epidemiology of autism

Although incidence rates measure autism prevalence directly, most epidemiological studies report other frequency measures, typically point or period prevalence, or sometimes cumulative incidence. Attention is focused mostly on whether prevalence is increasing with time.

Epidemiology defines several measures of the frequency of occurrence of a disease or condition:{{cite book |vauthors=Coggon D, Rose G, Barker DJ |title=Epidemiology for the Uninitiated |chapter=Quantifying diseases in populations |publisher=BMJ |date=1997 |isbn=0-7279-1102-3 |edition=4th}}

• The incidence rate of a condition is the rate at which new cases occurred per person-year, for example, "2 new cases per 1,000 person-years".
• The cumulative incidence is the proportion of a population that became new cases within a specified time period, for example, "1.5 per 1,000 people became new cases during 2006".
• The point prevalence of a condition is the proportion of a population that had the condition at a single point in time, for example, "10 cases per 1,000 people at the start of 2006".
• The period prevalence is the proportion that had the condition at any time within a stated period, for example, "15 per 1,000 people had cases during 2006".

When studying how conditions are caused, incidence rates are the most appropriate measure of condition frequency as they assess probability directly. However, incidence can be difficult to measure with rarer conditions such as autism. In autism epidemiology, point or period prevalence is more useful than incidence, as the condition starts long before it is diagnosed, bearing in mind genetic elements it is inherent from conception, and the gap between initiation and diagnosis is influenced by many factors unrelated to chance. Research focuses mostly on whether point or period prevalence is increasing with time; cumulative incidence is sometimes used in studies of birth cohorts.{{cite journal |vauthors=Newschaffer CJ, Croen LA, Daniels J, Giarelli E, Grether JK, Levy SE, Mandell DS, Miller LA, Pinto-Martin J, Reaven J, Reynolds AM, Rice CE, Schendel D, Windham GC |display-authors=6 |title=The epidemiology of autism spectrum disorders |journal=Annual Review of Public Health |volume=28 |pages=235–58 |year=2007 |pmid=17367287 |doi=10.1146/annurev.publhealth.28.021406.144007 |doi-access=free}}

The three basic approaches used to estimate prevalence differ in cost and in quality of results. The simplest and cheapest method is to count known autism cases from sources such as schools and clinics, and divide by the population. This approach is likely to underestimate prevalence because it does not count children who have not been diagnosed yet, and it is likely to generate skewed statistics because some children have better access to treatment.{{cite journal |vauthors=Scahill L, Bearss K |title=The rise in autism and the mercury myth |journal=Journal of Child and Adolescent Psychiatric Nursing |volume=22 |issue=1 |pages=51–3 |date=February 2009 |pmid=19200293 |doi=10.1111/j.1744-6171.2008.00152.x |doi-access=free}}

The second method improves on the first by having investigators examine student or patient records looking for probable cases, to catch cases that have not been identified yet. The third method, which is arguably the best, screens a large sample of an entire community to identify possible cases, and then evaluates each possible case in more detail with standard diagnostic procedures. This last method typically produces the most reliable, and the highest, prevalence estimates.

Estimates of the prevalence of autism vary widely depending on diagnostic criteria, age of children screened, and geographical location.{{cite journal |vauthors=Williams JG, Higgins JP, Brayne CE |title=Systematic review of prevalence studies of autism spectrum disorders |journal=Archives of Disease in Childhood |volume=91 |issue=1 |pages=8–15 |date=January 2006 |pmid=15863467 |pmc=2083083 |doi=10.1136/adc.2004.062083 |url=http://autismresearchcentre.com/docs/papers/2006_Williams_etal_ADC.pdf |archive-url=https://web.archive.org/web/20090418234727/http://www.autismresearchcentre.com/docs/papers/2006_Williams_etal_ADC.pdf |archive-date=2009-04-18}} Most recent reviews tend to estimate a prevalence of 1–2 per 1,000 for autism and close to 27.6 per 1,000 for ASD{{Cite web |last=CDC |date=2024-01-10 |title=Data and Statistics on Autism Spectrum Disorder {{!}} CDC |url=https://www.cdc.gov/ncbddd/autism/data.html |access-date=2024-02-18 |website=Centers for Disease Control and Prevention }};

PDD-NOS is the vast majority of ASD, Asperger syndrome is about 0.3 per 1,000 and the atypical forms childhood disintegrative disorder and Rett syndrome are much rarer.{{cite journal |vauthors=Fombonne E |title=Epidemiology of autistic disorder and other pervasive developmental disorders |journal=The Journal of Clinical Psychiatry |volume=66 |issue=Suppl 10 |pages=3–8 |date=2005 |pmid=16401144 |author-link=Eric Fombonne}}

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 ASD; these higher figures could be associated with broadening diagnostic criteria.{{cite journal |vauthors=Baird G, Simonoff E, Pickles A, Chandler S, Loucas T, Meldrum D, Charman T |title=Prevalence of disorders of the autism spectrum in a population cohort of children in South Thames: the Special Needs and Autism Project (SNAP) |journal=Lancet |volume=368 |issue=9531 |pages=210–5 |date=July 2006 |pmid=16844490 |doi=10.1016/S0140-6736(06)69041-7 |s2cid=23532364}} Studies based on more detailed information, such as direct observation rather than examination of medical records, identify higher prevalence; this suggests that published figures may underestimate ASD's true prevalence.{{cite journal |vauthors=Caronna EB, Milunsky JM, Tager-Flusberg H |title=Autism spectrum disorders: clinical and research frontiers |journal=Archives of Disease in Childhood |volume=93 |issue=6 |pages=518–23 |date=June 2008 |pmid=18305076 |doi=10.1136/adc.2006.115337 |s2cid=18761374}} A 2009 study of the children in Cambridgeshire, England used different methods to measure prevalence, and estimated that 40% of ASD cases go undiagnosed, with the two least-biased estimates of true prevalence being 11.3 and 15.7 per 1,000.{{cite journal |vauthors=Baron-Cohen S, Scott FJ, Allison C, Williams J, Bolton P, Matthews FE, Brayne C |title=Prevalence of autism-spectrum conditions: UK school-based population study |journal=The British Journal of Psychiatry |volume=194 |issue=6 |pages=500–9 |date=June 2009 |pmid=19478287 |doi=10.1192/bjp.bp.108.059345 |s2cid=6928284 |doi-access=free}}

A 2009 U.S. study based on 2006 data estimated the prevalence of ASD in eight-year-old children to be 9.0 per 1,000 (approximate range 8.6–9.3).{{cite journal |vauthors= |title=Prevalence of autism spectrum disorders - Autism and Developmental Disabilities Monitoring Network, United States, 2006 |journal=Morbidity and Mortality Weekly Report. Surveillance Summaries |volume=58 |issue=10 |pages=1–20 |date=December 2009 |pmid=20023608 |url=https://www.cdc.gov/mmwr/preview/mmwrhtml/ss5810a1.htm}} A 2009 report based on the 2007 Adult Psychiatric Morbidity Survey by the National Health Service determined that the prevalence of ASD in adults was approximately 1% of the population, with a higher prevalence in males and no significant variation between age groups;{{cite report |vauthors=Brugha T, McManus S, Meltzer H, etal |url=http://www.ic.nhs.uk/webfiles/publications/mental%20health/mental%20health%20surveys/APMS_Autism_report_standard_20_OCT_09.pdf |access-date=2010-02-16 |title=Autism Spectrum Disorders in adults living in households throughout England: Report from the Adult Psychiatric Morbidity Survey 2007 |publisher=NHS Information Centre for health and social care |date=2009 |archive-url=https://web.archive.org/web/20110103163841/http://www.ic.nhs.uk/webfiles/publications/mental%20health/mental%20health%20surveys/APMS_Autism_report_standard_20_OCT_09.pdf |archive-date=2011-01-03}} these results suggest that prevalence of ASD among adults is similar to that in children and rates of autism are not increasing.{{cite news |vauthors=Boseley S |url=https://www.theguardian.com/society/2009/sep/22/autism-rate-mmr-vaccine |title=Autism just as common in adults, so MMR jab is off the hook |work=Guardian |date=2009-09-22}}

