Autism diagnoses are rising. What do we actually know about why?

By Bloom · · 3 min read

The CDC tracks autism through its Autism and Developmental Disabilities Monitoring (ADDM) Network. Its latest report, published in April 2025, covers 2022. In the 16 areas it monitors, 32.2 of every 1,000 children aged 8, about 1 in 31, had been identified with autism [1]. When the network started in 2000 the figure was 1 in 150, and in 2020 it was 1 in 36 [1]. Two details matter. The rate varies widely by place, from 9.7 per 1,000 in Laredo, Texas to 53.1 per 1,000 in California [1]. And the CDC says its monitoring areas "are not nationally representative" [1].

How much of the rise is better diagnosis

The researchers who run ADDM point first to identification. They write that differences between areas "might be due to differences in availability of services for early detection and evaluation", and that recent patterns are "consistent with increased access to and provision of identification services among previously underserved groups" [1]. The National Institute of Environmental Health Sciences (NIEHS) says the 1 in 31 figure "reflects in part an increased ability to recognize and diagnose" autism earlier [2].

Denmark offers one of the few attempts to measure this. Danish researchers followed every child born there from 1980 to 1991. Two changes in how autism was recorded, broader diagnostic criteria in 1994 and the counting of outpatient diagnoses from 1995, explained about 60% of the rise in that group. The other 40% was left unexplained [3].

What researchers say about causes

NIEHS sums up the state of the science this way: "Studies indicate the rate of autism is rising, but causes are not well-understood" [2]. Genes play a clear part. Researchers have found both rare mutations and common small genetic variations in people with autism [2]. On the environment, NIEHS says the clearest evidence involves events before and during birth: older parents, "prenatal exposure to air pollution or certain pesticides", maternal obesity, diabetes or immune disorders, extreme prematurity, and birth complications that deprive the baby's brain of oxygen. It adds that "these factors alone are unlikely to cause autism" and appear to raise the likelihood in combination with genetic factors [2].

The pesticide studies

Two California studies are cited most often.

The larger, published in the BMJ in 2019, compared 2,961 children with autism born in the Central Valley between 1998 and 2010 with ten times as many children without it. Using state records of where pesticides were sprayed, the researchers estimated whether each mother lived within 2,000 metres of an application during pregnancy [4]. Prenatal exposure to several pesticides was associated with somewhat higher odds of autism: for glyphosate an odds ratio of 1.16, meaning about 16% higher odds, and for chlorpyrifos 1.13. For autism with intellectual disability the associations were about 30% stronger [4].

An earlier study from the CHARGE project at UC Davis compared 486 children with autism with 316 typically developing children. Living near organophosphate applications during pregnancy was associated with a 60% higher risk of autism, with larger estimates for some trimesters, though with wide uncertainty because the groups were small [5].

Both studies are observational. They show an association between living near sprayed fields and autism. They do not show that pesticides cause autism, and they did not measure what the mothers ate.

Where this leaves parents

The most solid facts are these. More children are being identified with autism, and better and earlier diagnosis explains a large part of that, though probably not all of it [1][2][3]. Genes matter, and researchers are studying how they interact with exposures before birth, including some pesticides [2][4][5]. None of the studies above shows a single cause.

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Sources

  1. Shaw KA, Williams S, Patrick ME, et al. "Prevalence and Early Identification of Autism Spectrum Disorder Among Children Aged 4 and 8 Years — Autism and Developmental Disabilities Monitoring Network, 16 Sites, United States, 2022." MMWR Surveillance Summaries 74, no. SS-2 (17 April 2025): 1-22 cdc.gov
  2. National Institute of Environmental Health Sciences. "Autism." niehs.nih.gov
  3. Hansen SN, Schendel DE, Parner ET. "Explaining the increase in the prevalence of autism spectrum disorders: the proportion attributable to changes in reporting practices." JAMA Pediatrics 169 (2015): 56-62 doi.org
  4. von Ehrenstein OS, Ling C, Cui X, et al. "Prenatal and infant exposure to ambient pesticides and autism spectrum disorder in children: population based case-control study." BMJ 364 (2019): l962 doi.org
  5. Shelton JF, Geraghty EM, Tancredi DJ, et al. "Neurodevelopmental disorders and prenatal residential proximity to agricultural pesticides: the CHARGE study." Environmental Health Perspectives 122 (2014): 1103-1109 doi.org

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