Fact sheet for information and discussion on the incidence of Autism, an Autism Spectrum Disorder


Autism is a very common disorder on the autism spectrum and it affects people equally regardless of ethnicity, intelligence, geographical location or socioeconomic background. Autism appears to be three to four times more likely to affect boys than girls.


gender differences in Autism

There is debate over whether boys are more likely to be diagnosed simply because girls generally have more developed social skills at a given age and will perform better on diagnostic tests. Studies have found much higher prevalence in boys at the high-functioning end of the autism spectrum, while the ratios appear to be closer to 1:1 at the low-functioning end. In addition, a study published in 2006 suggested that men over 40 are more likely than younger men to father a child with autism, and that the ratio of autism incidence in boys and girls is closer to 1:1 with older fathers.


Theories on increase of autism

There was a worldwide increase in reported cases of autism over the decade to 2006, which may echo the pattern following the description of schizophrenia in the twentieth century. There are several theories about the apparent sudden increase. Many epidemiologists argue that the rise in the incidence of autism is largely attributable to a broadening of the diagnostic concept, reclassifications and public awareness. However, some researchers indicate that the existence of an as yet unidentified contributing environmental risk factor cannot be ruled out.

The question of whether the rise in incidence is real or an artifact of improved diagnosis and a broader concept of autism remains controversial. Dr. Chris Johnson, a professor of pediatrics at the University of Texas Health Sciences Center at San Antonio and co-chair of the American Academy of Pediatrics Autism Expert Panel, sums up the state of the issue by saying, “There is a chance we’re seeing a true rise, but right now I don't think anybody can answer that question for sure.”For more information, see the Is there an autism epidemic? fact sheet.


Incidence of autism

Estimates of the prevalence of autism vary widely depending on diagnostic criteria, age of children screened, and geographical location.[109] Most recent reviews tend to estimate a prevalence of 1–2 per 1,000 for autism and close to 6 per 1,000 for Autism Spectrum Disorder; PDD-NOS is the vast majority of Autism Spectrum Disorder, Asperger's is about 0.3 per 1,000 and the atypical forms childhood disintegrative disorder and Rett Syndrome are much rarer.[1] A 2006 study of nearly 57,000 British nine- and ten-year-olds reported a prevalence of 3.89 per 1,000 for autism and 11.61 per 1,000 for Autism Spectrum Disorder; these higher figures could be associated with broadening diagnostic criteria.[2]


The risk of autism is associated with several prenatal and perinatal risk factors. A 2007 review of risk factors found associated parental characteristics that included advanced maternal age, advanced paternal age, and maternal place of birth outside Europe or North America, and also found associated obstetric conditions that included low birth weight and gestation duration, and hypoxia during childbirth.[3]


About 10–15% of autism cases have an identifiable Mendelian (single-gene) condition, chromosome abnormality, or other genetic syndrome, and Autism Spectrum Disorder is associated with several genetic disorders.[4] Autism is associated with intellectual disability: a 2001 British study of 26 autistic children found about 30% with intelligence in the normal range (IQ above 70), 50% with mild to moderate retardation, and about 20% with severe to profound retardation (IQ below 35). For Autism Spectrum Disorders other than autism the association is much weaker: the same study reported about 94% of 65 children with PDD-NOS or Asperger's had normal intelligence.[5] Autism Spectrum Disorder is also associated with epilepsy, with variations in risk of epilepsy due to age, cognitive level, and type of language disorder.[6] Boys are at higher risk for autism than girls. The Autism Spectrum Disorder sex ratio averages 4.3:1 and is greatly modified by cognitive impairment: it may be close to 2:1 with intellectual disability and more than 5.5:1 without. Recent studies have found no association with socioeconomic status, and have reported inconsistent results about associations with race or ethnicity. Phobias, depression and other psychopathological disorders have often been described along with Autism Spectrum Disorder but this has not been assessed systematically.[7]


Autism's incidence rate, despite its advantages for assessing risk, is less useful in autism epidemiology, as the disorder starts long before it is diagnosed, and the gap between initiation and diagnosis is influenced by many factors unrelated to risk. Attention is focused mostly on whether prevalence is increasing with time. Earlier prevalence estimates were lower, centering at about 0.5 per 1,000 for autism during the 1960s and 1970s and about 1 per 1,000 in the 1980s, as opposed to today's 1–2 per 1,000.


