Turn back the clock 40 years and autism was, at least officially, a rare condition. Case rates in the 1980s suggested that only a handful of people in 10,000 were diagnosed. The picture is radically different today: greater awareness and a broadening of the criteria mean that autism is now common, with an estimated 1% to 3% of people worldwide now falling on the autism spectrum.
History records some terrible errors about the underlying causes of the condition. One of the first studies of autistic children noted that many had “highly intelligent parents” and close relatives who were “limited in genuine interest in people.” The observation fueled the false and deeply damaging idea that autism was caused by “refrigerator parents”—a coldness in parenting. Further damage was done in the 1990s when researchers claimed that the MMR vaccine caused autism. The work has since been vigorously dismantled, but dozens of children continue to suffer, or worse, as a result of not being vaccinated.
So what causes autism? Twin studies suggest there’s a strong genetic component. About 80% of autism is thought to be due to DNA, which explains why it often runs in families. Genetics works much the same way as with height: the hundreds, if not thousands, of contributing gene variants are spread out across the population, concentrated in some people more than others. The more contributing variants a person has, the more likely he or she is to exhibit traits that meet the threshold for diagnosis.
But there’s more to autism than genetics. Researchers are investigating a number of potential factors that could play a role, such as having older parents, obesity, diabetes or an immune disorder. Exposure to air pollution or certain pesticides during pregnancy, complications during childbirth and low birth weight are also on the list that scientists need to study.
Another area of active research involves the gut microbiome, the trillions of bacteria, viruses, fungi and other microorganisms that call the gastrointestinal tract home. Many of these microbes break down food in their path and produce a wide range of substances, from enzymes and toxins to compounds that regulate brain chemicals. Studies suggest that the gut microbiome may have a substantial effect on the brain, contributing to conditions such as social anxiety disorder and depression.
Scientists discovered differences in the bacterial populations that live in the guts of autistic people decades ago. But what causes the disruption in these and other microbes, and what those differences mean, has been unclear. Autistic people are more susceptible to certain gut problems, and a disrupted microbiome could be part of the reason. Do gut microbes play a role in autism itself? The jury is still out, but some research points to the possibility.
In 2019, researchers at the California Institute of Technology collected gut microbes from autistic people and transferred them to the intestines of mice. According to the studythe mice did indeed exhibit autism-like behavior, leading the scientists to conclude that the microbes were colonizing the rodents’ intestines and producing neuroactive substances that altered the animals’ behavior.
But mice aren’t people. It’s still unclear whether reshaping the microbiome through diet, probiotics, prebiotics, or microbial implants will ever help alleviate some of the challenges that autistic people face. The good news is that research is underway, with scientists steadily identifying the specific microbes, and the specific compounds they produce, that might be worth targeting.