epidemiology of autism epidemiology of autism the epidemiology of autism is the study of the incidence and distribution of autism spectrum disorders (asd). a 2022 systematic review of global prevalence of autism spectrum disorders found a median prevalence of 1% in children in studies published from 2012 to 2021, with a trend of increasing prevalence over time. however, the study's 1% figure may reflect an underestimate of prevalence in low- and middle-income countries. further information: causes of autism asd averages a 4.3:1 male-to-female ratio in diagnosis, not accounting for asd in gender diverse populations, which overlap disproportionately with asd populations. the number of children known to have autism has increased dramatically since the 1980s, at least partly due to changes in diagnostic practice; it is unclear whether prevalence has actually increased; and as-yet-unidentified environmental risk factors cannot be ruled out. in 2020, the centers for disease control's autism and developmental disabilities monitoring (addm) network reported that approximately 1 in 54 children in the united states (1 in 34 boys, and 1 in 144 girls) is diagnosed with an autism spectrum disorder (asd), based on data collected in 2016. this estimate is a 10% increase from the 1 in 59 rate in 2014, 105% increase from the 1 in 110 rate in 2006 and 176% increase from the 1 in 150 rate in 2000. diagnostic criteria of asd has changed significantly since the 1980s; for example, u.s. special-education autism classification was introduced in 1994. asd is a complex neurodevelopmental disorder, and although what causes it is still not entirely known, efforts have been made to outline causative mechanisms and how they give rise to the disorder. the risk of developing autism is increased in the presence of various prenatal factors, including advanced paternal age and diabetes in the mother during pregnancy. in rare cases, autism is strongly associated with agents that cause birth defects. it has been shown to be related to genetic disorders and with epilepsy. asd is believed to be largely inherited, although the genetics of asd are complex and it is unclear which genes are responsible. asd is also associated with several intellectual or emotional gifts, which has led to a variety of hypotheses from within evolutionary psychiatry that autistic traits have played a beneficial role over human evolutionary history. other proposed causes, such as childhood vaccines, are controversial. the vaccine hypothesis has been extensively investigated and shown to be false, lacking any scientific evidence. andrew wakefield published a small study in 1998 in the united kingdom suggesting a causal link between autism and the trivalent mmr vaccine. after data included in the report was shown to be deliberately falsified, the paper was retracted, and wakefield was struck off the medical register in the united kingdom. it is problematic to compare autism rates over the last three decades, as the diagnostic criteria for autism have changed with each revision of the diagnostic and statistical manual (dsm), which outlines which symptoms meet the criteria for an asd diagnosis. in 1983, the dsm did not recognize pdd-nos or asperger's syndrome, and the criteria for autistic disorder (ad) were more restrictive. the previous edition of the dsm, dsm-iv, included autistic disorder, childhood disintegrative disorder, pdd-nos, and asperger's syndrome. due to inconsistencies in diagnosis and how much is still being learnt about autism, the most recent dsm (dsm-5) only has one diagnosis, autism spectrum disorder (asd), which encompasses each of the previous four disorders. according to the new diagnostic criteria for asd, one must have both struggles in social communication and interaction and restricted repetitive behaviors, interests and activities (rrbs). asd diagnoses continue to be over four times more common among boys (1 in 34) than among girls (1 in 154), and they are reported in all racial, ethnic and socioeconomic groups. studies have been conducted in several continents (asia, europe and north america) that report a prevalence rate of approximately 1 to 2 percent. a 2011 study reported a 2.6 percent prevalence of autism in south korea. frequency 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. incidence and prevalence epidemiology defines several measures of the frequency of occurrence of a disease or condition: 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. estimation methods 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. 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. frequency estimates estimates of the prevalence of autism vary widely depending on diagnostic criteria, age of children screened, and geographical location. most recent reviews tend to estimate a prevalence of 1–2 per 1,000 for autism and close to 6 per 1,000 for asd; 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. 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. 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. 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. 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). 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; these results suggest that prevalence of asd among adults is similar to that in children and rates of autism are not increasing. changes with time 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. reports of autism cases per 1,000 children grew dramatically in the u.s. from 1996 to 2007. it is unknown how much, if any, growth came from changes in autism's prevalence. the number of reported cases of autism increased dramatically in the 1990s and 2000s, prompting ongoing investigations into several potential reasons: 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. 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. the reported increase is largely attributable to changes in diagnostic practices, referral patterns, availability of services, age at diagnosis, and public awareness. a widely cited 2002 pilot study concluded that the observed increase in autism in california cannot be explained by changes in diagnostic criteria, 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. 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, suggesting that methodological factors may explain the observed increases in autism over time. 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. 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. 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. 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. 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. 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. africa the prevalence of autism in africa is unknown. the americas the prevalence of autism in the americas overall is unknown. canada the rate of autism diagnoses in canada was 1 in 450 in 2003. however, preliminary results of an epidemiological study conducted at montreal children's hospital in