What Causes Autism?

By Temma Ehrenfeld @temmaehrenfeld
August 09, 2019

Scientists do not yet know exactly what causes autism spectrum disorder (ASD), but suspect that several factors might be at work in each case. Learn more.

What causes autism spectrum disorder (ASD)? It’s easy to think that there is one clear cause that explains the recent increase in recorded cases. But the answer is more complex.

Our knowledge of this illness, which was first described in the 1940s, has grown over time, which explains some of the increase in the number of cases. We know that ASD runs in families and is related to genes. In twins, the chances of one child having ASD if the other does is about 70 percent. If your sibling has ASD, your risk is as high as 30 percent. But as with most illnesses, the cause is probably a combination of genetic vulnerability and triggers in your life. An infection or contact with chemicals could trigger symptoms.

After some suspicion that children developed ASD after vaccines, a good deal of research has concluded that there was no connection. Vaccines do not increase the risk of ASD, and they protect your child from serious infectious illnesses.


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Even looking at genes alone, there is no one pattern found in all people with ASD. More than a hundred genes on different chromosomes may contribute, affecting early brain development. These gene variations may cause specific symptoms in some people, or control the severity of those symptoms.

In about 1 percent of all cases, ASD is linked to a single gene in a syndrome associated with other physical symptoms: You may have heard of Fragile X or Rett’s syndrome. In other cases, more than one gene variation is at work. Other evidence suggests that more than 25 percent of all ASD cases involve a rare variation. However, in most cases genetic analysis does not reveal what causes autism. And people can have ASD-linked variations without developing the illness.

If either parent is unusually old when the child is born, the risk of ASD increases. Mothers can try to avoid ASD by spacing out pregnancies at least a year and taking folic acid before and at conception and through pregnancy. Children born in a multiple pregnancy or those born prematurely or with a low birthweight are more at risk.

Scientists studying what causes autism are also looking for clues in the body’s overall functioning and problems in the immune system, metabolism, or brain.

You may be reading this because you suspect ASD in your child or a child you know. ASD most often isn't diagnosed until the age of four, when we expect children to be able to communicate easily. One sign you could catch sooner: hearing loss in the range important for processing speech.

People with ASD may make little or inconsistent eye contact, be slow to respond when you try to get their attention, have trouble letting conversation flow and instead be silent or make speeches, make odd expressions, sound sing-song or flat and robot-like, and have trouble seeing other points of view or other people’s emotions.

They may get upset by slight changes in routine, be unusually sensitive to light or noise, and have trouble with sleep and their temper. On the good side, they may have an unusual memory for detail and great concentration.


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April 03, 2020

Reviewed By:  

Janet O’Dell, RN