The number of reported cases of autism increased dramatically in the 1990s and early 2000s. This increase is largely attributable to changes in diagnostic practices, referral patterns, availability of services, age at diagnosis, and public awareness,[8] though as-yet-unidentified contributing environmental risk factors cannot be ruled out.[3] A widely cited 2002 pilot study concluded that the observed increase in autism in California cannot be explained by changes in diagnostic criteria,[9] but a 2006 analysis found that special education data poorly measured prevalence because so many cases were undiagnosed, and that the 1994–2003 U.S. increase was associated with declines in other diagnostic categories, indicating that diagnostic substitution had occurred.[10] It is unknown whether autism's prevalence increased during the same period. An increase in prevalence would suggest directing more attention and funding toward changing environmental factors instead of continuing to focus on genetics.


Should research focus on genetic or environmental causes?

The answer to this question has significant ramifications on the direction of research, since a real increase would focus more attention and research funding on the search for environmental factors, while the alternative would focus more attention to genetics. On the other hand, it is conceivable that certain environmental factors (such as chemicals, infections, medicines, vaccines, diet and societal changes) may have a particular impact on people with a specific genetic constitution.


Autism - a disorder or a label?

One of the more popular theories is that there is a connection between “geekdom” and autism. This is hinted, for instance, by a Wired Magazine article in 2001 entitled “The Geek Syndrome”, which is a point argued by many in the autism rights movement. This article, many professionals assert, is just one example of the media’s application of mental disease labels to what is actually variant normal behavior – they argue that shyness, lack of athletic ability or social skills, and intellectual interests, even when they seem unusual to others, are not in themselves signs of autism, Asperger’s syndrome or other Autism Spectrum Disorder. Others assert that children who in the past would have simply been accepted as a little different or even labeled ‘gifted’ are now being labeled with mental disease diagnoses.

Due to the recent publicity surrounding autism and Autism Spectrum Disorders, an increasing number of adults are choosing to seek diagnoses of high-functioning autism or Asperger’s syndrome in light of symptoms they currently experience or experienced during childhood. Since the cause of autism is thought to be at least partly genetic, a proportion of these adults seek their own diagnosis specifically as follow-up to their children’s diagnoses. Because autism falls into the Pervasive Developmental Disorder category, an individual’s symptoms must have been present before age seven in order to make a strict differential diagnosis.



1. ^ Fombonne E (2005). "Epidemiology of autistic disorder and other pervasive developmental disorders". J Clin Psychiatry 66 (Suppl 10): 3–8. PMID 16401144.
2. ^ Baird G, Simonoff E, Pickles A et al. (2006). "Prevalence of disorders of the autism spectrum in a population cohort of children in South Thames: the Special Needs and Autism Project (SNAP)". Lancet 368 (9531): 210–5. doi:10.1016/S0140-6736(06)69041-7. PMID 16844490.
3. ^ Kolevzon A, Gross R, Reichenberg A (2007). "Prenatal and perinatal risk factors for autism". Arch Pediatr Adolesc Med 161 (4): 326–33. PMID 17404128.
4. ^ Zafeiriou DI, Ververi A, Vargiami E (2007). "Childhood autism and associated comorbidities". Brain Dev 29 (5): 257–72. doi:10.1016/j.braindev.2006.09.003. PMID 17084999.
5. ^ Chakrabarti S, Fombonne E (2001). "Pervasive developmental disorders in preschool children". JAMA 285 (24): 3093–9. PMID 11427137.
6. ^ Tuchman R, Rapin I (2002). "Epilepsy in autism". Lancet Neurol 1 (6): 352–8. doi:10.1016/S1474-4422(02)00160-6. PMID 12849396.
7. ^ Matson JL, Nebel-Schwalm MS (2007). "Comorbid psychopathology with Autism Spectrum Disorder in children: an overview". Res Dev Disabil 28 (4): 341–52. doi:10.1016/j.ridd.2005.12.004. PMID 16765022.
8. ^ Changes in diagnostic practices:
* Fombonne E (2003). "The prevalence of autism". JAMA 289 (1): 87–9. PMID 12503982.
* Wing L, Potter D (2002). "The epidemiology of autism spectrum disorders: is the prevalence rising?". Ment Retard Dev Disabil Res Rev 8 (3): 151–61. doi:10.1002/mrdd.10029. PMID 12216059.
9. ^ Byrd RS, Sage AC, Keyzer J et al. (2002). "Report to the legislature on the principal findings of the epidemiology of autism in California: a comprehensive pilot study" (PDF). M.I.N.D. Institute. Retrieved on 2006-09-18.
10. ^ Shattuck PT (2006). "The contribution of diagnostic substitution to the growing administrative prevalence of autism in US special education". Pediatrics 117 (4): 1028–37. doi:10.1542/peds.2005-1516. PMID 16585296. Lay summary (2006-04-03).

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Autism is one of the most common disorders on the autism spectrum and it affects people equally regardless of ethnicity, intelligence, geographical location or socioeconomic background. Autism appears to be three to four times more likely to affect boys than girls in terms of gender.