April is Autism Awareness Month, a time when families and communities gather to increase awareness and acceptance of autism spectrum disorder. Since the disorder was first described in 1943, scientists (and pseudoscientists) have proposed a litany of factors that might give rise to the range of communication and interaction disorders that we now call ASD. Vaccines. Power lines. Bad parenting. All have been soundly rejected. That means that even on Autism Awareness Day in 2019, a basic understanding of what causes autism remains elusive.
So what exactly causes autism—and what definitely does not? Here’s what we know.
What We’re Sure Does Not Cause Autism
Shortly after Leo Kanner first described children with “extreme autistic aloneness” in 1943, researchers rushed to blame bad parenting for the disorder. Kanner himself led the charge, with his suspicion that “genuine lack of maternal warmth” gave rise to children who struggled to interact normally with others.
This guilt trip was all but established science throughout the 1950s.
Then, in 1964, Bernard Rimland published Infantile Autism: The Syndrome and Its Implications for a Neural Theory of Behavior, a book that shifted the blame from parenting style to neurological development. As studies began to build around the theory that biological brain development, not parenting, was responsible for these disorders, the theory that parental coldness caused autism was slowly debunked. Experts now agree that parenting strategy does not cause autism. Indeed, if parental coldness caused autism, one would expect “cold” parents to have more autistic children than anyone else. Studies have confirmed that they do not.
But biology and brain development are unsatisfying explanations. So parents and less scrupulous researchers continued to look for someone — or something —t o blame. In 1998, Andrew Wakefield provided this with a fraudulent paper that linked the measles, mumps, and rubella vaccine to autism. There is perhaps no claim in the modern history of science that has been so thoroughly debunked — literally hundreds of studies on hundreds of thousands of children have failed to find any link between any vaccine or vaccine ingredient and autism.
So the hunt for an environmental enemy continued. In a 2006 working paper, economics researchers concluded that “just under forty percent of autism diagnoses … is the result of television watching”, a claim that was panned by scientists across academic disciplines. More recent quackery has given rise to claims that power lines, and Wi-Fi, cause autism, despite the fact that electromagnetic fields do not have enough energy to alter DNA and Wifi radiation is significantly lower than that of visible light. If Wifi causes autism, so do windows.
Which brings us back to Rimland, and his 1964 revelation that the key to understanding autism is learning more about the developing brain. Fortunately, while the public marched against vaccines and declared war on television and electricity, scientists spent the past half-century working the most plausible angle — simple, cruel, obdurate neurobiology and genetics.
Likely Causes of Autism: Genetics, Neurobiology, Parental Age
Genetics almost certainly contributes to autism. One indicator of a genetic root to the problem is the observation that families with one autistic child have a 1 in 20 chance of having a second child with an autism spectrum disorder. But even stronger evidence can be found in more recent studies of identical twins. Researchers have found that if one identical twin is diagnosed with ASD the other twin will have the same diagnosis. This happens between 36 and 95 percent of the time.
Meanwhile, roughly 20 genes have been linked to autism and almost all of them play a role in brain development and communication between brain cells. One of the most likely targets for future genetics research is chromosome 17. Children with a particular mutation on that chromosome have been shown to be 14 times more likely to develop autism than other kids.
Perhaps it is one, or many, of these genes and genetic mutations that alter the developing brain and kick off ASD. Imaging studies have shown that children with autism show marked, physical differences in the cerebral cortex and cerebellum (which control, among other things, concentration, and mood). The current thinking is that spontaneous genetic mutations rewire how brain cells communicate with one another in the early stages of development and growth of the brain. The ultimate consequences are the brain changes that are observed in people affected by autism.
So what causes these mutations in the first place? Scientists are not sure. One dominant theory is that parental age contributes to the risk of mutations across the board. Older parents have older sperm and egg cells, which may be more susceptible to de novo mutations — changes in DNA sequences that occur in each cell as a fertilized egg divides. Recent studies suggest that people with ASD have more de novo mutations than the general public. Mothers over the age of 40 are up to 50 percent more likely to have children with an autism spectrum disorder.
But there are likely other factors, perhaps even environmental factors, that contribute to the mutations that are thought to give rise to the brain changes that ultimately cause autism.
Will Knowing What Causes Autism Help Us Treat It?
That’s the plan. Since scientists began focusing on the neurobiological and genetic causes of autism in earnest, the National Institutes of Health have invested heavily in autism research that chases down potential biological leads.
Its Autism Coordination Committee has the stated goal of enhancing “the quality, pace, and coordination of efforts at the NIH to find a cure for autism”, and the NIH/ACC is currently funding research that explores links between autism and other, related diseases, including tuberous sclerosis complex, Fragile X Syndrome, Phelan-McDermid syndrome, and Rett syndrome. Studying the similarities between many of these conditions and autism is slowly leading researchers toward more precise understandings of how autism develops in the genome and in the brain.
Understanding what causes autism (and what most certainly does not) is instrumental in guiding researchers toward better early identification, more robust early intervention, and perhaps even a cure. And that’s something worth celebrating, this Autism Awareness Day. Because, while Autism Awareness Month is a time to increase our society’s awareness and acceptance of people with ASD, it is also a fine time to increase society’s awareness of how far we have come in the scientific study and treatment of autism — and how far we still have to go.