The autism spectrum or autistic spectrum describes a range of conditions classified as pervasive developmental disorders in the Diagnostic and Statistical Manual of Mental Disorders (DSM). Pervasive developmental disorders include autism, Asperger syndrome, pervasive developmental disorder not otherwise specified (PDD-NOS), childhood disintegrative disorder, and Rett syndrome, although usually only the first three conditions are considered part of the autism spectrum.[1] These disorders are typically characterized by social deficits, communication difficulties, stereotyped or repetitive behaviors and interests, and in some cases, cognitive delays. Although these diagnoses share some common features, individuals with these disorders are thought to be "on the spectrum" because of differences in severity across these domains.
Autism is characterized by delays or abnormal functioning before the age of three years in one or more of the following domains: (1) social interaction; (2) communication; and (3) restricted, repetitive, and stereotyped patterns of behavior, interests, and activities.[2] Social impairments are marked by poor use of nonverbal communication, difficulty in peer relations, lack of social-emotional reciprocity, and lack of shared enjoyment. Communication deficits may include failure to develop speech, use of stereotyped or delayed echolalia, and difficulties maintaining conversations. Social and communication impairments may also cause a lack of symbolic or imaginative play. Restricted and repetitive behaviors may include unusual preoccupations with narrow interests, inflexibility to nonfunctional routines, stereotyped and repetitive mannerisms, and preoccupations with parts of objections.
Asperger syndrome can be distinguished from autism by the lack of delay or deviance in early language development.[2] Additionally, individuals with Asperger syndrome do not have significant cognitive delays. An individual with Asperger syndrome typically demonstrates obsessive interest in a single topic or activity. Other symptoms include repetitive routines or rituals, peculiarities in speech and language, inappropriate affect or social behavior, problems with non-verbal communication, and clumsy or uncoordinated motor movements.[3] Because of these difficulties, individuals with Asperger syndrome often have trouble interacting with others.
PDD-NOS is considered "subthreshold autism" and "atypical autism" because it is often characterized by milder symptoms of autism or symptoms in only one domain (such as social difficulties).[4] Persons with PDD-NOS may demonstrate pervasive deficits in the development of reciprocal social interaction or stereotyped behaviors, but do not meet the criteria for a specific pervasive developmental disorder or other psychological disorders (such as schizophrenia or avoidant personality disorder).[2]
Autism Spectrum Disorder (ASD) is a proposed revision to the Diagnostic and Statistical Manual of Mental Disorders 5 (DSM-5), which will be released in May 2013.[5] This new diagnosis will encompass current diagnoses of autistic disorder, Asperger's disorder, childhood disintegrative disorder, and PDD-NOS. Rather than categorizing these diagnoses, the DSM-5 will adopt a dimensional approach to diagnosing disorders that fall underneath the autism spectrum umbrella. It is thought that individuals with ASDs are best represented as a single diagnostic category because they demonstrate similar types of symptoms and are better differentiated by clinical specifiers (i.e., dimensions of severity) and associated features (i.e., known genetic disorders, epilepsy and intellectual disability). An additional change to the DSM includes collapsing social and communication deficits into one domain. Thus, an individual with an ASD diagnosis will be described in terms of severity of social communication symptoms, severity of fixated or restricted behaviors or interests and associated features.
While a specific cause or specific causes of autism spectrum disorders has yet to be found, many risk factors have been identified in the research literature that may contribute to the development of an ASD. These risk factors include genetics, prenatal and perinatal factors, neuroanatomical abnormalities, and environmental factors.
The results of family and twin studies suggest that genetic factors play a role in the etiology of autism and other pervasive developmental disorders.[6] Studies have consistently found that the prevalence of autism in siblings of autistic children is approximately 15 to 30 times greater than the rate in the general population.[7] In addition, research suggests that there is a much higher concordance rate among monozygotic twins compared to dizygotic twins.[8] These studies suggest a strong genetic component in autism. It is estimated that autism involves 5-10 genes and possibly more.[9] It appears that there is no single gene that can account for autism. Instead, there seems to be multiple genes involved, each of which is a risk factor for part of the autism syndrome through various groups.[10] Possible susceptibility regions include chromosomes 1p, 2q, 7q, 13q, 16p, and 19q.[9]
A number of prenatal and perinatal complications have been reported as possible risk factors for autism. These risk factors include maternal gestational diabetes, maternal and paternal age over 30, bleeding after first trimester, use of prescription medication during pregnancy, and meconium in the amniotic fluid. While research is not conclusive on the relation of these factors to autism, each of these factors has been identified more frequently in children with autism compared to their non-autistic siblings and other normally developing youth.[11]
In general, neuroanatomical studies support the notion that autism is linked to a combination of brain enlargement in some areas and brain reduction in other areas.[12] These studies suggest that autism may be caused by abnormal neuronal growth and pruning during the early stages of prenatal and postnatal brain development, leaving some areas of the brain with too many neurons and other areas with too few neurons.[13] Some research has reported an overall brain enlargement in autism while others suggest abnormalities in several areas of the brain, including the frontal lobe, the mirror neuron system, the limbic system, the temporal lobe, and the corpus callosum.
