Autism Mostly Caused by Environmental Factors, Stanford Study Finds

A Stanford University School of Medicine study released Monday concludes that environmental factors have a much greater contribution to autism than originally thought, according to a co-author of the study.

“This is the largest ever twin study done to date on autism,” said Lisa Croen, Ph.D., co-author of the study and a senior research scientist and director of the Autism Research Program at Kaiser Permanente.

Dr. Croen published the "California Autism Twins Study" with principal investigator Dr. Joachim Hallmayer, M.D., a Stanford associate professor of psychiatry and behavioral science, specializing in child psychiatry.

The study, funded by the National Institutes of Health and Autism Speaks, focused on 192 pairs of identical and fraternal twins between 7 and 24 years of age. Twin pairs were found by linking California birth records to the Department of Developmental Services client files. 

In the study, participants received tests to establish a research diagnosis of autism. “We assessed them [study participants] very thoroughly,” Hallmayer said.

Researchers then used model fitting, a tactic used to estimate the degree to which a trait is detected by either environmental or genetic factors.

“The estimate of the environmental and genetic component is based on the concordance rates of the identical, the concordance rate of the non-identical twins and the population prevalence of the disorder,” Hallmayer said.

The study found that there was a higher rate of environmental factors that contributed to the participant’s autism than genetic factors.  “The non-shared environmental factors would place [the twins] apart,” she said, that is, one twin has autism and the other does not. T

Although the study did not outline specific environmental factors that contribute to autism, Croen did suggestethat the immune status of the mother, as well as different hormones and infections, are possible environmental factors.

Both Croen and Hallmayer agreed that the prenatal period of the child is a very important factor in the development of autism.  “We know autism starts very early,” Hallmayer said. 

Although the study concluded that environmental factors play a large role in autism development, neither Hallmayer nor Croen believe that their findings will renew concern over childhood vaccine use or mercury exposure.  “By and large most of the research doesn’t suggest that vaccines play a role,” Croen said. 

Hallmayer said that future studies, however, can be used to develop a greater understanding of which specific factors affect autism development.

Nonetheless, the conclusions of this study are bringing about a paradigm shift in the field of autism research, according to Croen.