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- See autism for general information about autism.
The reported incidence of autism varies considerably between countries and has shown large changes in limited time periods. There is uncertainty about the extent to which changes noted in the mid 1990's are accounted for by a change in the number of cases rather than by changed definitions, screening, improved recording and more frequent diagnoses including artifacts due to service provision.
BackgroundThe first diagnoses of autism were made in 1943 by psychiatrist Dr. Leo Kanner of the Johns Hopkins Hospital in Baltimore. Since then, the numbers diagnosed with autism have steadily increased. Data recording became more organised in the developed world in the 1990's. The organisation of information in general, and of interest and action groups became much easier, and individual participation more common in the latter half of the decade, after the introduction of the Web in 1993. Although diagnoses related to autism have increased, public health organizations and researchers have not yet determined the relative extents to which different aspects contribute to this :
- More complete pickup of autism (case finding), as a result of increased awareness and funding
- The diagnosis is being applied more broadly than before, as a result of the changing definition of the disorder, particularly changes in DSM-III-R and DSM-IV 
- The underlying condition may have a changing incidence with time
- Successively earlier diagnosis in each succeeding cohort of children including recognition in nursery (preschool)
The increase in the recorded prevalence of autism has led to several new treatment programs, advocacy groups and support programs. Attempts in the US and UK to sue vaccine companies may have also increased case-reporting .
When the rising prevalence of autism spectrum disorders sparked research in the late 1990's, the medical establishment primarily attributed the increase to improved diagnostic screening or changes in the definition of autism. In 1994, the fourth major revision of the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV) was published . It included updated criteria for the diagnosis of autism and autism spectrum disorders. Professional medical associations, including the American Academy of Pediatrics, say that this revision was an important factor in increasing the apparent prevalence of autism and a 2005 study by Mayo Clinic researchers found sharp increases in autism diagnoses followed the revisions in DSM criteria and changes in funding for special education programs. .
Some of the apparent increase might reflect increased awareness of autistic disorders by parents and pediatricians. The introduction of central funding for children diagnosed as autistic, but not for children with a similar degree of disability and need encourages case-substitution which does appear  to have occurred. Children who in the past would probably have been diagnosed as having a different condition, or not diagnosed at all are recorded as new cases of autistic spectrum disorder. Possible alternate diagnoses include mental retardation, learning disability and childhood schizophrenia . High-functioning autistic children are sometimes misdiagnosed ADHD  and it is possible that such misdiagnoses were more common in the recent past. Children who are not primarily autistic, e.g. those with Fragile-X Syndrome (with characteristics that fit the criteria for autism) and even Down's Syndrome may have the diagnostic group with the best funding assigned.
Incidence in sub-groups
There have been suggestions that the incidence of autism may vary amongst particular groups defined by occupation. Changes that made travel and communication easier, and the growth of the technological industries during the past decade, have been suggested as means for increase in the proportion of couples likely to produce an autistic child.   .
Areas with a general pattern of increase
The Yokohama study in Japan (2005) examined autism trends before and after the 1993 withdrawal of MMR, reporting 48 and 86 cases per 10,000 children in two sequential years before withdrawal, doubling to 97 and 161 per 10,000 afterwards in the two years afterwards.
An estimate of the UK incidence rate from the National Autistic Society estimated autism spectrum disorders in the total population at one in 110 . A 2001 review, by the Medical Research Council, yielded an estimate of one in 166 in children under eight .
According to statistics cited by Bernard Rimland[How to reference and link to summary or text], the autism rate in the UK spiked after the first introduction of the MMR vaccine in 1989, just as it had in the USA in the late 1970's. . This is not consistent with evidence published in the British Medical Journal.
After years of substantial annual increases, provisional data from the US Department of Education show a significant decrease in the number of new autism diagnoses recorded among children 3 to 5 years old. There were 1,451 new cases in 2001-2002; 1,981 in 2002-2003; 3,707 in 2003-2004; and 3,178 in 2004-2005, a drop of 529 new cases, or 14%.
A recent estimate by the National Autism Association is of approximately 500,000 autistic spectrum cases in the United States, including up to 1 in 150 children. Anne McElroy Dachel of the NAA noted that 80% of autistic Americans are under the age of 18. The incidence of autism increased by over 900% between 1992 and 2001, according to the United States Department of Education. In 1999, the autism incidence rate in the US was generally cited at 4.5 cases per 10,000 live births. By 2005, the US Centers for Disease Control (CDC) estimated one of every 250 babies was born with autism, or 40 cases per 10,000. The rising enrollments in special education classes in the nation's public school system provide another estimate of prevalence, subject to confounding factors. In Ohio in 1992-93 there were only 22 reported cases of autism, but by 2003-04 there were 5,146; in Illinois, there were five cases in 1992-93, and 6,005 in 2003-04; in Wisconsin, 18 cases of autism in 1992-93 and 3,259 in 2003-04. As many as 1.5 million Americans may have some form of autism, including milder variants, and the number is rising. Epidemiologists estimate the number of autistic children in the US could reach 4 million in the next decade. In Connecticut, the number of autistic children educated at public expense in Connecticut has increased 325 percent since 1996, according to the State Department of Education. Governor M. Jodi Rell included a 38% increase, to $25.5 million, in the State's budget, for reimbursement to local schools for special education costs.
