By LAURIE TARKAN
A new study of twins suggests that environmental factors, including conditions in the womb, may be at least as important as genes in causing autism.
The researchers did not say which environmental influences might be at work. But other experts said the new study, released online on Monday, marked an important shift in thinking about the causes of autism, which is now thought to affect at least 1 percent of the population in the developed world.
“This is a very significant study because it confirms that genetic factors are involved in the cause of the disorder,” said Dr. Peter Szatmari, a leading autism researcher who is the head of child psychiatry and behavioral neuroscience at McMaster University in Ontario. “But it shifts the focus to the possibility that environmental factors could also be really important.”
... Other experts have cited factors like parental age, multiple pregnancies, low birth weight and exposure to medications or maternal infection during pregnancy.
... In the new study, the largest of its kind among twins, researchers looked at 192 pairs of identical and fraternal twins whose cases were drawn from California databases. At least one twin in each pair had the classic form of autism, which is marked by extreme social withdrawal, communication problems and repetitive behaviors. In many cases, the other twin also had classic autism or a milder “autism spectrum” disorder like Asperger’s syndrome.
Identical twins share 100 percent of their genes; fraternal twins share 50 percent of their genes. So comparing autism rates in both types of twins can enable researchers to measure the importance of genes versus shared environment.
The study found that autism or autism spectrum disorders occurred in both twins in 77 percent of males and 50 percent of females. As expected, the rates among fraternal twins were lower: 31 percent of males and 36 percent of females.
But surprisingly, mathematical modeling suggested that only 38 percent of the cases could be attributed to genetic factors, compared with the 90 percent suggested by previous studies.
And more surprising still, shared environmental factors appeared to be at work in 58 percent of the cases.
“We, like everyone else, were very surprised because we didn’t expect it to be that high,” said a senior author of the study, Neil Risch, a geneticist and epidemiologist at the University of California, San Francisco.
In siblings who are not twins, the rate of autism is much lower, suggesting that the conditions the twins shared in the womb, rather than what they were exposed to after birth, contributed to the development of autism.
A second article in the same journal found an elevated risk of autism in children whose mothers took a popular type of antidepressant during the year before delivery. But the authors reassured women taking these drugs — so-called S.S.R.I.’s like Prozac, Zoloft, Celexa and Lexapro — that the risk was still quite low: 2.1 percent in children whose mothers used them in the year before delivery, and 2.3 percent in the first trimester of pregnancy.
You could make the argument that autism might be like sickle cell anemia, a hereditary disease that is fatal without medical care in people with two copies of the gene, but one copy makes you more likely to survive the most lethal form of malaria.