFS)
Note: UPD = uniparental disomy; VCFS = Velocardiofacial syndrome.
Given the heterogeneity of ASD, these genetic syndromes provide important perspectives that can fast-track candidate gene discoveries in ASD (see Muhle et al., 2004 for a thoughtful review). Examining two syndromes with high rates of co-occurring ASD, for example, Nishimura et al. (2007) identified several common genes that were dysregulated in subjects with autism and Fragile X syndrome, and autism due to 15q11-q13 duplications. Beyond their promising findings of shared molecular pathways in ASD, this study demonstrates the utility of syndromic-based research in autism.
Our purpose here is not to justify the scientific merit of examining “syndromic autism,” but to highlight a research practice that may not serve the field well. On the one hand, for researchers who examine behavior in people with syndromes, the syndrome is the focus of study, and phenotypic studies may or may not include assessments of ASD along with other syndrome characteristics. Thus, one might assess autism in Prader-Willi syndrome (Veltman et al., 2005), along with other salient characteristics of this syndrome such as hyperphagia (Dykens et al., 2007), jigsaw puzzle skills (Verdine et al., 2008), or associations between the behavioral phenotype and genetic subtypes and age (Dykens et al., 2008). Phenotypic studies are not rooted in the autism field, but will include ASD assessments to the extent needed to accurately depict the syndrome’s phenotypic expression.
In contrast, researchers rooted in the autism field, who focus exclusively on ASD, generally exclude participants with known etiologies or intellectual disabilities. Instead, these researchers examine individuals with what some refer to as “pure autism,” or idiopathic, primary, or nonsyndromic autism. While these studies are essential, such syndromic versus nonsyndromic research practices have led to a growing separation of research and disability communities. Such separation is reflected in different professional conferences and journals that cater to each of these groups, as well as in separate, national efforts to develop ASD versus syndrome -based databanks, biobanks, or patient registries. Thus, persons with Fragile X, Down, Prader-Willi, or other syndromes are either excluded from or not well represented in databases connected to the Autism Treatment Network or Simons Simplex or Sibling Studies. Though the Autism Genetic Resource Exchange (AGRE) includes representative proportions of individuals with genetic syndromes and autism, the overall numbers of these individuals are too low for meaningful analyses.
At the same time, many syndrome-specific advocacy or parent groups are now developing their own data repositories, biobanks, or research registries. On the one hand, this separation seems logical, as not all parents or researchers who study syndromes need be concerned with autism, especially in syndromes where autism is relatively infrequent. But to the extent that persons with genetic syndromes also have ASD or brush up against issues related to ASD, then sharing data, registries, and resources would accelerate the pace of discoveries and of more effective treatments for persons with (or without) syndromic autism.
What are the current practices regarding intellectual disabilities for researchers who publish studies pertaining to ASD? Are the IQs of participants routinely reported? How many studies include or exclude those with low IQs? To address these questions, we selected a long-standing and highly regarded autism journal, the Journal of Autism and Developmental Disorders (JADD), and reviewed all studies published in 2008. Although not an exhaustive review of ASD work, we reasoned that studies published in 2008 would likely be representative of other recent years, and that a leading journal in autism would pull for quality submissions that included such methodological details as IQ scores. For the journal review we identified how IQ was reported (as an average, range, or not at all) in child versus adult participants, and when possible, we tallied or extracted mean IQ scores from each study.
As shown in Table 15-3, the vast majority of publications for both children and adults with ASD included participants with high IQs. Regarding children, 60 studies used either high IQ or low IQ children, with 77% of articles including children with high IQs only. Typically these studies had a required IQ score of greater than 75, 80, or 90 as an inclusionary criterion. In contrast, 23% of articles included children with low IQs only. Examining the actual numbers of participants enrolled in these 60 articles, 83% of research subjects had high IQs, and 17% low IQs.
