While individuals with ASD have an intellectual disability in 35 % of cases, it is important to be able to rule out an Autism Spectrum Disorder when evaluating a child with an Intellectual Disability. As ASD is a diagnosis based on social communication and reciprocity and restricted repetitive behaviors or patterns, it is important to look at these areas in the context of ID. Intellectual disabilities are defined as including deficits in conceptual, social, and practical domains (APA, 2013; Trammell et al., 2013). These would include challenges in language, reasoning, knowledge, interpersonal skills, and adaptive skills. An individual with an intellectual disability is likely to have impaired social skills and impaired communication and may be repetitive speech or restricted interests. It is essential to discern when these characteristics are consistent with autism. When differentiating between autism and intellectual disability, the clinician must consider features of autism including social reciprocity, gesture, and play skills. Individuals who have limited conceptual skills like language and verbal communication, but do not have ASD, may use gestures and nonverbal communication, even more frequently than the typical child. Children with ASD may not make such efforts to communicate even when they have the intellectual ability to do so. A child with an intellectual disability may be repetitive and concrete in creating a play sequence, but he or she is likely to make eye contact, watch for the examiner’s reaction to play, share enjoyment, and work to connect on some level with the examiner. Thus, gestures, eye contact, nonverbally watching, connecting, and communicating, can be clear differentiating factors ruling out an ASD in children suspected of intellectual disabilities. Trammell et al. (2013) reports that adaptive skill deficits may be more pronounced in an individual with ASD and ID than in ID alone. Thus, adaptive skills can be compared to cognitive abilities to provide additional insight when differentiating autism and intellectual disabilities (Matson & Shoemaker, 2009; Trammell et al., 2013).

When differentiating between giftedness and ASD, the clinician should evaluate social communication including eye contact, facial expression, use of nonverbal communication, and reciprocal conversation. Children who are gifted but do not have ASD will show strengths in verbal and nonverbal communication rather than impairment. Although children with ASD tend to use formal language more than neurotypicals, gifted children may also use formal language because of their extensive vocabulary. However, gifted children tend to have strong receptive language and comprehension such that they understand the complex vocabulary they use, while children with autism may use words they do not understand. Children with autism may enjoy nonfiction more than fiction and may prefer reading to playing with toys. Gifted children may also show such preferences; however, this is likely due to the fact that they have a high proclivity for certain cognitively sophisticated topics and may have limited interest in playing games that do not call on this intellectual curiosity. Similarly, children with autism may prefer interacting with adults over peers. Gifted children may also prefer interacting with adults, but this is most likely due to the fact that adults may be more able to understand the intellectual complexity inherent in their communication. Alternatively, children with autism prefer socializing with adults because of their deficits in socializing with peers. Gifted children who do not have ASD will show appropriate social skills, adequate communication, and will not have restricted interests or repetitive behaviors.

Intellectual Disability is highly comorbid with ASD. As it was previously discussed, 35 % of individuals with ASD have Intellectual Disabilities. This means that in addition to having an intellectual disability, these individuals also show significant social communication weaknesses and restricted and repetitive behaviors. As noted above, it is possible that overall adaptive functioning is lower when Autism Spectrum Disorders and Intellectual Disabilities are comorbid. When considering comorbidity of ID and ASD, one important point is to reassess cognitive skills frequently as cognitive ability is more likely to change in those with Autism Spectrum Disorders and is particularly unstable at a young age (APA, 2013, p. 40).

In typically developing children, IQ scores are reported to become stable around age seven, but there may be more variability in autism. A study by Mayes and Calhoun in 1999 suggested that 33 % of young children with ASD who had IQ testing a year apart experienced significant change in IQ scores over time (in Mayes & Calhoun, 2003). A second study in 2013 found similar results showing that 32 % of children with ASD had a significant change of 15 IQ points or more from approximately age four to age seven (Barry, Moran, Anglim, Conway, & Guerin, 2013). In this study, researchers did not control for the intervention received; however, at time two 68 % were receiving ASD-specific educational supports which may have impacted their IQ.

