Photograph: Science Photo Library/Alamy
Wind the clock back 40 years and autism, at least officially, was a rare condition. According to case rates in the 1980s, only a handful of people in every 10,000 received a diagnosis. The picture today is radically different: better awareness and a broadening of the criteria mean autism is now common – 1% to 3% of people worldwide are now estimated to be on the autism spectrum.
History records some horrendous mistakes around the underlying causes of the condition. One of the earliest studies of autistic children remarked that many had “highly intelligent parents” and close relatives who were “limited in genuine interest in people”. The observation fuelled the false and deeply damaging idea that autism was caused by “refrigerator parents” – a coldness in the upbringing.
Yet more harm followed in the 1990s when researchers claimed that the MMR vaccine caused autism. The work has since been vigorously demolished, but scores of children still suffered, or worse, as a result of being unvaccinated.
So what does cause autism? Studies in twins suggest there is a strong genetic component. About 80% of autism is thought to be down to DNA, which explains why it often runs in families.
The genetics work much as they do for height: the hundreds, if not thousands, of gene variants that contribute are scattered through the population and concentrate in some people more than others. The more contributory variants a person has, the more likely they are to display traits that meet the threshold for diagnosis.
But there is more to autism than genetics. Researchers are investigating a number of potential factors that could play a role, such as having parents who are older, who are obese, or have diabetes or an immune disorder. Exposure to air pollution or certain pesticides during pregnancy, birth complications and low birth weight are also on the list for scientists to study.
Another area of active research involves the gut microbiome, the trillions of bacteria, viruses, fungi and other microorganisms that set up home in the gastrointestinal tract.
Many of these microbes break down food that comes their way, and produce a vast array of substances from enzymes and toxins to compounds that regulate brain chemicals. Studies suggest the gut microbiome can have a substantial effect on the brain, driving conditions such as social anxiety disorder and depression.
Scientists spotted 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, is unclear.
Autistic people are more prone to particular gut problems, and a disrupted microbiome could be part of the reason. Do gut microbes have a role in autism itself? The jury is still out, but some studies raise the possibility.
In 2019, researchers at the California Institute of Technology collected gut microbes from autistic people and transferred them into the guts of mice. According to the study, the mice duly displayed autism-like behaviours, leading the scientists to suggest that the microbes colonised the rodents’ guts and produced neuroactive substances that changed the animals’ behaviour.
But mice are not humans. It is still not clear whether reshaping the microbiome through diet, probiotics, prebiotics or microbial implants will ever help alleviate some of the challenges autistic people face. The good news is that research is afoot, with scientists steadily pinpointing the particular microbes, and the particular substances they produce, which may be worth targeting.