Table 15–3. Data on low versus high IQ in children and adults with ASD as published in the 2008 volume of JADD
Number of Studies
Number of Participants
Mean IQ, other IQ data
M IQ = 58.21 (based on 9 studies)
2 studies noted “profound or severe” ID
3 studies stipulated IQ < 70
M IQ = 102.10 (based on 35 studies)
6 studies stipulated IQ > 70 or 75
4 studies stipulated IQ > 80 or 90
1 study noted IQ range (85–115)
Low- & High-IQ Children
M IQ = 72.73 (based on 16 studies)
2 studies noted IQ range (53–124; 52–85)
1 study noted % no ID, mild, moderate, severe ID
% noted for mild, moderate, severe, profound ID
M IQ = 106.4 (based on 22 studies)
4 studies stipulated IQ > 70, 80, or 90
1 study noted IQ range (73 to 129)
Some studies of children with ASD included those with both low and high IQs: 19 studies included children with IQs less than 70 along with children with higher IQs. Of the 16 studies using low and high IQ children that reported actual IQ data, 10 reported a mean IQ greater than 70, and the mean IQ across these 10 studies was 77.80. Six studies reported mean IQs less than 70, with an overall average IQ of 66. Thus, while these 16 studies included some low IQ participants, on average most had IQs greater than 70, suggesting that the majority of participants were relatively high functioning.
Our review of 2008 JADD articles also indicates that adults are less often studied than children, and that the vast majority of adult studies only include high-functioning individuals. Of the 30 studies on adults with ASD, just 3 included persons with low IQs, and 27 with high IQs—the average IQ of these adults was 106.4. Thus, 9 times as many studies were published on high IQ adults than low IQ adults with ASD.
Examining the actual numbers of adult participants, 73% had high IQs, and surprisingly, the three studies of lower IQ adults represented over one quarter (27%) of adult subjects. This relatively high percentage is due to the fact that 2 of the 3 studies were based in large institutions for persons with intellectual disabilities. While such settings offer increased numbers of participants, they may not be representative of persons with ASD growing into adulthood today.
A final observation stemming from our 2008 JADD review is that the majority of studies (70%) included IQ data of some sort. Of the 47 articles that did not do so, 28% were parent surveys; 21% were case studies or single-subject interventions that may or may not have included formal developmental testing; 19% were genetic, medical, or physiological studies; 17% used large-scale or population-based datasets; and 15% sampled teachers or educators about their students with ASD. While there is room for improvement, the majority of articles that focused on children or adults with ASD included some index of intellectual functioning.
In summary, this “year in the life of an autism journal” confirms that published studies in ASD primarily include children and adults with relatively high IQs. While one could argue that 2008 may not be representative, there are no glaring or obvious cohort differences to suggest that 2008 differs dramatically from 2007 or 2009. As well, relative to other journals focused on autism, there is no reason to suspect that JADD is more or less likely to pull for submissions on high versus low functioning autism.
Based on studies summarized in Table 15-1, approximately 65% of persons with ASD also have an intellectual disability. Yet our journal review suggests that only 10% to 23% of published articles are focused on persons with ASD and intellectual disabilities. In contrast, from 77% to 90% of studies we sampled in 2008 were restricted to persons with high IQs. Assuming that studies on all persons on the autism spectrum are critically important steps toward discovery, why are those with lower IQs not well represented in today’s research landscape?
Reasons for Excluding Those with Intellectual Disabilities from ASD Research
Researchers present several explanations about why those with intellectual disabilities are often excluded from contemporary autism work. Some suggest that with the broadening of the autism diagnostic criteria, the field is now “catching up” or balancing out work based on a previous conceptual model of autism that primarily included lower functioning individuals. If this is indeed the case, we would expect to see an eventual shift toward a middle ground, with approximately equal numbers of studies that focus on low or high IQ participants, or that include a wider range of IQ in the same study.
A common explanation for excluding those with intellectual disabilities is that the demands of a particular research task or protocol are too difficult for those with low IQ. Tasks that require, for example, a high cognitive load, higher-order or abstract reasoning, or insight into one’s thoughts or emotions are all better suited for those with higher IQs. In addition, some methodologies, such as functional neuroimaging, require that participants comply, remain still, and respond to various button presses or stimuli while in the scanner, and these are more challenging for those with lower IQs. By virtue of their better-developed cognitive and communicative skills, higher-functioning persons with ASD have provided novel insights into the phenomenology of ASD, as well as on neural functioning and basic mechanisms associated with the wider autism spectrum.