Research consistently shows that high cognitive ability predicts optimal outcomes in ASD; however, IQ does not necessarily impact the severity of symptoms. Researchers found 2 years of early intensive behavioral intervention (EIBI) decreased Autism severity in both ID and non-ID groups (46 participants, mean age of 2) according to Ben-Itzchak, Watson, and Zachor (2014). The difference between ID and non-ID group was in their adaptive social and communication skills. Children with higher IQs had an increased rate of adaptive skill acquisition over 2 years. The study also found improvement in adaptive skills only occurred after 2 years of intervention suggesting that applying the skills required extensive practice before children could transfer them to day-to-day settings. Taken together, children who have a comorbid intellectual disability and autism show significantly more impairment across adaptive, social, and communicative domains.

Giftedness can also be “comorbid” with ASD. Giftedness for the purpose of this book is defined as an IQ score at or above the 90th to 95th percentile (or an IQ score in the superior to very superior range). Some school districts utilize a profile approach to consider other aspects of giftedness beyond IQ; in clinical settings, this model is seldom used. Individuals with this profile are often termed twice exceptional (also called “2E”). Gifted children with ASD have challenges in social communication and restricted/repetitive interests. They tend to have strong rote reading and math skills as well as exceptional skills in problem solving, analyzing, and conceptualizing.

This profile is highly complex, difficult to treat, and worthy of special consideration in clinical and educational settings . Often gifted individuals with ASD have significant emotional and behavioral symptoms due to a high level of intellectual understanding coupled with lagging emotional and social development . Treatment programs for twice exceptional individuals must teach skills relevant to the child’s developmental age (not chronological); and must consider these discrepant skills across domains. A gifted child may be able to access highly sophisticated academic materials ; however, may require social skills groups to focus on very basic skills such as eye contact, active listening, conducting a to-and-fro conversation, and expanding on the topics that are important to others. Clinicians may worry, and indeed initially the child might claim, that such ideas are too simplistic; but after some time working on social communication, the child often will come to find this type of skill practice engaging and helpful. Generally, gifted children with ASD tend to come to an understanding of these skill deficits and may desire to improve in order to make friends and participate socially.

Gifted children or adults with ASD may have difficulty with abstract reasoning and making inferences. These difficulties tend to show up academically in poor reading comprehension and written expression. Sometimes, individuals with weaknesses in comprehension and writing may prefer careers in engineering, architecture, science, or mathematics. These careers may allow them to utilize their strengths in problem solving without relying on social communication and inferencing .

Comorbidity rates for children with Autism Spectrum Disorders who meet the criteria to be considered “gifted” are difficult to find in the research. A Korean study with a target population of over 55,000 school-aged children found that approximately 2 % of the sample met criteria for an Autism Spectrum Disorder. This is slightly higher than the CDC proposed rate of 1 in 68 children impacted in the United States. Seven to 12 % of the children who met criteria for ASD also had an IQ in the Superior range as measured by the Korean WISC-III or the Leiter, a nonverbal intelligence test (Kim et al, 2011). These data would indicate that Superior IQ scores are approximately equally present in individuals with Autism Spectrum Disorders as they are in the population as a whole. Taken together, based on the research available, individuals with Autism Spectrum Disorders are much more likely to have an Intellectual Disability than the general population data would suggest (approximately 35 % to the general population 2.5 %), but those with Autism Spectrum Disorder are just as likely to be gifted as any other individual (approximately 9 % in the general population to 7–12 % in those with ASD). This research clearly suggests that IQ scores among ASD are highly variable and much more so than in the general population. A greater number of individuals with ASD have low intellectual ability and fewer individuals are Average. In conclusion, the authors of this text find that although it is reported that IQ scores are more variable and less stable in the ASD group relative to the general population, further research is needed.

Measures for cognitive assessment should be considered based on the language level of the child as this will impact performance. Children with average or advanced language are able to be assessed via measures such as the WISV-V, WJ-Cog, and K-ABC, while children with more limited language abilities may be assessed with the Leiter, UNIT Stanford Binet-5, or DAS-2 preschool (Fig. 13.1).

This table provides IQ scores with qualitative descriptions that adequately assess a child’s intellectual ability. Examiners must always consider IQ score in light of all other assessment data. For intellectual disabilities, an adaptive measure must also be administered with scores falling below 70 in order to meet criteria. Gifted individuals generally have either a Full Scale IQ score or an Index score in the Superior or Very Superior range. Some gifted schools accept children with an IQ score at or above the 90th or 95th percentile. Schools may also use a profile approach for assessing giftedness, which evaluates academic skills and other exceptional abilities, in addition to IQ. Clinically, this profile approach for giftedness is rarely used (Fig. 13.2).

