Autism's Most Common Cause: Environmental Factors

The largest and most rigorous twin study of its kind finds environmental exposures among the leading causes of autism.

July 7, 2011

Studying twins helps researchers tease out the causes of autism.

A large study released this week found that environmental factors are more responsible for autism than previously believed. In fact, environmental exposures are now believed to trigger autism more commonly than genetic factors. Researchers did not investigate which particular environmental exposures are linked to autism, although that is an emerging focus among autism experts. This latest research studied identical and fraternal sets of twins. Twins are important in autism research, because studying them helps researchers separate genetic and environmental factors that may affect autism. Identical twins have the same genes, while fraternal twins share just 50 percent of the same genes, but share the same environment in the womb.


Autism experts not involved in this particular study say the research is well done, and could help further turn the tide of autism research toward recognizing that environmental chemicals and certain drugs could be causes of autism. "It's very credible stuff," says pediatrician and epidemiologist Phil Landrigan, MD, professor and chair of the department of community and preventive medicine at Mount Sinai School of Medicine in New York City. "As new information comes in, it's becoming very clear that there's environmental contribution."

THE DETAILS: The study, published in Archives of General Psychiatry, studied nearly 200 sets of twins, some identical and some fraternal. At least one child in each set of twins had been diagnosed with autism. The researchers determined that fewer than 40 percent of the cases were caused strictly by genetics. Shared environmental factors, including exposures in the womb, were implicated in nearly 60 percent of the cases. A separate study published in the same journal issue found a doubled risk of autism in children whose mothers took SSRI antidepressants a year before giving birth.

WHAT IT MEANS: Previous estimates had labeled genetic factors as responsible for a whopping 90 percent of autism cases. Now scientists are starting to realize it's not just inherited or environmental exposure, but likely both.

"The proportion differs in different children," explains Dr. Landrigan. "Some cases are mainly environmental exposures, some are mainly genetic. And some are caused by an interplay between wrong exposures and genes."

Dr. Landrigan also notes that nothing is 100 percent, but as a general rule, exposures in early pregnancy are most dangerous. (Landrigan and other autism researchers are expected to publish a study listing the top chemicals of concern related to autism later this summer.)

While researchers are working on finding a link between autism and air pollution, BPA, and other chemicals, an article Landrigan published last year outlines the most strongly positive, "proof-of-concept" evidence supporting that autism is associated with certain environmental exposures. As previously reported by, these exposures during critical windows of fetal development are the ones most strongly linked to autism:

1. Thalidomide—Not commonly used in the U.S., this sedative drug has been linked to an increase in autism and other birth defects.

2. Misoprostol—This drug is licensed in the U.S. for use in preventing gastric ulcers, but it's used in other countries for abortions.

3. Valproic acid—This medication is used to control epilepsy.

4. Prenatal rubella infection—A woman infected while pregnant also faces a higher risk of having a child with other developmental problems, eye problems, deafness, or heart problems.

5. Chlorpyrifos pesticide—One of the most widely used pesticides in the U.S. Banned for residential uses, this pesticide is still legal in agriculture, and residues have turned up on apples, bell peppers, cranberries, kale, grapes, peaches, and dozens of other foods.

The report by Dr. Landrigan and his colleagues, expected this year, will likely add more culprits to that list.

Here's how to avoid exposure to possible autism triggers.

• Be sure to talk prenatal vitamins with your doctor. A new study published in the journal Epidemiology found mothers who reported taking prenatal vitamins during the three months before conception or the first month of pregnancy reported a nearly 40 percent lower risk of children who developed autism. For people with certain genes known to increase the risk of autism, the effect of prenatal vitamins on lower autism risk was even greater.

In response to the latest study finding a link between antidepressants and pregnancy, you should also discuss the use of those drugs if you're trying to get pregnant.

• Eat organic as much as possible. Pesticides have been linked to autism, ADHD, and other serious ailments. The good news is, previous studies have found a rapid drop-off of pesticides in the body once a person adopts an organic diet. "Generally, we know people who eat organic have much lower pesticide levels than those who eat conventional food," says Dr. Landrigan.

As you eliminate chemical-laced food from your diet, be sure to cut out the use of chemical pesticides in and around the home, too.

• Eat fish, but pick the right fish. Fish during pregnancy is recommended, but you want to make sure you're getting fish high in beneficial omega-3 fatty acids and low in harmful contaminants like PCBs and mercury. For a go-to list, check the Superfish List.

• Favor glass bottles. BPA is suspected to cause developmental problems in children, and although we don't know for sure yet if it's directly causing or triggering autism in certain children, Dr. Landrigan recommends using glass baby bottles. Since a recent study found that all types of plastic are harmful, it's probably a good idea for expecting moms to avoid plastic as much as possible, too.

• Be smart about remodeling. Don't sand down lead paint, and avoid toxic-plastic remodeling products, such as vinyl flooring.