Many factors may lead to symptoms of autism spectrum disorder (ASD). If a family already has a child diagnosed as having ASD, the chance that a sibling might also have ASD is 10 to 20 times higher than in the general population. Read on to learn more from the American Academy of Pediatrics about the causes of ASD.

Studies have shown that relatives of children with ASD are more likely to have some similar social and behavioral characteristics to those seen among children with ASD but not severe enough to merit a diagnosis. Behavioral symptoms may include social awkwardness, inflexibility, and repetitive behaviors.

ASD may occur more often in children with certain medical conditions or high risk factors. These medical conditions can include fragile X syndrome, tuberous sclerosis complex, Down syndrome, or other genetic disorders. However, most children with ASD do not have these or any other specific genetic conditions. Infants born preterm are another high-risk group. Considering whether a child with ASD has a genetic condition is an important part of the initial evaluation.

Potential environmental factors are also an important area for research and are not yet well understood.

Is there a link between vaccines and ASD?

Some people believe that vaccines cause ASD. However, many studies have been done, and there is no scientifically proven link between childhood vaccines—including the measles-mumps-rubella (MMR) vaccine—and ASD. In fact, the research article that first suggested a link between the MMR vaccine and ASD has been retracted (that is, removed permanently) because the research was done incorrectly. There is also no scientific proof to support a link between thimerosal (a mercury-containing preservative) and ASD. Even so, almost all vaccines given to children in the United States no longer contain mercury. Mercury was removed in the early 2000s.

The American Academy of Pediatrics urges parents to have their children fully immunized. Vaccines are a safe and effective way to protect children from diseases. Autism Speaks states, “Vaccines do not cause autism. We urge that all children be fully vaccinated.”

Families who remain concerned about vaccines and ASD should talk with their child’s pediatrician.

What is known about brain development in children with ASD?

The specific abnormalities in brain function that cause ASD are not known. However, research has shown that

• There is a difference in brain growth in children with ASD, with a tendency for brains to grow faster and be larger than usual in early childhood (often with a large head circumference).
• There are various microscopic abnormalities in brain areas of people with ASD, such as the cerebellum, limbic system, and cerebral cortex.
• There are differences or abnormalities in some brain chemicals in children with ASD. These brain chemicals are important regulators of brain development, nerve communication, and function. However, no single abnormality has been found in all people with ASD.
• There are some differences in function of certain parts of the brain in children with ASD, including how the brain recognizes faces, processes language, and allows for imitation.

Measurement of chemical neurotransmitters and imaging techniques of brain function are not currently appropriate in routine clinical practice for evaluation of children with autism. These and other studies may be important in research. Although our understanding of differences in brain structure and function is increasing, there is still much to be learned.

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