There are 74,000 Pennsylvanians diagnosed with autism spectrum disorders.
Areas without a clear pattern of increase
There are 1.8 million reported cases of autism in China with over 10,000 known cases in Shanghai. This gives a prevalence of 1 in 720 for the country (based on a population of 1.3 billion people ) and 1 in 1600 for Shanghai (based on 16 million population). The incidence now or over time is unclear.
In November 2002, a study reported a lower incidence of autism in Denmark than in the US and other countries. An incidence of 1 in 727 (738 out of 537,303) was reported, compared with up to 1 in 86 among primary school children in the United Kingdom and around 1 in 150 children in the USA. Danish authorities report a continued increase in the incidence of autism after 1992 . However, around that time Danish criteria for registration of children with autism changed, increasing the proportion (by a factor of 5 according to one source) .
The Russian autism rate did not change for at least a decade after 1985 .
California is considered to have the best reporting system for autism in the USA[How to reference and link to summary or text]. According to data released in late 2005 by the California Department of Developmental Services (DDS), new cases — of professionally diagnosed full syndrome DSM IV autism — entering the DDS system indicated a decline, from 734 new cases during the second quarter of 2005 to 678 new cases during the third quarter of 2005, a 7.5% decline in one quarter[How to reference and link to summary or text].
As of August 1993, a total of only 4,911 cases of autism had been logged in DDS's client-management system, a number excluding milder autism spectrum disorders such as Asperger's syndrome[How to reference and link to summary or text]. By April 29, 1999, the DDS reported a State-wide incidence rate of about 15 to 20 per 10,000, triggering alarms about the increase[How to reference and link to summary or text].
As of 2005, the DDS reported 28,046 cases, but that the rate of increase peaked in 2002 and has dropped slightly since. According to data released by DDS in January, 2006, the number of new cases of professionally diagnosed full syndrome autism entering California's developmental services system in 2005 was the lowest since 2001[How to reference and link to summary or text]. The DDS year end report documents that, in 2005, California added 2,848 new cases of autism to its system. Not since 2001, when 2,725 new cases were added, has California added fewer new cases of full syndrome autism to its system. Ever since the record year of 2002, there has been a slow, steady decline in the number of new autism cases entering the 37-year old DDS system, even though levels have still not yet reached the 1 in 166 reported by population-based studies[How to reference and link to summary or text].
The use of the term "New Cases" has come into question and DDS has documented that "New Cases" should not be calculated as the difference in the numbers between quarters . The total caseload handled by the state continues to increase much faster than population growth, but the recent trend points to a decrease in the caseload increase per quarter. The decline has been speculated to coincide with vaccines containing thimerosal being phased out. It could also indicate that the awareness curve is starting to level off. It has also been pointed out that the caseload does not yet meet the levels found in population studies.
According to a report by the DDS  the rate of children diagnosed with full-syndrome autism in California nearly doubled between 1999 and 2002, from 10,360 to 20,377. The report stated, "(B)etween Dec. 31, 1987, and Dec. 31, 2002, the population of persons with full-syndrome autism has increased by 634 percent."
California's increase in childhood autism was not due to flawed diagnosis, according to a 2002 study led by University of California, Davis pediatric epidemiologist Robert Byrd [How to reference and link to summary or text]. 1,685 newly diagnosed autistic children had entered the state's regional center system the previous year, marking a 273 percent increase over an 11-year period from 1988 to 1999. The data again included only children with classic autism, discounting those with PDD-NOS, Asperger's etc. "The sheer complexity of this phenomenon prevents any clear conclusions," the report stated. "What we do know is that the number of young children coming into the system each year is significantly greater than in the past."
Silicon Valley cluster
A 2002 BBC article indicated that one in 150 children in the region had an autistic spectrum disorder. A 2001 article in Wired suggested that the cluster is a result of a link between autistic disorders and computer skills.
A formal study has not been conducted, but efforts to assess the prevalence of autism in the Amish community revealed only a very few cases. About 22,000 Amish live in Lancaster County, yet only three or four Amish with autism have turned up so far in an informal survey of Lancaster County, whereas dozens would be expected at the 1-in-166 prevalence in society at large. Dr Frank Noonan, a Lancaster County family doctor, reported that other conditions were found in expected numbers. The Department of Health and Human Services may study the Amish community. There are substantial confounding factors among Amish lifestyle and genetic restriction and homogeneity.