There are numerous genetic disorders, to be discussed below, that may have ramifications for intellectual abilities and at times may need to be differentiated from or co-occur with ASD. However, first it is useful to mention a syndrome with purely environmental influence that may impact cognitive ability and could co-occur with or require differentiation from ASD. Fetal Alcohol Syndrome (FAS) is a pattern of mental and physical deficits that can develop when a mother consumes alcohol during pregnancy. Drinking can cause damage to the fetal brain and result in an intellectual disability and/or physical, behavioral, and emotional challenges. Cognition, attention, executive functioning, and adaptive skills can be impacted in FAS, and thus at times a child with FAS may be referred for an ASD evaluation. Clinicians should familiarize themselves with the physical features associated with FAS and take a thorough history including information on prenatal care, birth weight, and growth information. Low birth weight and growth deficiencies are often associated with FAS. This can be a tough diagnosis to make, particularly in a case where the biological mother may deny that alcohol was used during the prenatal period. In other cases a family who has adopted a child they suspect may have been exposed to alcohol may seek a comprehensive evaluation to include a rule out of FAS. FAS could occur with an Autism Spectrum Disorder, but in this case the clinician will need to give careful consideration to the primary symptoms of ASD (social communication and restricted/repetitive behaviors and interests). The assessment should include consideration of facial expression, eye contact, gesture, quality of speech, and social skills relative to an individual’s IQ. Play, empathy, and the absence of restricted topics or repetitive behaviors are all important areas to assess. Often significant behavior problems, executive functioning weaknesses, and attention problems can be secondarily present in ASD. In the case of FAS, these are also associated symptoms. Without clear core symptoms of autism present, it is not possible to make comorbid diagnoses of ASD and FAS.

Down syndrome occurs when an individual has an extra full or partial chromosome 21. Individuals with Down syndrome are often compared to those with ASD in research studies. Studies often look at the impact of intervention or examine adult outcomes because of the relatively high occurrence of Down syndrome in the population and the rate of intellectual disabilities occurring in both DS and ASD. Down syndrome is associated with cognitive delays that range from very mild to severe with most individuals born with Down syndrome having mild to moderate cognitive impairment. One study in 691 children is born with Down syndrome in the United States, making it the most common genetic condition (National Down Syndrome Society, webpage copyright 2012).

Individuals with Down syndrome may have low muscle tone, small stature, and upward slanting eyes. All have some level of cognitive impairment. Down syndrome is associated with physical growth delays and physical features. Differentiating ASD and DS would include a careful assessment of social communication similar to that noted in the ID section. Down syndrome alone does not result in impacted social communication, though cognitive delays can make this more difficult to differentiate.

Autism and Down syndrome can co-occur in a small percentage of children and the research would suggest that those with dual diagnoses have more severe cognitive delays (Howlin, Wing, and Gould, 1995). Between 1 and 10 % of children with Down syndrome may also have an ASD (Capone 1999). Dr. Capone notes that through Kennedy Krieger’s research studies on DS-ASD children with dual diagnoses had regression of language and social skills, poorer communication skills than other DS children, repetitive behaviors, self-injurious behaviors, unusual vocalization, unusual sensory responses, feeding challenges, increased anxiety, irritability, sleep disturbance, attention problems, and difficulty with transitions. Children with DS-ASD had less challenge with “social relatedness” than children with ASD alone but had more severe cognitive impairments on the whole (national Down syndrome society webpage 2012).

Dr. Susan Hepburn (2015), a recognized expert in the dual diagnosis of ASD and Down syndrome, describes that families often feel excluded in the greater community because their children do not share the same profile as would be expected for the stand alone disabilities. For example, children with Down syndrome generally do not have social deficits; however, children with comorbid ASD have significant social symptoms. These differences can make individuals feel as though they do not properly belong to either group. Further research is needed on treatment for individuals who have both disorders; as well as, advocacy efforts so that they can be appropriately included in the greater community of individuals with developmental disabilities.

Turner syndrome is a genetic condition caused by complete or partial absence of the second sex chromosome in women. It occurs in one in 2000 births and may be associated with nonverbal memory and attention deficits and challenges with spatial temporal processing. These may lead to impaired nonverbal learning and social skills. Physical signs and symptoms may include short stature, webbed neck, droopy eyes, strabismus, a broad chest, and flat feet (Turner syndrome society of the United States webpage 2013). Turner syndrome can be definitively ruled in or out with genetic testing.