Attention has been focused on whether the prevalence of autism 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 23 per 1000.{{cite web |url=https://www.cdc.gov/ncbddd/autism/data.html |title=ASD Data and Statistics |website=CDC.gov |access-date=23 Aug 2022 |archive-url=https://web.archive.org/web/20220809064946/http://www.cdc.gov/ncbddd/autism/data.html/ |archive-date=2022-08-09}}

The number of reported cases of autism increased dramatically in the 1990s and 2000s, prompting ongoing investigations into several potential reasons:{{cite web |url=http://www.nas.org.uk/nas/jsp/polopoly.jsp?d=364&a=2618 |title=Notes on the prevalence of autism spectrum disorders |vauthors=Wing L, Potter D |author-link1=Lorna Wing |publisher=National Autistic Society |date=1999 |access-date=2007-12-10 |archive-date=10 May 2008 |archive-url=https://web.archive.org/web/20080510064827/http://www.nas.org.uk/nas/jsp/polopoly.jsp?d=364&a=2618 |url-status=dead }}

• More children may have autism; that is, the true frequency of autism may have increased.
• There may be more complete pickup of autism (case finding), as a result of increased awareness and funding. For example, attempts to sue vaccine companies may have increased case-reporting.
• The diagnosis may be applied more broadly than before, as a result of the changing definition of the disorder, particularly changes in DSM-III-R and DSM-IV.
• An editorial error in the description of the PDD-NOS category of Autism Spectrum Disorders in the DSM-IV, in 1994, inappropriately broadened the PDD-NOS construct. The error was corrected in the DSM-IV-TR, in 2000, reversing the PDD-NOS construct back to the more restrictive diagnostic criteria requirements from the DSM-III-R.{{cite web |title=Clarification of the definition of Pervasive Developmental Disorder Not Otherwise Specified |url=http://www.psychiatry.org/practice/dsm/practice-relevant-changes-to-the-dsm-iv-tr |access-date=5 November 2012 |archive-date=30 September 2013 |archive-url=https://web.archive.org/web/20130930004907/http://www.psychiatry.org/practice/dsm/practice-relevant-changes-to-the-dsm-iv-tr |url-status=dead }}
• Successively earlier diagnosis in each succeeding cohort of children, including recognition in nursery (preschool), may have affected apparent prevalence but not incidence.
• A review of the "rising autism" figures compared to other disabilities in schools shows a corresponding drop in findings of intellectual disability.{{cite journal |vauthors=Shattuck PT |title=The contribution of diagnostic substitution to the growing administrative prevalence of autism in US special education |journal=Pediatrics |volume=117 |issue=4 |pages=1028–37 |date=April 2006 |pmid=16585296 |doi=10.1542/peds.2005-1516 |s2cid=26733990}}
• {{lay source |template=cite web |last1=Devitt |first1=Terry |title=Data provides misleading picture of autism |url=https://news.wisc.edu/data-provides-misleading-picture-of-autism/ |website=News.wisc.edu |publisher=University of Wisconsin–Madison |access-date=9 February 2024}}

The reported increase is largely attributable to changes in diagnostic practices, referral patterns, availability of services, age at diagnosis, and public awareness.Prevalence and changes in diagnostic practice:

• {{cite journal |vauthors=Fombonne E |title=The prevalence of autism |journal=JAMA |volume=289 |issue=1 |pages=87–9 |date=January 2003 |pmid=12503982 |doi=10.1001/jama.289.1.87}}
• {{cite journal |vauthors=Wing L, Potter D |title=The epidemiology of autistic spectrum disorders: is the prevalence rising? |journal=Mental Retardation and Developmental Disabilities Research Reviews |volume=8 |issue=3 |pages=151–61 |year=2002 |pmid=12216059 |doi=10.1002/mrdd.10029}} A widely cited 2002 pilot study concluded that the observed increase in autism in California cannot be explained by changes in diagnostic criteria,{{cite web |vauthors=Byrd RS, Sage AC, Keyzer J, etal |website=M.I.N.D. Institute |title=Report to the legislature on the principal findings of the epidemiology of autism in California: a comprehensive pilot study |year=2002 |url=http://www.ucdmc.ucdavis.edu/mindinstitute/newsroom/study_final.pdf |archive-url=https://web.archive.org/web/20080414094255/http://www.ucdmc.ucdavis.edu/mindinstitute/newsroom/study_final.pdf |archive-date=2008-04-14}} 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.

There are claims that there is an "autism epidemic" based on the increased number of diagnosed cases. A 2007 study that modeled autism incidence found that broadened diagnostic criteria, diagnosis at a younger age, and improved efficiency of case ascertainment, can produce an increase in autism diagnoses ranging up to 29-fold depending on the frequency measure, suggesting that methodological factors may explain what appears to be an increase in autism over time.{{cite journal |vauthors=Wazana A, Bresnahan M, Kline J |title=The autism epidemic: fact or artifact? |journal=Journal of the American Academy of Child and Adolescent Psychiatry |volume=46 |issue=6 |pages=721–730 |date=June 2007 |pmid=17513984 |doi=10.1097/chi.0b013e31804a7f3b}} This observation is often mistakenly described as an "autism epidemic".{{cite web | last=Gerrard | first=Rachel Burr | title=There's no autism epidemic. But there is an autism diagnosis epidemic | website=STAT | date=February 10, 2022 | url=https://www.statnews.com/2022/02/10/theres-no-autism-epidemic-but-there-is-an-autism-diagnosis-epidemic/ | access-date=April 30, 2025}} Robert F. Kennedy Jr. has claimed that there is an "autism epidemic", which he calls a "holocaust",{{cite news |last=White |first=Jeremy B. |date=April 7, 2015 |url=http://www.sacbee.com/news/politics-government/capitol-alert/article17814440.html |title=Robert Kennedy Jr. warns of vaccine-linked 'holocaust' |work=The Sacramento Bee |archive-url=https://web.archive.org/web/20150411002553/http://www.sacbee.com/news/politics-government/capitol-alert/article17814440.html |archive-date=April 11, 2015 |url-status=live}} and he has repeated the false claim that autism is caused by vaccines. He has been heavily criticized for such statements.{{cite web | last=Kessler | first=Glenn | title=RFK Jr.'s absurd statistic on the spike in chronic diseases in the U.S. | website=The Washington Post | date=April 25, 2025 | url=https://www.washingtonpost.com/politics/2025/04/25/rfk-jr-chronic-diseases-false/ | access-date=April 30, 2025 | quote=He's long been a purveyor of the fiction that vaccines cause autism, and one of his key points of evidence is that the percentage of people with autism has increased. But the percentage of people diagnosed with autism spectrum disorder has gone up mainly because of expanded definitions and better detection. There is no blood test for autism, so a diagnosis is based on observations of a person’s behavior. Indeed, while autism diagnoses have increased, those of intellectual disability have decreased, indicating that previously, children may have been misdiagnosed with other conditions.}}{{cite web | last=Wadman | first=Meredith | title=Claiming autism 'epidemic,' RFK Jr. describes NIH initiative to find environmental causes | website=Science | date=April 16, 2025 | url=https://www.science.org/content/article/claiming-autism-epidemic-rfk-jr-describes-nih-initiative-find-environmental-causes | access-date=April 30, 2025}}{{cite web | last=Geddie | first=Becky | title=Statement on Robert F. Kennedy Jr.'s Comments Regarding the Cause of Autism and Misleading Deadline | website=Autism Society of America | date=April 11, 2025 | url=https://autismsociety.org/statement-on-robert-f-kennedy-jr-s-comments-regarding-the-cause-of-autism-and-misleading-deadline/ | access-date=April 30, 2025}}{{cite web | last=Wendling | first=Mike | title=RFK Jr pledges to find the cause of autism 'by September' | website=BBC | date=April 11, 2025 | url=https://www.bbc.com/news/articles/cj0z9nmzvdlo | access-date=April 30, 2025}}