In neuroanatomical studies, it has been shown that for individuals with Autism spectrum disorder there is reduced activation in the primary and secondary somato-sensory cortices during Theory of Mind and facial emotion response tasks when compared to control. This is consistent with reports of patterns of abnormal cortical thickness and grey matter volume in those regions in individuals with Autism Spectrum Disorder.[14] (Sugranyes) In normal children, there is a bias of the left lateralized network that is essential for language development, as shown by magnetoencephalography. Specifically, there was a left dominance of parieto-temporal coherence in the theta band that was correlted with higher performance on language related tasks. This was not correlated with head circumference or chronological age.[15]
The frontal lobe is central to many functions that are associated with autism, such as language and executive functions. For instance, Broca's area, which is related to language production, is located in the inferior prefrontal lobe. Other important areas of the frontal lobe include: the prefrontal cortex (involved with aspects of executive function such as working memory, inhibition, planning, organizing, set-shifting and cognitive flexibility), the orbitofrontal cortex (involved in social cognition and theory of mind) and the inferior frontal gyrus (part of the mirror neuron system). Current research suggests that dysfunction in the frontal lobe may be associated with some of the deficits observed in individuals with ASD, including social cognition, imitation, face processing, language, attention, working memory, and problem-solving. For example, it has been found that individuals with autism have decreased concentrations of N-acetyl-asparate (NAA) and reduced glutaminergic neurons in the frontal lobe, suggesting some dysfunction in this region.[16] Another study using fMRI found that boys with high-functioning autism had reduced activity in the pars opercularis when observing and imitating emotions.[17] Orbitofrontal cortex deficits have also been implicated with autism, as individuals with high-functioning autism have shown decreased functioning in this area when participating in a task that involved the perception of fearful faces.,[18] Finally, individuals with ASD have shown decreased activation in the medial prefrontal cortex relative to a control group during a theory of mind task.[19]
The mirror neuron system (MNS) consists of a network of brain areas that have been associated with empathy processes in both animals[20] and humans.[21] In humans, the MNS has been identified in the inferior frontal gyrus (IFG) and the inferior parietal lobule (IPL) and is thought to be activated during imitation or observation of behaviors.[22] It has been suggested that the MNS generates internal representations of the self and others, which facilitates an understanding of other people. Many researchers have hypothesized that the MNS is related to cognitive processes such as imitative learning, "mind-reading", and empathy; all of which are necessary for social-communication.[23] Several studies using functional brain-imaging have found evidence of mirror neuron dysfunction in autism,[24][25] suggesting this neural system is associated with social impairments in individuals with ASDs. Specifically, it has been found that reduced mirror neuron activity and MNS cortical thinning[26] are highly correlated with autism severity.
Social skills impairments in autism have been theorized to reflect abnormal functioning in the limbic system. In animal models, it has been found that monkeys with lesions in the medial temporal lobe (e.g., the amygdala and hippocampus) demonstrate autistic-like behaviors, such as a failure to develop normal social relationships, stereotyped movements, and poor eye-contact.[27] Notably, it was found that that the most severe autistic symptoms resulted from lesions in the amygdala and hippocampus whereas less severe forms resulted from lesions to the amygdala alone. Human autopsy studies have also found evidence for limbic system abnormalities in individuals with ASDs. These studies revealed reduced neuronal cell size and increased cell-packing density in the hippocampus and amygdala.[28] However, MRI studies have not found any evidence for abnormalities in the hippocampus.[29]
Functions of the temporal lobe are related to many of the deficits observed in individuals with ASDs, such as receptive language, social cognition, joint attention, action observation and empathy. The temporal lobe also contains the superior temporal sulcus (STS) which may mediate facial processing. It has been argued that dysfunction in the STS underlie the social deficits that characterize autism. Compared to typically developing individuals, one fMRI study found that individuals with high functioning autism had reduced activity in the STS when viewing pictures of faces.[30] Other studies have suggested that the role of the STS may be more complex than simple face processing, as research has found that individuals with ASDs have shown reduced functioning when viewing fear-provoking faces; implying that the STS is involved in understanding the emotions of others.[18] Other areas of the temporal lobe have also been implicated in ASDs. For instance, fMRI research suggests that individuals with ASDs have reduced activity in the right temporoparietal junction and other regions during imitation and observation tasks.[31]
Studies have found evidence of reduced size of the corpus callosum in individuals with autism.[32] These findings suggest that there may be a link between autism and impaired communication between brain hemispheres.
