The epidemiology of autism is the study of the incidence and distribution of autism spectrum disorders (ASD). A 2012 review of global prevalence estimates of autism spectrum disorders found a median of 62 cases per 10,000 people.[1]In contrast, a 2016 review of global prevalence estimates of autism spectrum disorders found a median of 18.5 cases per 10,000 people.[2] However there is a lack of evidence from low- and middle-income countries.[1]
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.[3] 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;[4] and as-yet-unidentified environmental risk factors cannot be ruled out.[5] 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.[6] 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.[6] Diagnostic criteria of ASD has changed significantly since the 1980s; for example, U.S. special-education autism classification was introduced in 1994.[4]
Autism is a complex neurodevelopmental disorder. Many causes have been proposed, but its theory of causation is still questionable and ultimately unknown.[4][7] The possibility of autism is associated with several prenatal factors, including advanced paternal age and diabetes in the mother during pregnancy.[8] ASD is 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.[9][10] Some individuals perceive it to be connected to genetic disorders[11] and with epilepsy.[12] Autism is believed to be largely inherited, although the genetics of autism are complex, and it is unclear which genes are responsible.[13] Little evidence exists to support associations with specific environmental exposures.[4]
In rare cases, autism is strongly associated with agents that cause birth defects.[14] Other proposed causes, such as childhood vaccines, are controversial. The vaccine hypothesis has been extensively investigated and shown to be false,[15] lacking any scientific evidence.[5] 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.[16][17][18]
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).