Because Turner syndrome and autism have some commonalities, genetic research has been funded by the National Alliance for Autism Research (NAAR) . Studies by Eric Lander, Ph.D. and David Skuse, Ph.D. research the inability to recognize facial expressions that may be common to both disorders. Turner syndrome has a clear genetic makeup while ASD is genetically more complex but researchers feel that with some similar characteristics, looking at genes of those with Turner syndrome can help isolate genes involved in ASD. A study by Feigenberg et al. suggests that individuals with Turner syndrome should be routinely screened for ASD. Research indicates that as many as 25 % of women with Turner met criteria for a diagnosis on the Spectrum and 5 % met full autism criteria. This study found that 50 % of the 28 patients screened had at least mild Autism Spectrum symptoms (Inbar-Feigenberg et al., 2013). While Turner syndrome is relatively easy to diagnose, the presence may warrant an evaluation for ASD because of a high comorbidity rate.

Rett syndrome was once classified a neurodevelopmental disorder caused by a gene mutation on the X chromosome that almost exclusively affects females. It is rare and was, before DSM-5, classified as an Autism Spectrum Disorder. Because of the known etiology of Rett syndrome, it has been removed from the category Neurodevelopmental Disorders. It is characterized by developmental regression of language and motor milestones impacting movement, speech, and cognitive development. It is also characterized by reduced head circumference and repetitive hand movements. Individuals with Rett syndrome often live until adulthood but need significant 24 h care and support with activities of daily living including feeding, dressing, bathing, and toileting. Rett syndrome is not an Autism Spectrum Disorder as classified by the DSM-5 (International Rett Syndrome Foundation).

Williams syndrome is a genetic condition impacting 1 in 10,000 people worldwide. It stems from a deletion of genetic material from chromosome 7. According to the NIH, it is characterized by medical problems, developmental delays, intellectual disabilities, and learning disabilities. Those with Williams syndrome may have “elfin”-like features and are said to be “social, friendly, and endearing.” Some have called Williams syndrome the “anti-autism,” and research has found differences in the amygdala, with greater activation in those with Williams, which is associated with emotional and social functioning (Coe et al., 1999, Insights for Autism from Williams Syndrome SFARI). Autism Spectrum Disorders are not comorbid with Williams syndrome. Careful assessment of social communication should make the disorders fairly easy to differentiate. Research into genetics related to Williams syndrome may provide insights into genetics and ASD because of the “opposite” symptom profile and clear genetic makeup of Williams syndrome.

Fragile X is a genetic condition causing intellectual disability, behavioral and learning challenges, and occurring more frequently in males. There are physical and behavioral characteristics and Fragile X can include characteristics of AD/HD, Autism Spectrum Disorders and Anxiety (National Fragile X Foundation 2014). Fragile X is identified through genome sequencing and shares a complicated relationship with autism. One study found that as many as 47 % of individuals with Fragile X met criteria for autism (Demark, Feldman, & Holden, 2003). Rates of shared genes between individuals with both disorders to range from 0 to 12.5 %, with most studies finding about 3 % of cells in the autism group that code for Fragile X (Demark, n.d.). Although the disorders share some common features, individuals with Fragile X display symptoms that are distinctly different from the typical presentation of ASD and can be differentially diagnosed (Jessica Wright, author for SFARI, wrote in 2014 that the director of the UC Davis MIND Institute in Sacramento, Leonard Abbeduto). Researchers Joseph Piven and Stephen Warren note that extreme social anxiety, hand flapping behaviors, and low IQ scores are characteristic of Fragile X and autism. However, the more complex autism symptoms such as lining up toys and restricted interests are seldom seen in Fragile X. Children with Fragile X have fewer social deficits when compared to their cognitive abilities than do those diagnosed with Autism Spectrum Disorder (Simon’s Foundation 2014).

In conclusion, genetic disorders can be comorbid with Autism Spectrum Disorders including Down syndrome, Turner syndrome, and Fragile X. Studying genetic conditions and their relationship to Autism Spectrum Disorders may lead researchers to better understand the genetic mechanisms involved in ASD. Autism Spectrum Disorders are not thought to co-occur with Rett syndrome or Williams syndrome.