Epidemiology of autism

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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. 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; 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. 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. It has been shown to be related to genetic disorders 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 of autism have been controversial. The vaccine hypothesis has been extensively investigated and shown to be false, lacking any scientific evidence. 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.

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. 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.

A 2008 study in Venezuela reported a prevalence of 1.1 per 1,000 for autism and 1.7 per 1,000 for ASD.

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.

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.

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. A 2004 Japanese autism association reported that about 360.000 people have typical Kanner type autism.

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.

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. (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.

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. 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). 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. Those figures are considered as underrated as the WHO gives figures between 30 and 60 per 10,000. The French Minister of Health gives a prevalence of 4.9 per 10,000 on its website but it counts only early infantile autism.

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%.

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.

The incidence and changes in incidence with time are unclear in the United Kingdom. The reported autism incidence in the UK rose starting before the first introduction of the MMR vaccine in 1989. However, a perceived link between the two arising from the results of a fraudulent scientific study has caused considerable controversy, despite being subsequently disproved. 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.

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. The first studies of twins estimated heritability to be more than 90%; in other words, that genetics explains more than 90% of autism cases. 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. 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;

Genetic linkage analysis has been inconclusive; many association analyses have had inadequate power. 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. 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. Autism was found to be indirectly linked to prepregnancy obesity and low weight mothers. 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. However, recent studies have identified advancing paternal age as a significant indicator for ASD. Increased chance of autism has also been linked to rapid "catch up" growth for children born to mothers who had unhealthy weight at conception. 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. 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.

thumb|Share of males vs. females with autistic spectrum disorder, OWID
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. There is a statistically notable overlap between ASD populations and gender diverse populations. 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.

One theory behind autism is exposure to environmental chemicals before the age of two months. 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 environmental chemicals can be targeted and impacted by pollutants. Some systemic reviews have indicated that while significant correlations between mercury exposure and autism have been found, more research is needed.

Key symptoms of autism spectrum disorder are impaired social and communication abilities and having a narrow scope of interest and repeated behaviors. 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). 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. Research concerning the planum temporale and its role in language has been inconclusive. However, other research looks to pharmacology for treatment. Behavioral management therapy is another potential option. Overall however, treatment options are still in development.