A small 2008 study found that a significant number (40%) of people diagnosed with pragmatic language impairment as children in previous decades would now be given a diagnosis as autism.{{cite journal |vauthors=Bishop DV, Whitehouse AJ, Watt HJ, Line EA |title=Autism and diagnostic substitution: evidence from a study of adults with a history of developmental language disorder |journal=Developmental Medicine and Child Neurology |volume=50 |issue=5 |pages=341–5 |date=May 2008 |pmid=18384386 |doi=10.1111/j.1469-8749.2008.02057.x |s2cid=22025445 |doi-access=free}} A study of all Danish children born in 1994–99 found that children born later were more likely to be diagnosed at a younger age, supporting the argument that apparent increases in autism prevalence were at least partly due to decreases in the age of diagnosis.{{cite journal |vauthors=Parner ET, Schendel DE, Thorsen P |title=Autism prevalence trends over time in Denmark: changes in prevalence and age at diagnosis |journal=Archives of Pediatrics & Adolescent Medicine |volume=162 |issue=12 |pages=1150–6 |date=December 2008 |pmid=19047542 |doi=10.1001/archpedi.162.12.1150 |doi-access=}}

A 2009 study of California data found that the reported incidence of autism rose 7- to 8-fold from the early 1990s to 2007, and that changes in diagnostic criteria, inclusion of milder cases, and earlier age of diagnosis probably explain only a 4.25-fold increase; the study did not quantify the effects of wider awareness of autism, increased funding, and expanding support options resulting in parents' greater motivation to seek services.{{cite journal |vauthors=Hertz-Picciotto I, Delwiche L |title=The rise in autism and the role of age at diagnosis |journal=Epidemiology |volume=20 |issue=1 |pages=84–90 |date=January 2009 |pmid=19234401 |pmc=4113600 |doi=10.1097/EDE.0b013e3181902d15}}

• {{lay source |template=cite web |title=What Is Autism? |url=http://www.webmd.com/brain/autism/news/20090108/autism-cases-rise |date=2009-01-08 |website=WebMD}} Another 2009 California study found that the reported increases are unlikely to be explained by changes in how qualifying condition codes for autism were recorded.{{cite journal |vauthors=Grether JK, Rosen NJ, Smith KS, Croen LA |title=Investigation of shifts in autism reporting in the California Department of Developmental Services |journal=Journal of Autism and Developmental Disorders |volume=39 |issue=10 |pages=1412–9 |date=October 2009 |pmid=19479197 |doi=10.1007/s10803-009-0754-z |s2cid=7502270}}

Several environmental factors have been proposed to support the hypothesis that the actual frequency of autism has increased. These include certain foods, infectious disease, pesticides. There is overwhelming scientific evidence against the MMR hypothesis and no convincing evidence for the thiomersal (or Thimerosal) hypothesis, so these types of risk factors have to be ruled out.{{cite journal |vauthors=Rutter M |title=Incidence of autism spectrum disorders: changes over time and their meaning |journal=Acta Paediatrica |volume=94 |issue=1 |pages=2–15 |date=January 2005 |pmid=15858952 |doi=10.1111/j.1651-2227.2005.tb01779.x |s2cid=79259285 |author-link=Professor Sir Michael Rutter}} Although it is unknown whether autism's frequency has increased, any such increase would suggest directing more attention and funding toward addressing environmental factors instead of continuing to focus on genetics.{{cite journal |vauthors=Szpir M |title=Tracing the origins of autism: a spectrum of new studies |journal=Environmental Health Perspectives |volume=114 |issue=7 |pages=A412-8 |date=July 2006 |pmid=16835042 |pmc=1513312 |doi=10.1289/ehp.114-a412 |url=http://www.ehponline.org/members/2006/114-7/focus.html |archive-url=https://web.archive.org/web/20080708201740/http://www.ehponline.org/members/2006/114-7/focus.html |archive-date=2008-07-08}}

The COVID-19 pandemic may have impacted the current number of diagnoses. More assessments for ASD occurred among 4-year-olds than the current 8-year-olds when they were 4 years of age prior to the pandemic.{{Cite web |last=CDC |date=2023-03-23 |title=Key Findings from the ADDM Network |url=https://www.cdc.gov/ncbddd/autism/addm-community-report/key-findings.html |access-date=2024-02-18 |website=Centers for Disease Control and Prevention }} After the pandemic, the rate of current assessments has dropped, leading to possible delayed identification of ASD.

The prevalence of autism in Africa is unknown.{{cite journal |vauthors=Mankoski RE, Collins M, Ndosi NK, Mgalla EH, Sarwatt VV, Folstein SE |title=Etiologies of autism in a case-series from Tanzania |journal=Journal of Autism and Developmental Disorders |volume=36 |issue=8 |pages=1039–51 |date=November 2006 |pmid=16897390 |doi=10.1007/s10803-006-0143-9 |s2cid=22993074}}

The prevalence of autism in the Americas overall is unknown.

The Canadian government reported in 2019 that 1 in 50 children were diagnosed with autism spectrum disorder.{{Cite web |last=Canada |first=Public Health Agency of |date=2022-02-03 |title=Autism spectrum disorder: Highlights from the 2019 Canadian health survey on children and youth |url=https://www.canada.ca/en/public-health/services/publications/diseases-conditions/autism-spectrum-disorder-canadian-health-survey-children-youth-2019.html |access-date=2024-04-14 |website=www.canada.ca}} However, preliminary results of an epidemiological study conducted at Montreal Children's Hospital in the 200–2004 school year found a prevalence rate of 0.68% (or 1 per 147).{{cite web |last1=Norris |first1=Sonya |last2=Paré |first2=Jean-Rodrigue |last3=Starky |first3=Sheena |title=Childhood autism in Canada: some issues relating to behavioural intervention |url=https://www.publications.gc.ca/site/eng/295807/publication.html |format=PDF |publisher=Library of Parliament; Parliamentary Research Branch |series=[Modular backgrounders] |id=PRB 05-93E |location=Ottawa |date=January 26, 2006}}

A 2001 review of the medical research conducted by the Public Health Agency of Canada concluded that there was no link between MMR vaccine and either inflammatory bowel disease or autism.{{cite journal |last1=Strauss |first1=B |last2=Bigham |first2=M |title=Does measles-mumps-rubella (MMR) vaccination cause inflammatory bowel disease and autism? |journal=Canada Communicable Disease Report |date=2001 |volume=27 |issue=8 |pages=65–72 |pmid=11338656 |issn=1481-8531}} The review noted, "An increase in cases of autism was noted by year of birth from 1979 to 1992; however, no incremental increase in cases was observed after the introduction of MMR vaccination." After the introduction of MMR, "A time trend analysis found no correlation between prevalence of MMR vaccination and the incidence of autism in each birth cohort from 1988 to 1993."

According to a report by the Centers for Disease Control and Prevention in 2020, 1 in 36 children have ASD (27.6 in every 1,000).

The number of diagnosed cases of autism grew dramatically in the U.S. in the 1990s and have continued in the 2000s. For the 2006 surveillance year, identified ASD cases were an estimated 9.0 per 1000 children aged 8 years (95% confidence interval [CI] = 8.6–9.3). These numbers measure what is sometimes called "administrative prevalence", that is, the number of known cases per unit of population, as opposed to the true number of cases. This prevalence estimate rose 57% (95% CI 27%–95%) from 2002 to 2006.

