Which Weighs More in Autism Etiology: Genes or Environment?

Autism is mostly due to genes, right? Wrong. It can be blamed about half on genes and half on the environment, says a new study published online July 4 in the Archives of General Psychiatry.

The study's lead investigator was Joachim Hallmayer, M.D., an associate professor of psychiatry at Stanford University.

One strength of the new study, the researchers explained, is that it was a large population study conducted in California with diverse ethnic minority participation and with the participation of people of various socioeconomic statuses. Previous autism gene studies focused exclusively on people from a European background, they noted. Another strength of the study is that structured clinical interviews were used to diagnose autism, which was not the case with many previous studies.

The implications of their findings, the investigators pointed out, are that more attention should be paid to possible environmental contributions to autism such as those that occur prenatally or during the first year of life, since signs of autism usually emerge by the end of the first year (see Pediatricians Taught to Screen for Autism Disorders). Possible risk factors could be, for example, paternal age, low birth weight, viral infections during pregnancy, or perhaps even use of SSRI antidepressants during pregnancy, although the results here need to be replicated and extended (Psychiatric News, August 5).

The study included 192 twin pairs with at least one twin diagnosed with an autism spectrum disorder who were born between 1987 and 2004. The twins were identified through the California Department of Developmental Services. The investigators then determined the differences in autism concordance rates between the identical and fraternal twins to estimate the contribution to autism of genes and of a shared environment.

They found that the genetic contribution was 38 percent and that the shared environmental contribution was 58 percent. Also, when they focused only on cases of autism, not on autism plus Asperger's syndrome, the genetic and environmental contributions turned out to be similar—37 percent versus 55 percent.

Thus, while genes play an important role in autism spectrum disorder, it looks as if they are less important than previous studies have suggested.

The study was funded by the National Institute of Mental Health and by Autism Speaks.