A wide variety of environmental risk factors have been proposed as contributing to autism. These include gastrointestinal or immune system abnormalities, allergies, and exposure of children to drugs, vaccines, infection, certain foods, or heavy metals. The evidence for these risk factors is anecdotal and has not been confirmed by reliable studies.[33] The subject remains controversial and extensive further searches for environmental factors are underway.
There has been a great deal of controversy over the years surrounding various theories of the etiology of autism spectrum disorders. In the 1950s, the "refrigerator mother theory" emerged as an explanation for autism. This theory was based on the idea that autistic behaviors stem from the emotional frigidity, lack of warmth, and cold, distant, rejecting demeanor of a child's mother.[34] Naturally, parents of children with an autism spectrum disorder suffered from blame, guilt, and self-doubt, especially as the theory was embraced by the medical establishment and went largely unchallenged into the mid-1960s. While the "refrigerator mother theory" has been rejected in the research literature, its effects have lingered into the 21st century. Another controversial theory suggests that watching extensive amounts of television may cause autism. This theory is largely based on research suggesting that the increasing rates of autism in the 1970s and 1980s were due to the growth of cable television at this time.[35] This theory has not been supported in the research literature. Probably the biggest and most widely circulated controversial theory of autism etiology is the "vaccine theory". This theory suggests that autism results from brain damage caused either by (1) the measles, mumps, rubella (MMR) vaccine itself, or by (2) thimerosal, an MMR vaccine stabilizer that is 50% ethylmercury.[36] The current scientific consensus is that no convincing scientific evidence supports these claims, based on various lines of evidence including the observation that the rate of autism continues to climb despite elimination of thimerosal from routine childhood vaccines.[35] Major scientific and medical bodies such as the Institute of Medicine and World Health Organization as well as governmental agencies such as the Food and Drug Administration and the CDC reject any role for thimerosal in autism or other neurodevelopmental disorders.
Theory of mind tasks assess the ability of an individual to understand the capabilities of others with respect to cognitive abilities and perception. Theory of mind tasks assess attribution of epistemic mental states, intentions and motivations, and affective states.[37] In the field of cognitive development, these tasks are usually pursued with children over the age of three, in coordination with language development for interaction with the researcher. These tasks are presented as behavioral tasks to the subject. Theory of mind difficulties seem to be universal among individuals that present on the autism spectrum. When asked to differentiate between mental and physical activities, children ages 3–4 had difficulty assessing which was which accurately. Additionally, they tend to have issues understanding figurative speech or appropriate speech with respect to the social context.[38]
As adults, only half of autistic patients are capable of speech and their linguistic abilities are lower than normal subjects. Autistic children do not seem to acquire language by immersion. Mean length utterance, a measure of linguistic productivity, evolves rapidly in autistic children. A possible rationale is that they do not have enough referential vocabulary to trigger syntax. Referential words, the understanding of what a specific word refers to, is contrasted by relational vocabulary. Limited grammatical acquisition may be attributed to a decreased lexicon of referential words.[15]
Linguistic and non linguistic elements evolve with isolated and specific social and cognitive elements, with autism there is mutual interference. Gestural means of communication is used by both non-verbal and verbal autistic children. The mother may also have an influence on development of communication.[39]
There is a division of the Autism spectrum into three categories for patients. Individuals with Asperger who acquire language to a normal degree but with a slight delay in the process of acquisition. Verbal autistic children who show delay in language acquisition and individuals who are never capable of language acquisition, they are considered nonverbal. Difference in language acquisition may be linked to different levels of impairment at theory of mind tasks.[15]
