What’s known about contributing factors:

Genes.

About 15% to 20% of autistic children have a genetic mutation that causes their disorder, Insel says. Certain genetic disorders, such as Fragile X syndrome and Rett syndrome, are wellknown for increasing the risk of autism. Even when genes are the main contributor to autism, however, it’s possible that most children have a unique mutation or set of mutations, says David Amaral, research director of the University of California-davis MIND Institute.

Family history.

If parents have one child with autism, the risk of having a second child diagnosed with the disorder is nearly 20%, according to a landmark study from U.c.-davis. Among those with two autistic children, the risk of having a third is 32%, study author Sally Ozonoff says.

Environmental pollution.

One California study published last year found that babies whose mothers lived near a highway while pregnant were more likely to be diagnosed as autistic.

Older parents.

Both older fathers and mothers are at higher risk of having autistic children, Newschaffer says. Research from Israel and the Harvard School of Public Health also suggests that infertility treatments, which are more often used among older patients, are linked to a higher risk of autism. Prematurity, low birthweight. An October study in Pediatrics found that, among babies born weighing less than about 4½ pounds, 5% had been diagnosed as autistic by age 21.

Medications.

Many studies now show that a seizure treatment called valproic acid can increase the risk of autism in children exposed before birth. A single study published last year found a higher risk among children exposed prenatally to antidepressants. Using prenatal vitamins is linked to a lower risk of autism.

Closely spaced pregnancies.

In a 2011 study, children who were born less than one year after an older sibling were three times as likely to be diagnosed with autism, compared with children born three years after their mother’s last pregnancy.