The National Health Interview Survey for 2014–2016 studied 30,502 US children and adolescents and found the weighted prevalence of ASD was 2.47% (24.7 per 1,000); 3.63% in boys and 1.25% in girls. Across the 3-year reporting period, the prevalence was 2.24% in 2014, 2.41% in 2015, and 2.76% in 2016.{{cite journal |vauthors=Xu G, Strathearn L, Liu B, Bao W |title=Prevalence of Autism Spectrum Disorder Among US Children and Adolescents, 2014-2016 |journal=JAMA |volume=319 |issue=1 |pages=81–82 |date=January 2018 |pmid=29297068 |pmc=5833544 |doi=10.1001/jama.2017.17812}}

The number of new cases of autism spectrum disorder in Caucasian boys is roughly 50% higher than found in Hispanic children, and approximately 30% more likely to occur than in Non-Hispanic white children in the United States.{{cite web |url=https://www.cdc.gov/ncbddd/autism/data.html |title=ASD Data and Statistics |website=CDC.gov |access-date=5 Apr 2014 |archive-url=https://web.archive.org/web/20140418153648/http://www.cdc.gov/ncbddd/autism/data.html |archive-date=2014-04-18}}

A further study in 2006 concluded that the apparent rise in administrative prevalence was the result of diagnostic substitution, mostly for findings of intellectual disability and learning disabilities. "Many of the children now being counted in the autism category would probably have been counted in the mental retardation or learning disabilities categories if they were being labeled 10 years ago instead of today", said researcher Paul Shattuck of the Waisman Center at the University of Wisconsin–Madison, in a statement.{{cite web |last1=Devitt |first1=Terry |title=Data provides misleading picture of autism |url=https://news.wisc.edu/data-provides-misleading-picture-of-autism/ |website=News.wisc.edu |publisher=University of Wisconsin–Madison |access-date=9 February 2024}}

A population-based study in Olmsted County, Minnesota county found that the cumulative incidence of autism grew eightfold from the 1980–83 period to the 1995–97 period. The increase occurred after the introduction of broader, more-precise diagnostic criteria, increased service availability, and increased awareness of autism.{{cite journal |vauthors=Barbaresi WJ, Katusic SK, Colligan RC, Weaver AL, Jacobsen SJ |title=The incidence of autism in Olmsted County, Minnesota, 1976-1997: results from a population-based study |journal=Archives of Pediatrics & Adolescent Medicine |volume=159 |issue=1 |pages=37–44 |date=January 2005 |pmid=15630056 |doi=10.1001/archpedi.159.1.37 |doi-access=free}} During the same period, the reported number of autism cases grew 22-fold in the same location, suggesting that counts reported by clinics or schools provide misleading estimates of the true incidence of autism.{{cite journal |vauthors=Barbaresi WJ, Colligan RC, Weaver AL, Katusic SK |title=The incidence of clinically diagnosed versus research-identified autism in Olmsted County, Minnesota, 1976-1997: results from a retrospective, population-based study |journal=Journal of Autism and Developmental Disorders |volume=39 |issue=3 |pages=464–70 |date=March 2009 |pmid=18791815 |pmc=2859841 |doi=10.1007/s10803-008-0645-8}}

A 2008 study in Venezuela reported a prevalence of 1.1 per 1,000 for autism and 1.7 per 1,000 for ASD.{{cite journal |vauthors=Montiel-Nava C, Peña JA |title=Epidemiological findings of pervasive developmental disorders in a Venezuelan study |journal=Autism |volume=12 |issue=2 |pages=191–202 |date=March 2008 |pmid=18308767 |doi=10.1177/1362361307086663 |s2cid=11235638}}

A journal reports that the median prevalence of ASD among 2–6-year-old children who are reported in China from 2000 upwards was 10.3/10,000.{{cite journal |vauthors=Sun X, Allison C |title=A review of the prevalence of Autism Spectrum Disorder in Asia |journal=Research in Autism Spectrum Disorders |date=April 2010 |volume=4 |issue=2 |pages=156–167 |doi=10.1016/j.rasd.2009.10.003}}

A 2008 Hong Kong study reported an ASD incidence rate similar to those reported in Australia and North America, and lower than Europeans. It also reported a prevalence of 1.68 per 1,000 for children under 15 years.{{cite journal |vauthors=Wong VC, Hui SL |title=Epidemiological study of autism spectrum disorder in China |journal=Journal of Child Neurology |volume=23 |issue=1 |pages=67–72 |date=January 2008 |pmid=18160559 |doi=10.1177/0883073807308702 |s2cid=29590911}}

A 2005 study of a part of Yokohama with a stable population of about 300,000 reported a cumulative incidence to age 7 years of 48 cases of ASD per 10,000 children in 1989, and 86 in 1990. After the vaccination rate of the triple MMR vaccine dropped to near zero and was replaced with MR and M vaccine, the incidence rate grew to 97 and 161 cases per 10,000 children born in 1993 and 1994, respectively, indicating that the combined MMR vaccine did not cause autism.{{cite journal |vauthors=Honda H, Shimizu Y, Rutter M |title=No effect of MMR withdrawal on the incidence of autism: a total population study |journal=Journal of Child Psychology and Psychiatry, and Allied Disciplines |volume=46 |issue=6 |pages=572–9 |date=June 2005 |pmid=15877763 |doi=10.1111/j.1469-7610.2005.01425.x |doi-access=free}}

• {{lay source |template=cite web |title=Autism in the absence of MMR vaccine |url=http://www.medicine.ox.ac.uk/bandolier/booth/Vaccines/noMMR.html |archive-url=https://web.archive.org/web/20080804110713/http://www.medicine.ox.ac.uk/bandolier/booth/Vaccines/noMMR.html |archive-date=2008-08-04 |date=2005 |website=Bandolier}} A 2004 Japanese autism association reported that about 360.000 people have typical Kanner-type autism.

Across all ages, 1 in 70 Australians identify as being autistic (or a person with autism). 1 in 23 children (or 4.36%) aged 7 to 14 years have an autism diagnosis.{{Cite web |title=Understanding autism prevalence |url=https://taxpolicy.crawford.anu.edu.au/sites/default/files/publication/taxstudies_crawford_anu_edu_au/2023-11/complete_wp_m_ranjan_nov_2023.pdf |access-date=2024-06-04 |date=2023 |website=The Australian National University}}

A 2009 study reported that the annual incidence rate of Israeli children with a diagnosis of ASD receiving disability benefits rose from zero in 1982–1984 to 190 per million in 2004. It was not known whether these figures reflected true increases or other factors such as changes in diagnostic measures.{{cite journal |vauthors=Senecky Y, Chodick G, Diamond G, Lobel D, Drachman R, Inbar D |title=Time trends in reported autistic spectrum disorders in Israel, 1972-2004 |journal=The Israel Medical Association Journal |volume=11 |issue=1 |pages=30–3 |date=January 2009 |pmid=19344009 |url=http://www.ima.org.il/imaj/ar09jan-05.pdf |archive-url=https://web.archive.org/web/20110716095534/http://www.ima.org.il/imaj/ar09jan-05.pdf |archive-date=2011-07-16}}

Studies of autism frequency have been particularly rare in the Middle East. One rough estimate is that the prevalence of autism in Saudi Arabia is 18 per 10,000, slightly higher than the 13 per 10,000 reported in developed countries.{{cite journal |vauthors=Al-Salehi SM, Al-Hifthy EH, Ghaziuddin M |title=Autism in Saudi Arabia: presentation, clinical correlates and comorbidity |journal=Transcultural Psychiatry |volume=46 |issue=2 |pages=340–7 |date=June 2009 |pmid=19541755 |doi=10.1177/1363461509105823 |s2cid=26992006}}(compared to 168 per 10,000 in the USA)

In 1992, thiomersal-containing vaccines were removed in Denmark. A study at Aarhus University indicated that during the chemical's usage period (up through 1990), there was no trend toward an increase in the incidence of autism. Between 1991 and 2000 the incidence increased, including among children born after the discontinuation of thimerosal.{{cite journal |vauthors=Madsen KM, Lauritsen MB, Pedersen CB, Thorsen P, Plesner AM, Andersen PH, Mortensen PB |title=Thimerosal and the occurrence of autism: negative ecological evidence from Danish population-based data |journal=Pediatrics |volume=112 |issue=3 Pt 1 |pages=604–6 |date=September 2003 |pmid=12949291 |doi=10.1542/peds.112.3.604}}