Although autism spectrum disorders are thought to follow two possible developmental courses, most parents report that symptom onset occurred within the first year of life.[40] One course of development follows a gradual course of onset in which parents tend to report concerns in development over the first two years of life and diagnosis is made around 3–4 years of age. Some of the early signs of ASDs in this course include decreased looking at faces, failure to turn when name is called, failure to show interests by showing or pointing, and delayed pretend play (see Table 1).[41] A second course of development is characterized by normal or near-normal development followed by loss of skills or regression in the first 2–3 years. Regression may occur in a variety of domains, including communication, social, cognitive, and self-help skills; however, the most common regression is loss of language.[41] There continues to be a debate over the differential outcomes based on these two developmental courses. Some studies suggest that regression is associated with poorer outcomes and others report no differences between those with early gradual onset and those who experience a regression period.[42] Overall, the prognosis for individuals with autism is poor with respect to academic achievement and independent living abilities, particularly for those who have not received early intervention.[42] However, many individuals show improvements as they grow older. The two best predictors of favorable outcome in autism are the absence of intellectual disability and the development of some communicative speech prior to 5 years of age.[43] Overall, the literature stresses the importance of early intervention in achieving positive longitudinal outcomes.[44]
Table 1: Early Symptoms of Autism[42]
| Social behavior |
Typically develops |
Behavior in children with autism compared to typically developing children |
| Looking at faces |
Birth |
Less at 12 months |
| Following person's gaze |
6–9 months |
Less at 18 months |
| Turning when name called |
6–9 months |
Less at 9 and 12 months |
| Showing objects to others |
9–12 months |
Less at 12 months |
| Pointing at interesting objects |
9–12 months |
Less at 12 months and 18 months |
| Pointing to request |
9–12 months |
Not delayed at 18 months |
| Symbolic play |
14 months |
Absent at 18 months |
Autism spectrum disorders tend to be highly comorbid with other disorders. Comorbidity may increase with age and may worsen the course of youth with ASDs and make intervention/treatment more difficult.[45] Distinguishing between ASDs and other diagnoses can be challenging because the traits of ASDs often overlap with symptoms of other disorders and the characteristics of ASDs make traditional diagnostic procedures difficult.[46] [47]In spite of these difficulties, comorbid disorders are readily identified and tend to fall into six categories, medical conditions, intellectual disabilities, anxiety disorders, mood disorders, behavior-related disorders, and sensory processing disorders.
A variety of medical conditions commonly occur in individuals with ASDs. The most common is seizure disorder or epilepsy, which occurs in 11-39% of individuals with ASD.[48] Typically, onset of epilepsy occurs before age five or during puberty.[49] and is more common in females and individuals who also have a comorbid intellectual disability. Tic disorder is another common medical condition seen in individuals with ASDs. While only about 6.5% of individuals with an ASD have full blown Tourette syndrome, nearly 30% show some form of tics.[50] Tuberous sclerosis, a medical condition in which non-malignant tumors grow in the brain and on other vital organs, occurs in 1-4% of individuals with ASDs.[51] Sleep disorders are also commonly reported by parents of children with ASDs, including late sleep onset, early morning awakening, and poor sleep maintenance.[49]
Intellectual disabilities are some of the most common comorbid disorders with ASDs. Recent estimates suggest that 40-69% of individuals with ASD have some degree of mental retardation,[42] with females more likely to be in severe range of mental retardation. Learning disabilities are also highly comorbid in individuals with an ASD. Approximately 25-75% of individuals with an ASD also have some degree of learning disability,[52] although the types of learning disability vary depending on the specific strengths and weaknesses of the individual.