- ↑ Epidemiology for the Uninitiated 4th Ed: Quantifying diseases in populations BMJ
- ↑ Incidence National Autistic Society
- ↑ [http://ajp.psychiatryonline.org/cgi/content/abstract/145/11/1404 DSM3 changes
- ↑ DSM4 changes
- ↑ Summary item in "Overlawyered archive"
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- ↑ Am. Assoc. paed. Case substitution accounts for some of the increase in reported incidence
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- ↑ CNN report Dr Fred Volkmar, an autism researcher from Yale said "Autism is a kind of fashionable diagnosis".
- ↑ Parental type
- ↑ BBC report Simon Baron-Cohen believes that "it has become easier for systemizers to meet each other, with the advent of international conferences, greater job opportunities and more women working in these fields."
- ↑  Assortative mating has not been demonstrated in humans. The spouses of identical twins tended to find the other twin annoying rather than attractive.
- ↑ Mearns, Int. Paed. autistic individuals have a higher proportion of engineers as close family members than the rest of the population. Speculation on job choice and phenotype.
- ↑ Number of disabled students soars Sydney Children's Hospital
- ↑ Hideo Honda, Yasuo Shimizu and Michael Rutter (2005). No effect of MMR withdrawal on the incidence of autism: a total population study. Journal of Child Psychology and Psychiatry 46 (6): 572. PMID 15877763 DOI:10.1111/j.1469-7610.2005.01425.x.
- ↑ 
- ↑ Kaye JA, del Mar Melero-Montes M, Jick H (2001). Mumps, measles, and rubella vaccine and the incidence of autism recorded by general practitioners: a time trend analysis. BMJ 322 (7284): 460-3. PMID 11222420.
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- ↑ SHanghai Daily News reported in China View - www.chinaview.cn 2005-11-29 viewed April 2006
- ↑ U.S. Central Intelligence Agency World Factbook
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- ↑ Danish measurement artifact USA Today article by Dan Olmsted
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- ↑ Autistic Spectrum Disorders, Changes in the California Caseload: 1999-2002
- ↑ includeonly>"Autism link to 'geek genes'", BBC, 14 August 2002.
- ↑ includeonly>Steve Silberman. "The Geek Syndrome", Wired, December 2001.
- ↑ Amish
- CPA-APC.org - Diagnosis and Epidemiology of Autism Spectrum Disorders Lee Tidmarsh, MD, Fred R Volkmar, MD, The Canadian Journal of Psychiatry, Vol 48 pp 517–525, 2003
- NIH.gov - 'The changing prevalence of autism in California', L.A. Croen, J.K. Grether, J Hoogstrate, S Selvin, Journal of Autism Developmental Disorders Vol 32, No 3, pp 207-15, June, 2002
- NIH.gov -'The epidemiology of autistic spectrum disorders: is the prevalence rising?', Lorna Wing, D. Potter, Ment Retard Dev Disabil Res Rev, Vol 8, No 3, pp 151-61, 2002
- NIH.gov - 'Prevalence of autistic spectrum disorders in Lothian, Scotland: An estimate using the 'capture-recapture' technique', M.J. Harrison, A O'Hare, H. Campbell, A. Adamson, J McNeillage, Arch Dis Child. May 10, 2005
- NIH.gov - 'The incidence of autism in Olmsted County, Minnesota, 1976-1997: results from a population-based study', W.J. Barbaresi, S.K Katusic, R.C. Colligan, A.L. Weaver, S.J. Jacobsen, Arch Pediatr Adolesc Med, Vol 159, No 1, pp 37-44, January, 2005
- California DDS figures and reports: http://www.dds.ca.gov/FactsStats/quarterly.cfm , http://www.dds.ca.gov/autism/autism_main.cfm
- NeuroDiversity.com - Although labelled 'Prevalence of Autism' this lists references to incidence as well.
- About.com - 'The Puzzling Reality of an Autism Epidemic', Floyd Tilton
- PediatricServices.com - 'The Autism Epidemic'
- ScienceDaily.com - 'The Age of Autism: What epidemic?', Dan Olmsted, Science Daily (August 1, 2005)
- ScienceDaily.com - 'The Age of Autism: The Amish anomaly', Dan Olmsted, Science Daily (April 18, 2005)
- SFGate.com - 'State autism rate confounds experts: 273% increase in 11-year span', Katherine Seligman, San Francisco Chronicle (October 18, 2002)
- TheAge.com.au - 'Number of disabled students soars' Chee Chee Leung The Age (April 26, 2005)
- TMCNet.com - 'Parents say autism is an issue across the globe' (January 11, 2006)
- Wired.com - 'The Geek Syndrome: Autism - and its milder cousin Asperger's syndrome - is surging among the children of Silicon Valley. Are math-and-tech genes to blame?' Steve Silberman Wired (December, 2001)
Pervasive developmental disorders / Autism spectrum
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