{{Main|Autism in France}}

France made autism the national focus for the year 2012 and the Health Ministry estimated the rate of autism in 2012 to have been 0.67%, i.e. 1 in 150.{{cite web |url=http://www.autismegrandecause2012.fr/ |title=Autisme Grande Cause |archive-url=https://web.archive.org/web/20120404202501/http://www.autismegrandecause2012.fr/ |archive-date=4 April 2012}}

Eric Fombonne made some studies in the years 1992 and 1997. He found a prevalence of 16 per 10,000 for the global pervasive developmental disorder (PDD).{{cite journal |vauthors=Fombonne E, du Mazaubrun C |title=Prevalence of infantile autism in four French regions |journal=Social Psychiatry and Psychiatric Epidemiology |volume=27 |issue=4 |pages=203–10 |date=August 1992 |pmid=1411750 |doi=10.1007/bf00789007 |s2cid=20081218}}{{cite journal |vauthors=Fombonne E, Du Mazaubrun C, Cans C, Grandjean H |title=Autism and associated medical disorders in a French epidemiological survey |journal=Journal of the American Academy of Child and Adolescent Psychiatry |volume=36 |issue=11 |pages=1561–9 |date=November 1997 |pmid=9394941 |doi=10.1016/S0890-8567(09)66566-7}}

The INSERM found a prevalence of 27 per 10,000 for the ASD and a prevalence of 9 per 10,000 for the early infantile autism in 2003.Expertise collective. Troubles mentaux. Dépistage et prévention chez l'enfant et chez l'adolescent. Inserm, 2003, 8 Those figures are considered as underrated as the WHO gives figures between 30 and 60 per 10,000.{{cite web |url=http://www.sante-sports.gouv.fr/IMG/pdf/Plan_autisme_2008-2010.pdf |title=Plan autisme 2008–2010: Construire une nouvelle étape de la politique des troubles envahissants du développement (TED) et en particulier de l'autisme |trans-title=Autism Plan 2008-2010: Building a new stage in the policy of pervasive developmental disorders (PDD) and in particular autism |publisher=Republic of France |language=French |archive-url=https://web.archive.org/web/20100327023230/http://www.sante-sports.gouv.fr/IMG/pdf/Plan_autisme_2008-2010.pdf |archive-date=2010-03-27}} The French Minister of Health gives a prevalence of 4.9 per 10,000 on its website but it counts only early infantile autism.{{Cite web |url=http://lesrapports.ladocumentationfrancaise.fr/BRP/034000590/0000.pdf |title=Jean-François Chossy, La situation des autistes en France, besoins et perspectives, rapport remis au Premier ministre, La Documentation française : Paris, Septembre 2003. |access-date=29 April 2010 |archive-date=19 April 2023 |archive-url=https://web.archive.org/web/20230419112626/http://lesrapports.ladocumentationfrancaise.fr/BRP/034000590/0000.pdf |url-status=dead }}

A 2008 study in Germany found that inpatient admission rates for children with ASD increased 30% from 2000 to 2005, with the largest rise between 2000 and 2001 and a decline between 2001 and 2003. Inpatient rates for all mental disorders also rose for ages up to 15 years, so that the ratio of ASD to all admissions rose from 1.3% to 1.4%.{{cite journal |vauthors=Bölte S, Poustka F, Holtmann M |title=Trends in autism spectrum disorder referrals |journal=Epidemiology |volume=19 |issue=3 |pages=519–20 |date=May 2008 |pmid=18414094 |doi=10.1097/EDE.0b013e31816a9e13 |doi-access=free}}

A 2009 study in Norway reported prevalence rates for ASD ranging from 0.21% to 0.87%, depending on assessment method and assumptions about non-response, suggesting that methodological factors explain large variances in prevalence rates in different studies.{{cite journal |vauthors=Posserud M, Lundervold AJ, Lie SA, Gillberg C |title=The prevalence of autism spectrum disorders: impact of diagnostic instrument and non-response bias |journal=Social Psychiatry and Psychiatric Epidemiology |volume=45 |issue=3 |pages=319–27 |date=March 2010 |pmid=19551326 |doi=10.1007/s00127-009-0087-4 |s2cid=24751946}}

The incidence and changes in incidence with time are unclear in the United Kingdom.{{cite web |url=http://www.autism.org.uk/nas/jsp/polopoly.jsp?a=5576 |title=Incidence of autism |publisher=National Autistic Society |date=2004 |access-date=2007-12-10}}{{dead link|date=December 2019 |bot=InternetArchiveBot |fix-attempted=yes }} The reported autism incidence in the UK rose starting before the first introduction of the MMR vaccine in 1989.{{cite journal |vauthors=Kaye JA, del Mar Melero-Montes M, Jick H |title=Mumps, measles, and rubella vaccine and the incidence of autism recorded by general practitioners: a time trend analysis |journal=BMJ |volume=322 |issue=7284 |pages=460–3 |date=February 2001 |pmid=11222420 |pmc=26561 |doi=10.1136/bmj.322.7284.460}} However, a perceived link between the two arising from the results of a fraudulent scientific study has caused considerable controversy, despite being subsequently disproved.{{cite news |url=http://www.thesundaytimes.co.uk/sto/public/news/article148992.ece |archive-url=https://web.archive.org/web/20140227222152/http://www.thesundaytimes.co.uk/sto/public/news/article148992.ece |archive-date=February 27, 2014 |title=MMR doctor Andrew Wakefield fixed data on autism |vauthors=Deer B |work=The Sunday Times |date=8 February 2009 |access-date=2009-02-09 |location=London}} A 2004 study found that the reported incidence of pervasive developmental disorders in a general practice research database in England and Wales grew steadily during 1988–2001 from 0.11 to 2.98 per 10,000 person-years, and concluded that much of this increase may be due to changes in diagnostic practice.{{cite journal |vauthors=Smeeth L, Cook C, Fombonne PE, Heavey L, Rodrigues LC, Smith PG, Hall AJ |title=Rate of first recorded diagnosis of autism and other pervasive developmental disorders in United Kingdom general practice, 1988 to 2001 |journal=BMC Medicine |volume=2 |pages=39 |date=November 2004 |pmid=15535890 |pmc=533883 |doi=10.1186/1741-7015-2-39 |doi-access=free}}

{{further|Heritability of autism}}

As late as the mid-1970s there was little evidence of a genetic role in autism; evidence from genetic epidemiology studies now suggests that it is one of the most heritable of all psychiatric conditions.{{cite book |chapter=Genetic epidemiology of autism spectrum disorders |vauthors=Szatmari P, Jones MB |author-link1=Peter Szatmari |pages=157–178 |title=Autism and Pervasive Developmental Disorders |edition=2nd |veditors=Volkmar FR |publisher=Cambridge University Press |year=2007 |isbn=978-0-521-54957-8}} The first studies of twins estimated heritability to be more than 90%; in other words, that genetics explains more than 90% of autism cases. When only one identical twin is autistic, the other often has learning or social disabilities. For adult siblings, the risk of having one or more features of the broader autism phenotype might be as high as 30%, much higher than the risk in controls.{{cite journal |vauthors=Bolton P, Macdonald H, Pickles A, Rios P, Goode S, Crowson M, Bailey A, Rutter M |display-authors=6 |title=A case-control family history study of autism |journal=Journal of Child Psychology and Psychiatry, and Allied Disciplines |volume=35 |issue=5 |pages=877–900 |date=July 1994 |pmid=7962246 |doi=10.1111/j.1469-7610.1994.tb02300.x}} About 10–15% of autism cases have an identifiable Mendelian (single-gene) condition, chromosome abnormality, or other genetic syndrome,{{cite journal |vauthors=Folstein SE, Rosen-Sheidley B |title=Genetics of autism: complex aetiology for a heterogeneous disorder |journal=Nature Reviews. Genetics |volume=2 |issue=12 |pages=943–55 |date=December 2001 |pmid=11733747 |doi=10.1038/35103559 |s2cid=9331084}} and ASD is associated with several genetic disorders.{{cite journal |vauthors=Zafeiriou DI, Ververi A, Vargiami E |title=Childhood autism and associated comorbidities |journal=Brain & Development |volume=29 |issue=5 |pages=257–72 |date=June 2007 |pmid=17084999 |doi=10.1016/j.braindev.2006.09.003 |s2cid=16386209}}

