From Genetics to the Environment—Risk Factors for Autism
- Gordon Cairns
- Jun 21
- 6 min read
When 89-year-old retired banker Donald Triplett died quietly at his home in Forest, Missouri, in 2023, his passing made headlines for one fact alone: He was the first person ever diagnosed with autism. He was only 10 years old when child psychologist Leo Kanner used the term to describe young Donald’s range of behaviors, which included an excellent memory, an inability to relate to others, and obsessively repetitive actions.
Within the span of Triplett’s life, worldwide diagnosis of autism increased dramatically. As of 2021, an estimated 62 million others globally have been diagnosed as being autistic, according to the most recent study by The Lancet, published this year. The rise in Autistic Spectrum Disorders (ASD) hasn’t been gradual: Global prevalence has actually doubled since the journal’s previous study published in 2018, in which over 31 million people were described as being on the autism spectrum.
An estimated 62 million others have been diagnosed as of 2021, according to the most recent study by The Lancet, published this year.
This worldwide increase is mirrored in the US. In a typical class of third-graders, it is statistically likely that one child will have an autism diagnosis, as an estimated 32 out of every 1,000 eight-year-olds were autistic in 2022. This is nearly five times the rate in 2000, according to the US Centers for Disease Control and Prevention.
Does this dramatic rise mean aspects of the modern world is causing children to be born with autism? Or does it point to improved awareness and advances in diagnostic practices in identifying the condition? Autism is broadly characterized as affected individuals lacking in empathy, difficulty interpreting nonverbal communication signals, or struggling to develop, keep, or understand relationships.
Improved Awareness and Diagnosis of Autism
The answer is a bit of both. Alison Singer, co-founder and president of the Autism Science Foundation, believes the startling increase is due to a wide range of factors. In a recent blog post for the National Foundation for Infectious Diseases, she listed a number of causes for the rise, including an increased general awareness of autism coupled with a broader definition of autism introduced just over a decade ago. There is also improved access to services that can help identify autism, and these services have better screening tools and processes available to them. Apart from making it easier to identify, Singer also included other elements such as parents having children at a later age and improved survival rates for preterm babies.
Expanding Genetic Factors of Autism in the Family
However, the main determining factor of autism has remained the same—one’s genes. The scope of associated genes was updated in a Nature study published in December 2024 by researchers at the University of Texas (UT) Southwestern Medical Center. In the study, the researchers indicated they found 92 potential variants in 73 known neurodevelopmental disorder genes and 158 potential variants in 120 autism candidate genes. This was based on 222 people with autism from 195 families.
“Research suggests that there may be up to 1,000 genes associated with ASD susceptibility, and the genetic variants identified to date represent just a fraction of the disease burden.”
“Research suggests that there may be up to 1,000 genes associated with ASD susceptibility, and the genetic variants identified to date represent just a fraction of the disease burden,” said senior author associate professor Maria Chahrour, PhD, in UT Southwestern Medical Center’s press release.
Autism also has an estimated heritability of 70% to 90%, causing it to run in the family. The National Fragile X Foundation, a nonprofit that supports families living with Fragile X syndrome, states that “an identifiable single-gene condition may account for 15% to 20% of all cases of autism” and recommends genetic testing that includes Fragile X syndrome.
In addition, autism is estimated to be about 10 times more prevalent in children with an autistic sibling. A 2024 study by researchers at the University of California Davis found that 20.2% of siblings of children with autism developed autism themselves, based on 1,605 infants with an older autistic sibling. This probability increased to 37% for a child with multiple siblings with autism.
Environmental Risk Factors for Autism
But genetics doesn’t explain all the causes of autism. Scientists have also been looking at the environment to see if there is a connection between environmental pollution and autism.
Someone working in this field is Heather Volk, associate professor at Johns Hopkins University, who investigates how interactions between environmental factors, such as air pollution, and genetic factors, like mutations, lead to ASD in children.
One investigation led by Volk in 2011 found children born of mothers living less than 1,014 feet from a freeway during their third trimester of pregnancy were twice as likely to develop ASD. Speaking on a webinar in 2023 released by Spark, a US-based organization that researches autism, she said, “Broadly, what we can see is that neurodevelopment is impacted by air pollution exposure that happens during the prenatal period in early life.”
She has examined 23 papers published within the last 10 years that looked at the relationship between air quality and autism and concludes: “Outdoor air quality in the neighborhood based on an individual’s address often around the time of birth does seem to be associated with autism.”
She adds that higher levels of pollution have an even more detrimental effect. “Among kids with autism,” she explains, “children who live in more polluted areas, particularly in their first year of life, are more likely to have more problems in daily life, communication in particular, in overall functioning as well as in their daily living skills.”
Prenatal Health and Autism
Volk has also examined five studies on how the diet of the expectant mother might work to reduce the impact of the environment on the possibility of a child developing autism. “For example,” she says, “perhaps moms who take a prenatal vitamin and moms who might live in an area where they might be exposed to a higher level of air pollution or a higher level of pesticides, those two factors might work against each other.”
“Maybe, if you take your prenatal vitamin even if you live in an area of poor air quality, that could help protect your child subsequently in your pregnancy. So much research needs to be done here.”
“Maybe, if you take your prenatal vitamin even if you live in an area of poor air quality, that could help protect your child subsequently in your pregnancy. So much research needs to be done here.”
A review published in 2024 of a number of studies highlights potential links between autism and the health of the pregnant mother, as well as environmental conditions. A low intake of zinc, magnesium, and selenium during pregnancy was found to be linked with increased risk of neurodevelopmental disorders, including autism. One study in the United States and another in Norway reported a nearly 40% decline in autism risk associated with prenatal vitamin use, while the US study also found a significant decrease in ASD tendency with a daily folic acid intake. On the other hand, a Danish study could not find any association between an expectant mother taking these supplements and a reduced risk for their child.
Therapy for Autism through Nature
Just as the urban environment can impact the probability of a child being born with autism, so too can the natural environment provide support for parents with autistic children, both for the young person themselves and adults who are on the spectrum.
While there is little research into this area, a report from Iran found family-centered nature therapy was effective in improving the parents’ relationship with their children, including reducing conflicts. The offspring showed lower dependence on their parents after receiving the treatment. Another small-scale study found participants with autism experiencing considerable positive psychological outcomes from engaging in outdoor exercise.
For families dealing with autism, consult support groups in one’s community, educational materials, and healthcare providers for more information and recommendations.
*Gordon Cairns is a freelance journalist and teacher of English at the Forest Schools, based in Scotland. For almost two decades, he has been taking groups of teenagers with autism out into the woods, where they build fires and dens, swing in hammocks, and set up camp. Anecdotally, the young people report sleeping better the following night and feeling calmer after spending time in nature and away from restricting walls, artificial lighting and electronic hums.
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