A variety of anxiety disorders tend to co-occur with autism spectrum disorders, with overall comorbidity rates of 7-84%.[42] Specific phobia is the most common comorbid condition over the lifetime for those with ASD, with comorbidity rates of 38-63%.[45] Common phobias for children with ASD include the fear of certain places or situations, the fear of medically related people, places, or things, and the fear of loud noises.[53] Obsessive-compulsive disorder (OCD) occurs in 11-35% of individuals with ASD, with 16-81% showing features of the disorder without a full diagnosis. While individuals with ASD exhibit rigid thinking and compulsions, similar to OCD, the repetitive behaviors displayed in ASD (i.e., flapping, spinning, repeating phrases) are distinct and serve alternate functions than the repetitive behaviors displayed in OCD (i.e., checking, cleaning, counting).[45] Social Phobia or Social Anxiety Disorder is seen in approximately 7.4% of individuals with ASD, but is more common in higher-functioning individuals who have a desire for social interactions, but are also aware of their social deficits.[45]
Rates of comorbid depression in individuals with an ASD range from 4–58%.[54] The presentation of depression in ASDs can depend on level of cognitive functioning, with lower functioning children displaying more behavior issues and higher functioning children displaying more traditional depressive symptoms.[46] Depression is thought to develop and occur more in high-functioning individuals during adolescence, when they develop greater insight into their differences from others.[42] Bipolar disorder may also be comorbid with an ASD, although it is far less common than many other disorders. Rates of comorbidity vary greatly, but tend to be around 2-8%.[53]
Deficits in ASD are often linked to behavior problems, such as difficulties following directions, being cooperative, and doing things on other people's terms [55] These behavior problems are also characteristic of Oppositional Defiant Disorder (ODD) and Conduct Disorder (CD), but the reasons for the behaviors often differ.[53] For this reason, comorbidity rates of ODD and CD range from 7%[53] to 73%.[45] Attention Deficit Hyperactivity Disorder (ADHD) has ASD comorbidity rates of 30-80%; however, current diagnostic guidelines prevent diagnosing ADHD along with ASD.[53] Rather, ADHD-like symptoms are seen to be part of the ASD diagnosis.[56]
Sensory processing disorder is also comorbid with ASD, with comorbidity rates of 42-88%.[57] Three patterns of sensory processing difficulties are commonly seen, hyperresponsiveness (behavioral over-reactivity to sensory stimuli), hyporesponsiveness (behavioral under-reactivity to sensory stimuli), and sensory-seeking (craving or fascination with certain stimuli). These patterns of processing difficulties may be present with auditory, visual, or tactile stimuli.[58]
Typical characteristic of adolescent with minor Autism spectrum disorder, notice the Latitudinous
Philtrum
Facial Phenotypes in Prepubescent boys indicate that the face and brain develop in coordination; researchers have found distinct abnormalities in facial characteristics in a clinically significant percentage of boys with ASD as compared to those of normally developing boys. The distinguishing irregularities are typically: a broader upper face and wider eyes, a shorter middle region of the face, including the cheeks and nose, and a broader mouth and Philtrum.[59]
The diverse expressions of ASD symptoms pose diagnostic challenges to clinicians. Individuals with an ASD may present at various times of development (e.g., toddler, child, or adolescent) and symptom expression may vary over the course of development.[60] Furthermore, clinicians are required to differentiate among the different pervasive developmental disorders as well as other disorders such as mental retardation not associated with a pervasive developmental disorder, specific developmental disorders (e.g. language), and early onset schizophrenia.
Considering the unique challenges associated with diagnosing ASD, specific practice parameters for the assessment of ASD have been published by the American Academy of Neurology,[61] the American Academy of Child and Adolescent Psychiatry,[60] and a consensus panel with representation from various professional societies.[62] The practice parameters outlined by these societies include an initial screening of children by general practitioners (i.e., "Level 1 screening") and for children who fail the initial screening, a comprehensive diagnostic assessment by experienced clinicians (i.e. "Level 2 evaluation"). Furthermore, it has been suggested that assessments of children with suspected ASD be evaluated within a developmental framework, include multiple informants (e.g., parents and teachers) from diverse contexts (e.g., home and school), and employ a multidisciplinary team of professionals (e.g., clinical psychologists, neuropsychologists, and psychiatrists).[63]
After a child fails an initial screening, psychologists administer various psychological assessment tools to assess for ASD.[63] Amongst these measurements, the Autism Diagnostic Interview-Revised (ADI-R) and the Autism Diagnostic Observation Schedule (ADOS) are considered the "gold standards" for assessing children with autism. The ADI-R is a semi-structured parent interview that probes for symptoms of autism by evaluating a child's current behavior and developmental history. The ADOS is a semistructured interactive evaluation of ASD symptoms that is used to measure social and communication abilities by eliciting a number of opportunities (or "presses") for spontaneous behaviors (e.g., eye contact) in standardized context. Various other questionnaires (e.g., The Childhood Autism Rating Scale) and tests of cognitive functioning (e.g., The Peabody Picture Vocabulary Test) are typically included in an ASD assessment battery.
