What Causes Autism?

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Though autism research has come a long way in the past decade, in most diagnosed cases the cause of an individual’s Autism Spectrum Disorder (ASD) is not known. Family and large scale research studies suggest that genetics are strongly involved; however, only about 15-20% of cases have a known genetic disorder in addition to ASD.

Research shows that genes are not the only cause and that environmental factors must be present in the population too. Epigenetic research focuses on the ways in which environmental factors regulate the way genes work. This is an emerging area of environmental autism research. Future research may help us understand how genes and environmental factors interact with each other to cause ASD.

What does it mean to ‘cause’ autism?

Causation means that one action or event is the direct cause of another. Sometimes causation is easy to prove, as in when water is exposed to freezing temperatures it turns from a liquid to a solid. Other times, causation is much more complicated – particularly when more than one factor is at play. More effort is devoted in autism research to find the causes than for any other research goal. This is because scientists have learned in other branches of medicine that the best way to find truly effective treatments is by first understanding the biological causes of a medical condition.

Often times, people confuse causation with correlation. Correlation is a concept from statistics that measures the extent to which two things increase or decrease together. But this does not mean that one causes the other. For example, a person’s age and height are correlated – as one increases the other also increases. But one’s age does not cause one’s height. Generally, it is much easier to find a correlation than to prove a causation.  A famous saying in research is “Correlation does not equal causation.”

What is a gene-environment interaction?

Scientists believe that certain genetic makeups lead some people to be predisposed to autism, when exposed to certain environmental factors. “Environment” is defined broadly by scientists, and the term can include not just chemicals in the environment, but also factors such as premature birth and developmental experiences.

What are some of the environmental factors that have been suggested to cause ASD?

  • Refrigerator Mothers: Historically, parents – particularly mothers – were accused of causing ASD. In the 1950s and 1960s, a child psychologist named Bruno Bettelheim theorized that “refrigerator mothers,” those who lacked maternal warmth, caused their children’s autism by inattention and failure to show love and emotion. It is now widely understood that whatever causes autism must be biological in nature, and not from any particular parenting style.
  • Vaccinations: In 1998, a British researcher named Andrew Wakefield published a paper proposing a connection between the MMR (measles, mumps, and rubella) vaccine and autism in eight children. That paper set off general concerns over vaccines, including not just the MMR vaccine, but also the use of thimerosal (a preservative containing mercury) in vaccines, and the notion that babies receive too many vaccines too soon. Since Wakefield’s paper in 1998, scientists across the globe have conducted dozens of studies comparing hundreds of thousands of children who received vaccines to those who did not. They have consistently found neither correlation nor causation between vaccines and autism. Furthermore, the Lancet, the British journal that published Wakefield’s initial paper, has since published an editorial repealing the article based on evidence that Wakefield falsified data, and Wakefield’s license to practice medicine has been revoked. Unfortunately, the Lancet article and the distrust in vaccines it produced has caused some parents to stop getting their children immunized. Vaccines prevent many diseases that can lead to serious illness and/or death. Because of the high number of children who are currently unvaccinated, the risk of illness to these children and to others in their communities has increased steadily. People who are too young or too weak to be vaccinated themselves rely on high vaccination rates in their communities (“herd immunity”) to protect them from these illnesses. High vaccination rates are also critical to eradicating these life-threatening diseases.
  • PANDAS: PANDAS stands for Pediatric Autoimmune Neuropsychiatric Disorders Associated with Streptococcal Infections. It is associated with the sudden onset of obsessive compulsive disorder (OCD) or tics following a strep infection, such as strep throat or scarlet fever. While PANDAS symptoms may sometimes look a lot like autism, it is a separate disorder. In addition to sudden onset of OCD or tics, symptoms of PANDAS may include hyperactivity, inattention, separation anxiety, changes in mood, sleep disturbance, urinary frequency (including bed wetting), fine or gross motor changes (for example, changes in handwriting), and joint pain.
  • Prematurity/Low Birth Weight: Babies born prematurely and/or with very low birth weights often have medical or developmental problems early in life. Research has shown that babies born at less than 4 pounds, 7 ounces are about 5 times more likely than other children to be diagnosed with ASD. Despite this correlation, causation has not been proven as of yet. In fact, for some premature babies, the early birth might be caused by an existing genetic or biological problem in the baby.
  • Food Allergies: Some parents and researchers have suggested that allergies to milk and wheat products might cause ASD. Research has shown that casein (found in dairy products) and gluten (wheat) allergies are not more prevalent in individuals with ASD compared to the general population. Nevertheless, some families have changed the diets of their children with ASD to eliminate casein and gluten (known as the GF/CF diet). Research is mixed regarding whether the GF/CF diet reduces symptoms associated with ASD over a long enough time period to result in change. As of now, food allergies are not considered a cause of autism, nor has a correlation between food allergies and autism been established.

Research studies have also pointed to epigenetic factors that may contribute to an increased risk of ASD and other neurodevelopmental  disorders. Epigenetics refers to nongenetic factors that affect how genes work, but which do not change the DNA sequence itself. For example:

  • Maternal Health during Pregnancy: Some research has suggested that serious infections during pregnancy may increase the risk of ASD in a child born from that pregnancy. Most studies tend to agree, however, that mild infections, particularly those not requiring hospitalization, do not increase the risk of ASD. Research is ongoing to determine whether the correlations observed are causative factors. Similarly, obesity, gestational diabetes, and high blood pressure during pregnancy and exposure to thalidomide, valproic acid, misoprostol, and alcohol (in quantities sufficient to cause Fetal Alcohol syndrome) during pregnancy have been correlated with an increased risk of ASD, but these factors have not as of yet been shown to be causative.
  • Parental Age: Several studies have found that parents over a certain age (studies differ, but one study found age 40 to be the dividing line) are at a higher risk of having children with ASD than younger parents. One study even found that a grandfather’s age when his child was born is correlated with having a grandchild with autism. While there are theories as to why age may matter (for example, DNA mutations may be more prevalent with age), at this point advanced parental age is only a risk factor for ASD, and causation has not been established.
  • Pollution and Exposure to Environmental Toxins:  Exposures to environmental toxins such as mercury, lead, arsenic, polychlorinated biphenyls (PCBs), and toluene are known causes of neurodevelopmental disorders. For example, scientists have shown that methylmercury, which occurs when mercury reacts with bacteria in the environment, damages the nervous symptom, thus leading to neurodevelopmental disorders. For all toxins, however, it is difficult to establish precise causation because humans are constantly exposed to chemicals through the environment, not only over the lifespan, but also (in utero) before birth. Determining which exposures (including exposures to the parent as well as the individual) are causative is not possible to do at this time. Nonetheless, many studies have established correlation between autism and pesticides, phthalates, polychlorinated biphenyls (PCBs), solvents, toxic waste sites, air pollutants and heavy metals, with the strongest correlations found for air pollutants and pesticides.

Research into the causes of ASD is important because knowing the cause(s) is the best way to develop treatments. Unfortunately federal funding for autism research is relatively low compared to the frequency of autism. Nevertheless, research centers across the globe continue to work to understand the complex causation of ASD, both with federal funding and private support.

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Last Updated: November 18, 2014

The Center for Autism Research and The Children's Hospital of Philadelphia do not endorse or recommend any specific person or organization or form of treatment. The information included within the CAR Autism Roadmap™ and CAR Resource Directory™ should not be considered medical advice and should serve only as a guide to resources publicly and privately available. Choosing a treatment, course of action, and/or a resource is a personal decision, which should take into account each individual's and family's particular circumstances.


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