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updated 1/11/2011 6:28:05 PM ET 2011-01-11T23:28:05

The idea that vaccines cause autism has received yet another blow, with a new article in the British Journal of Medicine declaring the 1998 study by Andrew Wakefield, which originally found an autism-vaccine link, an "elaborate fraud." The article is the latest criticism of a theory that has been widely discredited. But if vaccines are off the table, what does cause autism?

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While scientists are still investigating the issue, they say the disorder likely has a number of causes involving both our genes and our environment, or some combination of the two. For instance, people may have underlying genetic susceptibilities to autism that are triggered by something they encounter in the environment.

Making things even more complicated is the fact that autism, which is characterized by problems interacting and communicating with others, is not a single disorder, but a range of disorders that may have various causes. [See a time line of events: Vaccines and Autism: How the Truth Unfolded ]

"People are going to manifest the disorder in different ways, and that could be because there are different sets of genes, [or] different sets of environmental factors," that contribute to how the disorder presents itself, said Alycia Halladay, director of research for environmental sciences for Autism Speaks, an autism advocacy organization that funds autism research.

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Here are the latest findings and ideas from scientists about what might really cause this mysterious condition.

Genetics
There is strong evidence that changes in our genes contribute to autism.

For one thing, the disorder is highly heritable. Families that have one child with autism have a 1 in 20 chance of having a second child with autism, according to the National Institutes of Health. This is a higher risk than in the general population. And twin studies have found that if one twin has autism, the other twin as a 90 percent chance of having the disorder.

Research has also shown that the genetic changes that contribute to autism don't have to be inherited — they may also arise spontaneously.

In total, scientists have identified about 20 genes that may be involved in autism spectrum disorder, Halladay said. While these genes are located all over the human genome, they share a common theme, she said. Many have a role in brain development, brain growth and the way brain cells communicate.

For example, a recent study found that children with a genetic mutation on chromosome 17 were 14 times more likely to develop autism than those without the mutation.

Pesticides
Exposure to pesticides has also been linked to autism. Some studies have found that pesticides may interfere with genes involved in the central nervous system, said Dr. Alice Mao, an associate professor of psychiatry at Baylor College of Medicine in Houston.

Scientists think that chemicals in pesticides may adversely affect those who are genetically predisposed to autism, leading them to develop the full-blown disorder, Mao said.

"Maybe they were born with a vulnerability to autism, but then exposure to the pesticides might have cause the presentation of autism," Mao told MyHealthNewsDaily.

Pharmaceuticals
Babies that have been exposed to certain pharmaceuticals in the womb, including valproic acid and thalidomide, have been found to have a higher risk of autism.

Thalidomide is a drug that was first used in the 1950s to treat morning sickness, anxiety and insomnia. The drug was withdrawn from the market after it was linked with birth defects, but is currently prescribed for a severe skin disorder and as a treatment for cancer.

Valproic acid is a medication prescribed for seizures, mood disorders and bipolar disorder, according to the NIH.

Parental age
As parents grow older, they have a higher risk of having children with autism, Halladay said.

A study published last February found that women who are 40 years old have a 50 percent greater risk of having a child with autism than women who are between 20 and 29 years old.

Researchers aren't sure why parental age may influence autism risk, but it might be related to genetic mutations that occur in the sperm or the egg as parents grow older, Halladay said.

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The development of the brain
Particular areas of the brain, including the cerebral cortex and the cerebellum, have been implicated in autism, Mao said. These brain areas are thought to be responsible for concentration, movement and mood regulation.

Irregularities in the levels of neurotransmitters, such as dopamine and serotonin, have also been tied to autism, Mao said. Problems regulating dopamine can lead to problems with concentration and movement disabilities, while troubles controlling serotonin levels can result in mood problems.

Research advances
While scientists cannot say definitively what causes autism, they have come a long way in the last decade, Halladay said.

For example, researchers originally thought there might be just a single gene or a few environmental factors that are linked to autism, but now evidence has shown there are likely to be more.

"I think our knowledge has increased, and the way that we go about looking for potential genes and environmental candidates has improved," Halladay said.

"We're thinking about a new model — it's not just going to be just one gene or one environmental factor, it's more complex than that," she said.

Pass it on: Autism likely has a number of causes, both genetic and environmental.

Follow MyHealthNewsDaily staff writer Rachael Rettner on Twitter @Rachael_MHND.

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