Since heritability is less than 100% and symptoms vary markedly among identical twins with autism, environmental factors are most likely a significant cause as well. If some of the risk is due to gene-environment interaction the 90% heritability may be too high; However, in 2017, the largest study, including over three million participants, estimated the heritability at 83%.{{cite journal |title=The Heritability of Autism Spectrum Disorder |date=26 September 2017 |pmc=5818813 |last1=Sandin |first1=S. |last2=Lichtenstein |first2=P. |last3=Kuja-Halkola |first3=R. |last4=Hultman |first4=C. |last5=Larsson |first5=H. |last6=Reichenberg |first6=A. |journal=JAMA |volume=318 |issue=12 |pages=1182–1184 |doi=10.1001/jama.2017.12141 |pmid=28973605}}

Genetic linkage analysis has been inconclusive; many association analyses have had inadequate power.{{cite journal |vauthors=Sykes NH, Lamb JA |title=Autism: the quest for the genes |journal=Expert Reviews in Molecular Medicine |volume=9 |issue=24 |pages=1–15 |date=September 2007 |pmid=17764594 |doi=10.1017/S1462399407000452 |s2cid=45880191}} Studies have examined more than 100 candidate genes; many genes must be examined because more than a third of genes are expressed in the brain and there are few clues on which are relevant to autism.

{{main|Causes of autism}}

{{Summary too long|causes of autism|date=January 2022}}

A few studies have found an association between autism and frequent use of acetaminophen (e.g. Tylenol, Paracetamol) by the mother during pregnancy.{{cite journal |vauthors=Parker W, Hornik CD, Bilbo S, Holzknecht ZE, Gentry L, Rao R, Lin SS, Herbert MR, Nevison CD |display-authors=6 |title=The role of oxidative stress, inflammation and acetaminophen exposure from birth to early childhood in the induction of autism |journal=The Journal of International Medical Research |volume=45 |issue=2 |pages=407–438 |date=April 2017 |pmid=28415925 |pmc=5536672 |doi=10.1177/0300060517693423}}{{cite journal |vauthors=Seneff S, Davidson RM, Liu J |title=Empirical Data Confirm Autism Symptoms Related to Aluminum and Acetaminophen Exposure |journal=Entropy |date=7 November 2012 |volume=14 |issue=11 |pages=2227–2253 |doi=10.3390/e14112227 |bibcode=2012Entrp..14.2227S |doi-access=free |hdl=1721.1/77575 |hdl-access=free}} Autism is also associated with several other prenatal factors, including advanced age in either parent, and diabetes, bleeding, or use of psychiatric drugs in the mother during pregnancy.{{cite journal |vauthors=Gardener H, Spiegelman D, Buka SL |title=Prenatal risk factors for autism: comprehensive meta-analysis |journal=The British Journal of Psychiatry |volume=195 |issue=1 |pages=7–14 |date=July 2009 |pmid=19567888 |pmc=3712619 |doi=10.1192/bjp.bp.108.051672}} Autism was found to be indirectly linked to prepregnancy obesity and low weight mothers.{{cite journal |vauthors=Moss BG, Chugani DC |title=Increased risk of very low birth weight, rapid postnatal growth, and autism in underweight and obese mothers |journal=American Journal of Health Promotion |volume=28 |issue=3 |pages=181–8 |year=2014 |pmid=23875984 |doi=10.4278/ajhp.120705-QUAN-325 |s2cid=25497329}} It is not known whether mutations that arise spontaneously in autism and other neuropsychiatric disorders come mainly from the mother or the father, or whether the mutations are associated with parental age.{{cite journal |vauthors=Schubert C |title=Male biological clock possibly linked to autism, other disorders |journal=Nature Medicine |volume=14 |issue=11 |pages=1170 |date=November 2008 |pmid=18989289 |doi=10.1038/nm1108-1170a |s2cid=6695070 |doi-access=free}} However, recent studies have identified advancing paternal age as a significant indicator for ASD.{{cite journal |vauthors=Geschwind DH |title=Advances in autism |journal=Annual Review of Medicine |volume=60 |issue=1 |pages=367–80 |date=February 2009 |pmid=19630577 |pmc=3645857 |doi=10.1146/annurev.med.60.053107.121225}} Increased chance of autism has also been linked to rapid "catch-up" growth for children born to mothers who had unhealthy weight at conception.

A large 2008 population study of Swedish parents of children with autism found that the parents were more likely to have been hospitalized for a mental disorder, that schizophrenia was more common among the mothers and fathers, and that depression and personality disorders were more common among the mothers.{{cite journal |vauthors=Daniels JL, Forssen U, Hultman CM, Cnattingius S, Savitz DA, Feychting M, Sparen P |title=Parental psychiatric disorders associated with autism spectrum disorders in the offspring |journal=Pediatrics |volume=121 |issue=5 |pages=e1357-62 |date=May 2008 |pmid=18450879 |doi=10.1542/peds.2007-2296 |s2cid=7493945}}

• {{lay source |template=cite press release |title=Mental disorders in parents linked to autism in children |url=http://uncnews.unc.edu/news/health-and-medicine/mental-disorders-in-parents-linked-to-autism-in-children.html |archive-url=https://web.archive.org/web/20080612225902/http://uncnews.unc.edu/news/health-and-medicine/mental-disorders-in-parents-linked-to-autism-in-children.html |archive-date=2008-06-12 |date=2008-05-05 |website=UNC News}}

It is not known how many siblings of autistic individuals are themselves autistic. Several studies based on clinical samples have given quite different estimates, and these clinical samples differ in important ways from samples taken from the general community.{{cite journal |vauthors=Rogers SJ |title=What are infant siblings teaching us about autism in infancy? |journal=Autism Research |volume=2 |issue=3 |pages=125–37 |date=June 2009 |pmid=19582867 |pmc=2791538 |doi=10.1002/aur.81}}

Autism has also been shown to cluster in urban neighborhoods of high socioeconomic status. One study from California found a three to fourfold increased risk of autism in a small 30 by 40 km region centered on West Hollywood, Los Angeles.{{cite journal |vauthors=Mazumdar S, King M, Liu KY, Zerubavel N, Bearman P |title=The spatial structure of autism in California, 1993-2001 |journal=Health & Place |volume=16 |issue=3 |pages=539–46 |date=May 2010 |pmid=20097113 |pmc=2835822 |doi=10.1016/j.healthplace.2009.12.014}}

{{Main|Sex and gender differences in autism}}

File:Share of males vs. females with autistic spectrum disorder, OWID.svg

Boys have a higher chance of being diagnosed with autism than girls. The ASD 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.

RORA deficiency may explain some of the difference in frequency between males and females. RORA protein levels are higher in the brains of typically developing females compared to typically developing males, providing females with a buffer against RORA deficiency. This is known as the female protective effect. RORA deficiency has previously been proposed as one factor that may make males more vulnerable to autism.{{cite journal |vauthors=Hu VW, Sarachana T, Sherrard RM, Kocher KM |title=Investigation of sex differences in the expression of RORA and its transcriptional targets in the brain as a potential contributor to the sex bias in autism |journal=Molecular Autism |volume=6 |pages=7 |year=2015 |pmid=26056561 |pmc=4459681 |doi=10.1186/2040-2392-6-7 |doi-access=free}}

There is a statistically notable overlap between ASD populations and gender diverse populations.

