Scientists don't know the exact causes of autism spectrum disorder (ASD), but research suggests that both genes and environment play important roles.
In identical twins who share the exact same genetic code, if one has ASD, the other twin also has ASD in nearly 9 out of 10 cases. If one sibling has ASD, the other siblings have 35 times the normal risk of also developing the disorder. Researchers are starting to identify particular genes that may increase the risk for ASD.
Still, scientists have only had some success in finding exactly which genes are involved. For more information about such cases, see the information below about Fragile X syndrome and tuberous sclerosis.
Most people who develop ASD have no reported family history of autism, suggesting that random, rare, and possibly many gene mutations are likely to affect a person's risk. Any change to normal genetic information is called a mutation. Mutations can be inherited, but some arise for no reason. Mutations can be helpful, harmful, or have no effect.
Having increased genetic risk does not mean a child will definitely develop ASD. Many researchers are focusing on how various genes interact with each other and environmental factors to better understand how they increase the risk of this disorder.
In medicine, "environment" refers to anything outside of the body that can affect health. This includes the air we breathe, the water we drink and bathe in, the food we eat, the medicines we take, and many other things that our bodies may come in contact with. Environment also includes our surroundings in the womb, when our mother's health directly affects our growth and earliest development.
Researchers are studying many environmental factors such as family medical conditions, parental age and other demographic factors, exposure to toxins, and complications during birth or pregnancy.
As with genes, it's likely that more than one environmental factor is involved in increasing risk for ASD. And, like genes, any one of these risk factors raises the risk by only a small amount. Most people who have been exposed to environmental risk factors do not develop ASD. The National Institute of Environmental Health Sciences is also conducting research in this area. More information is available on their website .
Scientists are studying how certain environmental factors may affect certain genes—turning them on or off, or increasing or decreasing their normal activity. This process is called epigenetics and is providing researchers with many new ways to study how disorders like ASD develop and possibly change over time.
ASD and vaccines
Health experts recommend that children receive a number of vaccines early in life to protect against dangerous, infectious diseases, such as measles. Since pediatricians in the United States started giving these vaccines during regular checkups, the number of children getting sick, becoming disabled, or dying from these diseases has dropped to almost zero.
Children in the United States receive several vaccines during their first 2 years of life, around the same age that ASD symptoms often appear or become noticeable. A minority of parents suspect that vaccines are somehow related to their child's disorder. Some may be concerned about these vaccines due to the unproven theory that ASD may be caused by thimerosal. Thimerosal is a mercury-based chemical once added to some, but not all, vaccines to help extend their shelf life. However, except for some flu vaccines, no vaccine routinely given to preschool aged children in the United States has contained thimerosal since 2001. Despite this change, the rate of children diagnosed with ASD has continued to rise.
Other parents believe their child's illness might be linked to vaccines designed to protect against more than one disease, such as the measles-mumps-rubella (MMR) vaccine, which never contained thimerosal.
Many studies have been conducted to try to determine if vaccines are a possible cause of autism. As of 2010, none of the studies has linked autism and vaccines.
Following extensive hearings, a special court of Federal judges ruled against several test cases that tried to prove that vaccines containing thimerosal, either by themselves or combined with the MMR vaccine, caused autism. More information about these hearings is available on the U.S. Court of Federal Claims' website .
The latest information about research on autism and vaccines is available from the Centers for Disease Control and Prevention . This website provides information from the Federal Government and independent organizations.