(CNN) -- The controversy surrounding a retracted study that linked autism to childhood vaccines has been fueled by the fact that no one knows what really causes autism.
"If we knew, it would change this whole debate altogether," said CNN chief medical correspondent Dr. Sanjay Gupta.
Researchers believe both genetics and environmental factors play a role in the development of autism spectrum disorders. According to the National Institutes of Health, researchers have "identified a number of genes associated with the disorder. Studies of people with ASD have found irregularities in several regions of the brain. Other studies suggest that people with ASD have abnormal levels of serotonin or other neurotransmitters in the brain."
The abnormalities suggest that ASDs could be caused by a disruption in brain development during the fetal stage, "possibly due to the influence of environmental factors on gene function."
But these findings are only preliminary and require further study, the NIH says.
Two studies strongly suggest some people have a genetic predisposition to autism, the NIH says. "Identical twin studies show that if one twin is affected, there is a 90 percent chance the other twin will be affected."
Dr. Eric Hollander, who was director of the Seaver and New York Autism Center of Excellence in 2008, told CNN.com at the time that "of all neuropsychiatric disorders, there's a stronger genetic predisposition for autism. It's also clear that early environmental experiences can play an important role in modifying how these genes develop."
But which environmental experiences and why? Theories abound, but no answers. Lee Grossman, president and CEO of the Autism Society of America, said, "What is the involvement of the environment, working with these genetic predispositions for a person to get autism? We're just starting to scratch the surface on this right now."
Researchers have been engaged in a wide range of studies, looking at all sorts of possibilities for what may trigger, or show a greater likelihood of, autism spectrum disorders.
For example, in October, Danish researchers suggested full-term babies who develop jaundice have a 67% higher risk of developing autism. That finding, published in the journal Pediatrics, contradicted a similar study done five years earlier. The NIH notes that jaundice is present to some degree in most newborns, but usually doesn't cause any problems and goes away within one to two weeks.
Another study, published in 2009, said children born more than three months premature have double the expected rate of autism.
The Centers for Disease Control and Prevention website notes that "some harmful drugs taken during pregnancy have been linked with a higher risk of ASDs, for example, the prescription drugs thalidomide and valproic acid."
One thing researchers are adamant about: Parents aren't to blame. "The theory that parental practices are responsible for ASD has long been disproved," the NIH says.
Still, the lack of answers leaves many parents wondering. "I questioned all things," said Maria Collazo of Belleville, New Jersey, whose daughter was diagnosed with autism. "Did I eat something I shouldn't have? Did I expose myself to something? You question yourself. You question and you don't know -- and it's like a state of mind."
"There's no evidence to say the child is autistic because the parents did something wrong during their pregnancy or early years," said Patricia Robinson, a therapist for people with autism.
It is estimated that an average of about 1 in 110 children in the United States have autism, according to the CDC. Cases are found among all ethnic and socioeconomic groups. But boys are 4 to 5 times more likely to be reported to have autism than girls.
In some cases, parents don't realize their children have autism. ASDs sometimes go unrecognized in mildly affected children "or when it is masked by more debilitating handicaps," the NIH notes.
The CDC calls autism spectrum disorders "an urgent public health concern."
"Just like the many families affected in some way by ASDs," the group says, "CDC wants to find out what causes the disorder."
CNN Medical Producer Miriam Falco contributed to this report.