Over the past two decades, there has been increasing attention to the development of evidenced-based interventions for young children with ASDs. Additionally, many unresearched alternative therapies have also been implemented (e.g., vitamin therapy and acupuncture). Although evidenced-based interventions for children with autism vary in their methods, many adopt a psychoeducational approach to enhancing cognitive, communication and social skills while minimizing problem behaviors. It has been argued that no single treatment is best and treatment is typically tailored to the child's needs. Available approaches include Treatment and education of autistic and related communication handicapped children (TEACCH Structured Teaching , Picture Exchange Communication Systems (PECS) an Augmentative and Alternative Communication system, Pivotal Response Training (PRT), Discrete Trial Training (DTT), Incidental Teaching, Positive Behavior Supports, Verbal Behavior Analysis (VBA) all of which have roots in applied behavior analysis (ABA); speech and language therapy, social skills therapy, occupational therapy, and parent-communication training.[64] One of the most empirically supported intervention approaches is ABA, particularly in regard to early intensive home-based therapy. It is recommended that children receiving early intervention ABA therapy receive approximately 40 hours of therapy per week for about two years. This intensive approach underlies the UCLA Young Autism Project, originally developed by Lovaas and colleagues, and children in these programs have demonstrated gains in IQ that are maintained until adolescence.[65][66] Although ABA therapy has a strong research base, other studies have found that this approach may be limited by diagnostic severity and IQ.[67][68][69]
- ABA AND EIBI
Early Intensive Behavioral Intervention, through the use of Applied Behavioral Analysis, has been researched for over 40 years in its effectiveness. Most EIBI programs recognize that all skills appropriate for each specific age are teachable and should be taught. General curriculum areas that are addressed are language, social skills, play skills, motor skills, pre-academic and academic skills, and independent living skills. ABA models of intervention for preschool age include two main approaches to teaching. "Discrete Trial Teaching" or DTT, includes multiple discrete opportunities that are presented across the day or session. A discrete trial consists of the therapist presenting an instruction, the child responding, and the therapist responding to that by presenting a consequence. If the child responds incorrectly, the reinforcer is not given and the therapist will follow up with a error correction procedure, followed by another trial. A strength of this way of teaching is the child receives a large number of trials in a short time, allowing for a large amount of learning opportunities. A potential weakness may be that the skills learned in this structured setting are not easily generalized in less strict settings. "Natural Environment Teaching" consists of maximizing naturally occurring learning opportunities. It involves a more child-directed format that allows for the child to initiate learning, and the therapist to recognize this and follow it by prompting the child for a desired behavior before giving the reinforcer.[70]
All successful early intervention programs should focus on the TRIAD of impairments in ASDs.[71] This includes: a) Joint attention and communication, b) Social understanding and relationships, and c) Flexibility in thinking and behavior. It also includes a strong, positive partnership with parents, siblings, and co-therapists to keep consistency throughout the childs day. Research for more effective treatments should focus on the extent to which E.I Programs are adapting to the child's pattern of strengths and weaknesses and take into account all family circumstances. The importance and effectiveness of treatment (especially ABA formatted treatment) relies heavily on both child and family circumstances. At the child's level the age at entry into treatment and the severity of impairment on a cognitive, language, and behavioral level are crucial. At the family level, Socio-Economic status, level of income, presence of parents and proximity to outside family and resources, organization, extent of stress of the parents and ability to work alongside the therapist are most important for success.[72]
The Early Start Denver Program integrates applied behavior analysis with relationship and developmental based approaches. Autistic children who participated in the Early Start Denver Program showed significant increase in IQ, adaptive behavior and diagnostic status when compared to community interventions.[73]
Many popular therapies including auditory integration training, GFCF diets, and chelation are not considered evidence-based practices. Notably, research suggests that children with ASDs on GFCF diets do not differ from control groups in terms of their symptoms.[74]
Reviews tend to estimate a prevalence of 6 per 1,000 for autism spectrum disorders as a whole,[75] however prevalence rates vary for each of the developmental disorders in the spectrum. Autism prevalence has been estimated at 1-2 per 1,000, Asperger syndrome at roughly 0.6 per 1,000, childhood disintegrative disorder at 0.02 per 1,000, and PDD-NOS at 3.7 per 1,000.[75] These rates are consistent across cultures and ethnic groups, as autism is considered a universal disorder.[42]
While rates of autism spectrum disorders are consistent across cultures, they vary greatly by gender, with boys being affected far more frequently than girls. The average male-to-female ratio for ASDs is 4.2:1,[76] affecting 1 in 70 males, but only 1 in 315 females.[77] Females, however, are more likely to have associated cognitive impairment. Among those with an ASD and mental retardation, the sex ratio may be closer to 2:1.[78] Prevalence differences might be accounted by gender differences in the expression of clinical symptoms, with females showing less atypical behaviors and, therefore, less likely to receive an ASD diagnosis. [79]
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