There exists behavioral differences and differences in brain structures between males and females with autism. Females often either mask their symptoms more (called camouflaging) or need to display more prominent symptoms to receive a diagnosis.{{Cite journal |last=Shuck R. K., Flores, R. E., & Fung, L. K. |date=June 2019 |title=Brief Report: Sex/Gender Differences in Symptomology and Camouflaging in Adults with Autism Spectrum Disorder |journal=J Autism Dev Disord |volume=49 |issue=6 |pages=2597–2604 |doi= 10.1007/s10803-019-03998-y|pmid= 30945091|pmc= 6753236}}{{Cite journal |last1=Kirkovski |first1=Melissa |last2=Enticott |first2=Peter G. |last3=Fitzgerald |first3=Paul B. |date=November 2013 |title=A review of the role of female gender in autism spectrum disorders |url=https://pubmed.ncbi.nlm.nih.gov/23525974/ |journal=Journal of Autism and Developmental Disorders |volume=43 |issue=11 |pages=2584–2603 |doi=10.1007/s10803-013-1811-1 |issn=1573-3432 |pmid=23525974}}{{Cite journal |last1=Kreiser |first1=Nicole L. |last2=White |first2=Susan W. |date=March 2014 |title=ASD in females: are we overstating the gender difference in diagnosis? |url=https://pubmed.ncbi.nlm.nih.gov/23836119/ |journal=Clinical Child and Family Psychology Review |volume=17 |issue=1 |pages=67–84 |doi=10.1007/s10567-013-0148-9 |issn=1573-2827 |pmid=23836119}} Males tend to demonstrate common symptoms of autism such as repetitive and restricted behaviors more so than females. This difference is hypothesized to be part of why females are more likely to be underdiagnosed.

A common misconception is that vaccinations are the cause of children developing ASD. This is partly due to the concern of a former ingredient called thiomersal, which is a substance that contains mercury.{{Cite web |date=2022-01-25 |title=Autism and Vaccines {{!}} Vaccine Safety {{!}} CDC |url=https://www.cdc.gov/vaccinesafety/concerns/autism.html |access-date=2024-02-18 |website=www.cdc.gov }} Scientific literature demonstrates that there is no causal link between thimerosal and ASD. Though the ingredient is not as prevalent in vaccines anymore, there is still concern about the link between autism and vaccinations, but there is no evidence to support this notion.

File:Thiomersal.svg

One hypothesis is that exposure to environmental chemicals before the age of two months contributes to development of ASD.{{Cite journal |last1=Pelch |first1=Katherine E. |last2=Bolden |first2=Ashley L. |last3=Kwiatkowski |first3=Carol F. |date=April 2019 |title=Environmental Chemicals and Autism: A Scoping Review of the Human and Animal Research |journal=Environmental Health Perspectives |language=en |volume=127 |issue=4 |page=46001 |doi=10.1289/EHP4386 |issn=0091-6765 |pmc=6785231 |pmid=30942615|bibcode=2019EnvHP.127d6001P }}

Human studies have mainly focused on particulate matter or mercury. Other studies investigated the effects of pollutants in the air or lead. Additionally, studies involving rodent animal models also investigated the effects of chlorpyrifos. Research suggests that some genes involved in ASD may be targeted by environmental chemicals and pollutants.{{Cite journal |last1=Carter |first1=C. J. |last2=Blizard |first2=R. A. |date=2016-10-27 |title=Autism genes are selectively targeted by environmental pollutants including pesticides, heavy metals, bisphenol A, phthalates and many others in food, cosmetics or household products |url=https://pubmed.ncbi.nlm.nih.gov/27984170/ |journal=Neurochemistry International |volume=101 |pages=S0197–0186(16)30197–8 |doi=10.1016/j.neuint.2016.10.011 |issn=1872-9754 |pmid=27984170}}

One longitudinal study had found elevated levels of perfluorodecanoic acid and zearalenone in the cord serum of children that later developed autism, suggesting that prenatal exposure may affect ASD.{{cite journal |last1=Ahrens |first1=Angelica P. |last2=Hyötyläinen |first2=Tuulia |last3=Petrone |first3=Joseph R. |last4=Igelström |first4=Kajsa |last5=George |first5=Christian D. |last6=Garrett |first6=Timothy J. |last7=Orešič |first7=Matej |last8=Triplett |first8=Eric W. |last9=Ludvigsson |first9=Johnny |title=Infant microbes and metabolites point to childhood neurodevelopmental disorders |journal=Cell |date=April 2024 |volume=187 |issue=8 |pages=1853–1873.e15 |doi=10.1016/j.cell.2024.02.035|doi-access=free |pmid=38574728 }}

Key symptoms of autism spectrum disorder are impaired social and communication abilities and having a narrow scope of interest and repeated behaviors.{{Cite journal |last1=Cermak |first1=Carly A. |last2=Arshinoff |first2=Spencer |last3=Ribeiro de Oliveira |first3=Leticia |last4=Tendera |first4=Anna |last5=Beal |first5=Deryk S. |last6=Brian |first6=Jessica |last7=Anagnostou |first7=Evdokia |last8=Sanjeevan |first8=Teenu |date=2022-02-01 |title=Brain and Language Associations in Autism Spectrum Disorder: A Scoping Review |url=https://doi.org/10.1007/s10803-021-04975-0 |journal=Journal of Autism and Developmental Disorders |language=en |volume=52 |issue=2 |pages=725–737 |doi=10.1007/s10803-021-04975-0 |pmid=33765302 |issn=1573-3432|url-access=subscription }} An impairment of language used to be a key diagnostic factor, but research has led to categorizing this symptom as a specifier. One scoping review has determined multiple brain structures that appear to play a role in language related symptoms in autism spectrum disorder. For example, having a larger sized right inferior frontal gyrus is correlated with those diagnosed with autism, specifically categorized in the language impairment subgroup (but not in those without).{{Cite journal |last1=De Fossé |first1=Lies |last2=Hodge |first2=Steven M. |last3=Makris |first3=Nikos |last4=Kennedy |first4=David N. |last5=Caviness |first5=Verne S. |last6=McGrath |first6=Lauren |last7=Steele |first7=Shelley |last8=Ziegler |first8=David A. |last9=Herbert |first9=Martha R. |last10=Frazier |first10=Jean A. |last11=Tager-Flusberg |first11=Helen |last12=Harris |first12=Gordon J. |date=December 2004 |title=Language-association cortex asymmetry in autism and specific language impairment |url=https://pubmed.ncbi.nlm.nih.gov/15478219/ |journal=Annals of Neurology |volume=56 |issue=6 |pages=757–766 |doi=10.1002/ana.20275 |issn=0364-5134 |pmid=15478219}} Some research yields conflicting results however related to different structures and total language scores, in which a possible factor for this could be age.

As for temporal regions, increased rightward radial diffusivity might have an association with receptive language scores.{{Cite journal |last1=Lange |first1=Nicholas |last2=DuBray |first2=Molly B. |last3=Lee |first3=Jee Eun |last4=Froimowitz |first4=Michael P. |last5=Froehlich |first5=Alyson |last6=Adluru |first6=Nagesh |last7=Wright |first7=Brad |last8=Ravichandran |first8=Caitlin |last9=Fletcher |first9=P. Thomas |last10=Bigler |first10=Erin D. |last11=Alexander |first11=Andrew L. |last12=Lainhart |first12=Janet E. |date=December 2010 |title=Atypical diffusion tensor hemispheric asymmetry in autism |journal=Autism Research |language=en |volume=3 |issue=6 |pages=350–358 |doi=10.1002/aur.162 |issn=1939-3792 |pmc=3215255 |pmid=21182212}} Research concerning the planum temporale and its role in language has been inconclusive. The cerebellum may also be a factor in whether a person has language impairment or not.{{Cite journal |last1=Hodge |first1=Steven M. |last2=Makris |first2=Nikos |last3=Kennedy |first3=David N. |last4=Caviness |first4=Verne S. |last5=Howard |first5=James |last6=McGrath |first6=Lauren |last7=Steele |first7=Shelly |last8=Frazier |first8=Jean A. |last9=Tager-Flusberg |first9=Helen |last10=Harris |first10=Gordon J. |date=March 2010 |title=Cerebellum, language, and cognition in autism and specific language impairment |journal=Journal of Autism and Developmental Disorders |volume=40 |issue=3 |pages=300–316 |doi=10.1007/s10803-009-0872-7 |issn=1573-3432 |pmc=3771698 |pmid=19924522}}

