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Autism Causes

Factors Affecting Brain Development

What causes normal brain development to go awry? Some NIMH researchers are investigating genetic causes-the role that heredity and genes play in passing the disorder from one generation to the next. Others are looking at medical problems related to pregnancy and other factors.

Heredity. Several studies of twins suggest that autism  -- or at least a higher likelihood of some brain dysfunction -- can be inherited. For example, identical twins are far more likely than fraternal twins to both have autism. Unlike fraternal twins, which develop from two separate eggs, identical twins develop from a single egg and have the same genetic makeup.

It appears that parents who have one child with autism are at slightly increased risk for having more than one child with autism. This also suggests a genetic link. However, autism does not appear to be due to one particular gene. If autism, like eye color, were passed along by a single gene, more family members would inherit the disorder. NIMH grantees, using state-of-the-art gene splicing techniques, are searching for irregular segments of genetic code that the autistic members of a family may have inherited.

Some scientists believe that what is inherited is an irregular segment of genetic code or a small cluster of three to six unstable genes. In most people, the faulty code may cause only minor problems. But under certain conditions, the unstable genes may interact and seriously interfere with the brain development of the unborn child.

A body of NIMH-sponsored research is testing this theory. One study is exploring whether parents and siblings who do not have autism show minor symptoms, such as mild social, language, or reading problems. If so, such findings would suggest that several members of a family can inherit the irregular or unstable genes, but that other as yet unidentified conditions must be present for the full-blown disorder to develop.

Pregnancy and other problems. Throughout pregnancy, the fetal brain is growing larger and more complex, as new cells, specialized regions, and communication networks form. During this time, anything that disrupts normal brain development may have lifelong effects on the child's sensory, language, social, and mental functioning.

For this reason, researchers are exploring whether certain conditions, like the mother's health during pregnancy, problems during delivery, or other environmental factors may interfere with normal brain development. Viral infections like rubella (also called German measles), particularly in the first three months of pregnancy, may lead to a variety of problems, possibly including autism and retardation. Lack of oxygen to the baby and other complications of delivery may also increase the risk of autism. However, there is no clear link. Such problems occur in the delivery of many infants who are not autistic, and most children with autism are born without such factors.

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