Decades of research into autism has revealed this much: There is no simple answer for what causes the condition.
In each person with autism, genes and their environment combined in a unique way, experts say.
“What causes autism in one person is unlikely to be the cause in another person,” says Laura Grofer Klinger, PhD, executive director of the UNC TEACCH Autism Program. “It’s a gene-environment interaction, and that’s what makes it so complicated.”
Of course, people with autism and their families want a better understanding of why their brains work the way they do, and misinformation runs rampant, making a complex topic even more confusing.
Fortunately, evidence-backed research provides some clues as to why someone might develop autism. Dr. Klinger and Rebecca Baum, MD, chief of UNC’s Division of Developmental Behavioral Pediatrics, explain.
Causes of Autism: The Evidence for Genetics and Environmental Factors
Researchers at UNC have been studying autism for more than 50 years. Together with other autism experts, they have learned:
- Autism runs in families.
Research has found that about 20 percent of babies with an older sibling who has autism will develop autism themselves; when one identical twin has autism, the other is very likely to have it too. Children born with certain genetic disorders, such as Fragile X syndrome, are more likely to have autism.
Scientists have identified some genetic mutations that are associated with autism, but the full picture isn’t known. Genes and environmental causes interact in ways that are not yet fully understood.
“There are hundreds of different genetic changes that have been associated with autism,” Dr. Baum says. “Sometimes those changes are inherited; sometimes they happen out of the blue.”
UNC-led research has shown that brain matter in 6-month-old infant siblings of children with autism, who had not yet shown symptoms but later went on to have autism themselves, looked different under MRI.
“That is important for families who wonder if something happened at 12 months or 18 months to cause their child’s autism,” Dr. Klinger says. “That may be when behavioral signs emerge, but the differences in the brain are there long before.”
- Exposures during pregnancy could play a role.
Babies in the womb can be exposed to environmental factors that may contribute to the development of autism. There is evidence that air pollution could be a risk factor, as well as pregnancy complications and infections and fevers in the mother during pregnancy.
Certain medications also increase risk, but there isn’t reliable evidence that over-the-counter drugs long considered safe for pregnant women, such as acetaminophen (Tylenol), cause autism.
There is some evidence that acetaminophen taken by the mother is associated with autism in the child, but the mother might have been taking the drug to counter a fever or infection that is more likely to be the cause, Dr. Klinger says. Associations are not the same as finding a cause. Other studies have found no association between Tylenol and autism; in particular, the largest study to date showed no link.
“The risk of having complications due to untreated fever are probably much greater than the risk of complications related to taking Tylenol,” Dr. Klinger says.
- Older parents are a focus of study.
Research has found that a child’s risk for autism is higher if they are born to older parents. There are many theories for the increased risk, including that older moms may be more likely to experience pregnancy complications that can lead to autism, and older dads may have sperm that carries genetic mutations.
The overall risk of an older parent conceiving a child with autism is still low, but it’s an area that needs more research, Dr. Klinger says.
“I wouldn’t say every older parent should be worrying, but there has been some research suggesting an association—though an association doesn’t make it a causation.”
- Vaccines do not cause autism.
In the 1990s, a medical journal article blamed the measles, mumps and rubella (MMR) vaccine for autism, but it was later debunked and retracted, and many large, rigorous studies since have shown that there is no relationship, Dr. Baum says.
Early symptoms of autism typically emerge between 15 months and 3 years of age, and this is also the time children get a number of vaccines. Sometimes parents think a vaccine is to blame for changes that would have happened regardless, Dr. Baum says.
Why Autism is on the Rise
It’s true that autism rates have skyrocketed: In 2000, about 1 in 150 children were diagnosed with the condition; today it’s 1 in 31, according to public health officials.
Part of that increase is because today’s parents and pediatricians are much more aware of autism than in previous generations. And the “spectrum” approach to viewing autism as something that can range from mild to debilitating is a fairly recent development; in previous generations, only children with the most severe disabilities would be diagnosed with autism.
“We used to define autism as requiring a co-occurring intellectual or cognitive delay, and now, most people with autism don’t have an intellectual or cognitive delay,” Dr. Klinger says. “We’ve broadened what we consider autism, and that accounts for some of this dramatic rise.”
Researchers who examined data sets from the 1980s say that many more of those children would be diagnosed under today’s standards; some of them are being diagnosed as adults.
If you have questions about your child’s health, talk to your doctor or find one near you