Much research centered around those with ASD that have impaired communication and seeking improvement in social interaction often investigate the use of psychosocial interventions.{{Cite journal |last1=Ameis |first1=S. H. |last2=Kassee |first2=C. |last3=Corbett-Dick |first3=P. |last4=Cole |first4=L. |last5=Dadhwal |first5=S. |last6=Lai |first6=M.-C. |last7=Veenstra-VanderWeele |first7=J. |last8=Correll |first8=C. U. |date=November 2018 |title=Systematic review and guide to management of core and psychiatric symptoms in youth with autism |url=https://onlinelibrary.wiley.com/doi/10.1111/acps.12918 |journal=Acta Psychiatrica Scandinavica |language=en |volume=138 |issue=5 |pages=379–400 |doi=10.1111/acps.12918 |pmid=29904907 |issn=0001-690X|url-access=subscription }} However, other research looks to pharmacology for treatment. Behavioral management therapy is another potential option.{{Cite web |date=2021-04-19 |title=Behavioral Management Therapy for Autism {{!}} NICHD - Eunice Kennedy Shriver National Institute of Child Health and Human Development |url=https://www.nichd.nih.gov/health/topics/autism/conditioninfo/treatments/behavioral-management |access-date=2024-04-24 |website=www.nichd.nih.gov |language=en}} Overall however, treatment options are still in development.

{{Further|Conditions comorbid to autism}}

Autism is associated with several other conditions:

• Genetic disorders. About 10–15% of autism cases have an identifiable Mendelian (single-gene) condition, chromosome abnormality, or other genetic syndrome, and ASD is associated with several genetic disorders.
• Intellectual disability. The fraction of autistic individuals who also meet criteria for intellectual disability has been reported as anywhere from 25% to 70%, a wide variation illustrating the difficulty of assessing autistic intelligence.{{cite book |chapter=Learning in autism |vauthors=Dawson M, Mottron L, Gernsbacher MA |title=Learning and Memory: A Comprehensive Reference |volume=2 |pages=759–72 |veditors=Byrne JH, Roediger III HL |publisher=Academic Press |date=2008 |doi=10.1016/B978-012370509-9.00152-2 |isbn=978-0-12-370504-4 |archive-url=https://web.archive.org/web/20120303191513/http://psych.wisc.edu/lang/pdf/Dawson_AutisticLearning.pdf |archive-date=2012-03-03 |chapter-url=http://psych.wisc.edu/lang/pdf/Dawson_AutisticLearning.pdf}}
• Gastrointestinal disease are a common comorbidity in patients with autism spectrum disorders, even though the underlying mechanisms are largely unknown. The most common gastrointestinal symptoms reported are abdominal pain, constipation, diarrhea and bloating, reported in at least 25 percent of patients.{{cite journal |last1=Mayer |first1=EA |last2=Padua |first2=D |last3=Tillisch |first3=K |title=Altered brain-gut axis in autism: comorbidity or causative mechanisms? |journal=BioEssays |date=October 2014 |volume=36 |issue=10 |pages=933–9 |doi=10.1002/bies.201400075 |pmid=25145752}}
• Anxiety disorders are common among children with ASD, although there are no firm data. Symptoms include generalized anxiety and separation anxiety,{{cite journal |journal=Res Autism Spectr Disord |year=2009 |volume=3 |issue=1 |pages=1–21 |title=Anxiety in children and adolescents with Autism Spectrum Disorders |vauthors=MacNeil BM, Lopes VA, Minnes PM |doi=10.1016/j.rasd.2008.06.001}} and are likely affected by age, level of cognitive functioning, degree of social impairment, and ASD-specific difficulties. Many anxiety disorders, such as social phobia, are not commonly diagnosed in people with ASD because such symptoms are better explained by ASD itself, and it is often difficult to tell whether symptoms such as compulsive checking are part of ASD or a co-occurring anxiety problem. The prevalence of anxiety disorders in children with ASD has been reported to be anywhere between 11% and 84%.{{cite journal |vauthors=White SW, Oswald D, Ollendick T, Scahill L |title=Anxiety in children and adolescents with autism spectrum disorders |journal=Clinical Psychology Review |volume=29 |issue=3 |pages=216–29 |date=April 2009 |pmid=19223098 |pmc=2692135 |doi=10.1016/j.cpr.2009.01.003}}
• Epilepsy, with variations in risk of epilepsy due to age, cognitive level, and type of language disorder; 5–38% of children with autism have comorbid epilepsy, and only 16% of these have remission in adulthood.{{cite journal |vauthors=Levisohn PM |title=The autism-epilepsy connection |journal=Epilepsia |volume=48 |issue=Suppl 9 |pages=33–5 |date=2007 |pmid=18047599 |doi=10.1111/j.1528-1167.2007.01399.x |s2cid=25651749 |doi-access=free}}
• Several metabolic defects, such as phenylketonuria, are associated with autistic symptoms.{{cite journal |vauthors=Manzi B, Loizzo AL, Giana G, Curatolo P |title=Autism and metabolic diseases |journal=Journal of Child Neurology |volume=23 |issue=3 |pages=307–14 |date=March 2008 |pmid=18079313 |doi=10.1177/0883073807308698 |s2cid=30809774}}
• Minor physical anomalies are significantly increased in the autistic population.{{cite journal |vauthors=Ozgen HM, Hop JW, Hox JJ, Beemer FA, van Engeland H |title=Minor physical anomalies in autism: a meta-analysis |journal=Molecular Psychiatry |volume=15 |issue=3 |pages=300–7 |date=March 2010 |pmid=18626481 |doi=10.1038/mp.2008.75 |s2cid=26087399 |doi-access=}}
• Preempted diagnoses. Although the DSM-IV rules out concurrent diagnosis of many other conditions along with autism, the full criteria for ADHD, Tourette syndrome, and other of these conditions are often present and these comorbid diagnoses are increasingly accepted.{{cite journal |vauthors=Steyaert JG, De la Marche W |title=What's new in autism? |journal=European Journal of Pediatrics |volume=167 |issue=10 |pages=1091–101 |date=October 2008 |pmid=18597114 |doi=10.1007/s00431-008-0764-4 |s2cid=11831418}} A 2008 study found that nearly 70% of children with ASD had at least one psychiatric disorder, including nearly 30% with social anxiety disorder and similar proportions with ADHD and oppositional defiant disorder.{{cite journal |vauthors=Simonoff E, Pickles A, Charman T, Chandler S, Loucas T, Baird G |title=Psychiatric disorders in children with autism spectrum disorders: prevalence, comorbidity, and associated factors in a population-derived sample |journal=Journal of the American Academy of Child and Adolescent Psychiatry |volume=47 |issue=8 |pages=921–9 |date=August 2008 |pmid=18645422 |doi=10.1097/CHI.0b013e318179964f |s2cid=205551553}} Childhood-onset schizophrenia, a rare and severe form, is another preempted diagnosis whose symptoms are often present along with the symptoms of autism.{{cite journal |vauthors=Rapoport J, Chavez A, Greenstein D, Addington A, Gogtay N |title=Autism spectrum disorders and childhood-onset schizophrenia: clinical and biological contributions to a relation revisited |journal=Journal of the American Academy of Child and Adolescent Psychiatry |volume=48 |issue=1 |pages=10–8 |date=January 2009 |pmid=19218893 |pmc=2664646 |doi=10.1097/CHI.0b013e31818b1c63}}

{{reflist}}

{{Pervasive developmental disorders}}

{{Autism resources}}

Category:Autism

Autism

Autism