Even so, generations of biobehavioral scientists have specialized in the field of intellectual disabilities, and used a variety of methodologies that assess cognitive, neural, social, emotional, and adaptive functioning. Further, functional and structural neuroimaging studies have now been conducted in persons with intellectual disabilities and with Fragile X, Prader-Willi, Williams, and other syndromes. Techniques to enhance valid imaging data from persons with intellectual disabilities include behavioral interventions to decrease movement in the scanner and increase compliance, the use of mock scanners, and new methods of acquiring imaging data. These approaches hold much promise for imaging work in persons with intellectual disabilities and ASD.
Some experimental tasks can also be adapted to those with low IQ, while other experimental tasks can simply be used “as is” in persons with intellectual disabilities. Examples of specific methodologies or tasks gleaned from the 2008 JADD review that were limited to high-functioning persons but could easily be used across the IQ spectrum include: wearing actigraphy watches; playing cards with a peer; deciding if a speaker was male or female; taking a hearing test; providing saliva samples for cortisol assays; and identifying moods on feeling thermometers. The inclusion of those with low IQs may not necessarily be methodologically challenging for these or other ASD studies.
A second issue, however, is that some researchers are understandably striving to recruit a homogeneous sample of high-functioning persons with ASD. Presumably, their hypotheses and research questions demand that they focus exclusively on this high-IQ subgroup. Surprisingly few investigators, however, specify why their questions pertain only to high-functioning individuals, nor do their publications necessarily explain the scientific rationale for excluding those with lower IQs. Without a well-articulated rationale for using IQ cutoffs, excluding those with intellectual disabilities seems arbitrary; over the long term, this practice may not reflect well on the broader ASD field.
A third point is that because autism is a spectrum disorder, some investigators assume that findings from high IQ individuals will automatically apply to those with lower IQs. By definition, findings from ASD in its more “pure” form must apply to those with ASD and intellectual disabilities. Given the heterogeneity of ASDs, however, it seems shaky at best to assume that processes in high IQ individuals will simply extend to those with ASD and intellectual disabilities. Actually, this is an empirical question, and data are needed from both low- and high-IQ persons with ASD to determine if findings from individuals with high IQ are applicable to those with intellectual disabilities. Of concern, however, is that most researchers do not seem to be tackling this question, and are increasingly shying away from studies that include individuals with both low and high IQ.
Ideas for ASD Research That Includes Intellectual Disabilities
Table 15-1 suggests that approximately 65% or more of persons with ASD also have an intellectual disability, yet these individuals are not well represented in national ASD research networks, data repositories, or in published articles on ASD. Although far from comprehensive, our journal review suggests that a minority of published articles (from 10% to 23%) focus on persons with ASD and intellectual disabilities. A further concern applies to IQ across autism spectrum diagnoses. Calculating means IQs from Witwer and Lecavalier’s (2008) review of ASD diagnoses, we found the expected high IQ in those with Asperger’s disorder (M = 103.1), a lower IQ in those with PDD-NOS (M = 86.1), and a surprisingly high average IQ in participants with autistic disorder (M = 82.3). Although autistic disorder often includes those with intellectual disabilities, the reliably diagnosed participants in this review were relatively high functioning. Although further work is needed to explore these trends, it appears that those with lower IQs are not well represented in published research. As this imbalance may impede new insights into ASD, how can we increase the inclusion of those with intellectual disabilities into current ASD research?
A first step is for investigators to ask if their study hypotheses only apply to those with ASD and high IQs. Do investigators know for certain that their data will be compromised by adding those with intellectual disabilities to their study? Or are preliminary data needed on how those with low IQs might perform on a given task in order to make an informed decision about IQ inclusionary criteria and data integrity?
