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See Autism for general information about autism. See Incidence (epidemiology) for a definition of incidence.

The reported incidence of autism varies considerably among countries and is complicated by varying criteria for diagnosing autism, different standards for reporting public health problems, and other variations.

Epidemiology defines several measures of the frequency of occurrence of a disease or condition:^[1]

• The incidence rate of a condition is the rate at which new cases occurred per person-year, for example, "2 new cases per 1000 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 1000 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 1000 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 1000 people had cases during 2006".

When studying how diseases are caused, incidence rates are the most appropriate measure of disease frequency as they assess risk directly. However, incidence can be difficult to measure with rarer chronic diseases such as autism.^[1] In autism epidemiology, point or period prevalence is more useful than incidence, as the disorder starts long before it is diagnosed, and the gap between initiation and diagnosis is influenced by many factors unrelated to risk. Research focuses mostly on whether point or period prevalence is increasing with time; cumulative incidence is sometimes used in studies of birth cohorts.^[2]

The number of autistic children served under the U.S. Individuals with Disabilities Education Act grew dramatically in the 1990s and early 2000s. It is unknown how much, if any, growth came from changes in autism's incidence or prevalence.

The number of reported cases of autism increased dramatically in the 1990s and early 2000s. There are several potential reasons for the reported increase:^[3]

• 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.
• Successively earlier diagnosis in each succeeding cohort of children, including recognition in nursery (preschool), may have affected apparent prevalence but not incidence.

The reported increase is largely attributable to changes in diagnostic practices, referral patterns, availability of services, age at diagnosis, and public awareness.^[4] For example, 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 the frequency of autism ranging up to 29-fold depending on the frequency measure (that is, a measure of annual incidence, or of prevalence by a certain age), suggesting that methodological factors may explain the observed increases in autism over time.^[5]

Several contributing environmental risk factors have been proposed to support the hypothesis that the actual frequency of autism has increased. These include certain foods, infectious disease, pesticides, MMR vaccine, and vaccines containing the preservative thiomersal, formerly used in several childhood vaccines in the U.S.^[2] Although there is overwhelming scientific evidence against the MMR hypothesis and no convincing evidence for the thiomersal hypothesis, other as-yet-unidentified contributing environmental risk factors cannot be ruled out.^[4] Although it is unknown whether autism's frequency has increased, any such increase would suggest directing more attention and funding toward changing environmental factors instead of continuing to focus on genetics.^[6]

A 2003 study reported that the cumulative incidence of autism in Denmark began a steep increase starting around 1990, and continued to grow until 2000, despite the withdrawal of thiomersal-containing vaccines in 1992. For example, for children aged 2–4 years, the cumulative incidence was about 0.5 new cases per 10,000 children in 1990 and about 4.5 new cases per 10,000 children in 2000.^[7]

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 MMR vaccine dropped to near zero, the incidence rate grew to 97 and 161 cases per 10,000 children in 1993 and 1994, respectively, indicating that MMR vaccine did not cause autism.^[8]

The incidence and changes in incidence with time are unclear in the UK.^[9] The reported autism incidence in the UK rose starting before the first introduction of the MMR vaccine in 1989.^[10]

The number of diagnosed cases of autism grew dramatically in the U.S. in the 1990s and early 2000s. For example, in 1996, 21,669 children and students aged 6–11 years diagnosed with autism were served under Part B of the Individuals with Disabilities Education Act (IDEA) in the U.S. and outlying areas; by 2001 this number had risen to 64,094, and by 2005 to 110,529.^[11] 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.^[12]

A population-based study of one 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.^[13]