Perhaps intellectual disabilities should be a human subject characteristic that investigators are required to address in IRB and grant applications. Just as investigators must specify their rationale for the inclusion or exclusion of children, women, and ethnic/minority participants, maybe they should also specify the scientific rationale for excluding those with low IQs. Although this idea is not likely to be formally adopted, being asked to justify the exclusion of those with intellectual disabilities could sharpen investigators thinking about the role of IQ in their design.
Assuming that study hypotheses could be extended to those with low IQs, a second step is for investigators to think creatively about their methodology. How could tasks or experimental manipulations be adapted in order to glean meaningful data from participants with ASD and intellectual disabilities? Are extra supports, costs, or accommodations needed in order to include those with intellectual disabilities? Specific ways of examining persons with a broad range of IQs may be extracted from research on the behavioral phenotypes of persons with genetic etiologies of intellectual disabilities. In this work, IQ is rarely used to truncate samples, and all persons with a given genetic syndrome are included in research, regardless of their IQ. IQ effects are assessed in phenotypic data, which then inform links between underlying molecular genetics of syndromes, neurological functioning, and behavioral phenotypes.
Recent recommendations for dimensional assessments of cognitive, linguistic, adaptive, and social functioning in ASDs (Volkmar, State, & Klin, 2009) are similar to approaches taken in most phenotypic studies, and hint at the idea of moving away from low IQ as an exclusionary criterion in ASD research. Instead, IQ could be cast as a continuous variable that informs the broad autism phenotype.
Intellectual disabilities remain a highly prevalent co-occurrence with ASD. Our review of recent, well-conducted ASD epidemiology studies suggests that approximately 65% of those with an ASD have intellectual disabilities. As well, certain genetic intellectual disability syndromes are characterized by relatively high rates of ASD. Even so, persons with ASD and intellectual disabilities or with specific genetic etiologies, are not well represented in today’s ASD research landscape. The exclusion, or relatively poor representation, of those with low IQs or genetic syndromes is reflected in large-scale databases, biobanks, and participant registries, as well as in published, peer-reviewed articles. Our admittedly limited sampling of “a year in the life of an autism journal” revealed that only 23% of articles on children, and 10% on adults, included persons with intellectual disabilities. Examining the numbers of children enrolled in these studies, 83% were high functioning, and 17% had intellectual disabilities.
The current research practice of truncating IQ and focusing primarily on those with high IQs does not represent the full scope of autism as a spectrum disorder, limits generalizability of findings, and does not represent the majority of persons who have autism spectrum disorder. This chapter describes several ideas for increasing the representation of those with intellectual disabilities into future ASD research.
This chapter sounds a strong cautionary note about research practices. It does not aim to detract from the critical importance of studies of “pure” autism, or those with high IQs, idiopathic autism, or no known genetic etiology. These samples are essential for ongoing, multisite efforts aimed at candidate gene discovery, or linking ASD candidate genes to specific aspects of the ASD phenotype. Similarly, the cautionary note does not aim to promote research focused exclusively on those with low IQs or with syndromic autism. Rather, a more equitable or balanced inclusion of individuals with low and high IQs is essential for future discoveries about the etiologies, mechanisms, and treatments of ASD.
Future Directions and Challenges
• Researchers should aim to increase numbers of individuals with lower IQ or syndromic autism in autism research programs and large-scale databases and biobanks. Studies of individuals with differing types of syndromic autism may provide information on common genetic or molecular pathways.
• Instead of setting IQ cutpoints, researchers should examine IQ as a continuous variable that can affect other areas of the phenotype.
• If researchers choose to exclude individuals with intellectual disability or syndromic autism, they should provide rationalizations for why these exclusions are necessary for their research question and should address whether or not their findings would extend to the broader phenotype.
• Researchers will need to think creatively about how to adapt research paradigms (if necessary) to include lower functioning individuals, which may include specific supports, accommodations, or additional costs. Researchers in the field of autism can look to the field of behavioral phenotypes in genetic syndromes for how these adaptations have been put into practice to successfully include lower functioning individuals in research.
This chapter was supported in part by NICHD Grants P30HD15052NICHD, and R01 R01HD135681. The authors thank Robert Hodapp for his helpful comments on an earlier draft of this manuscript.
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