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Large Study Suggests Genes & Environment Contribute Equally to Autism Risk

Largest-ever autism risk analysis provides further evidence that genetic and nongenetic factors both contribute powerfully to ASD
May 03, 2014

The largest-ever study of its kind suggests that the risk of autism is influenced equally by genetic and environmental factors. In scientific terms, environmental factors include a broad range of influences. In autism, these can be as varied as parental age, birth complications, maternal nutrition at conception and exposure to pollution during early brain development.

The new report appears in the Journal of the American Medical Association (JAMA).

In their study, researchers with the Karolinska Institute looked at the health records of all those born in Sweden between 1982 and 2006 - more than 2 million individuals. They identified 14,516 as having autism spectrum disorder (ASD).

Ten-fold risk if sibling affected
The study showed elevated risk of developing autism among children who had family members affected by the disorder, a finding consistent with earlier research. Among those with an affected sibling, the risk was ten-fold greater than in the general population.

The study was unique in extending this risk analysis to include extended family members as far out as cousins. Children with an affected cousin had double the autism rate of the general population.  

Equal contributions
By studying extended families, the researchers were better able to estimate the combined effects of inherited genes and environmental influences. Their analysis indicated that each contributed around half of the overall risk of autism in a population. What’s more, this 50/50 contribution remained consistent across the study’s 24-year span.

This should not be misconstrued to mean that half of autism cases are caused by genetic and half by environmental factors, comments Alycia Halladay, Autism Speaks senior director for environmental and clinical sciences. “It’s not an either/or scenario. A complex interaction of genetics and environmental factors, working together, underlie the development of ASD.”

The study’s large size is of particular importance in verifying the important role of both heredity and environment in autism, Dr. Halladay adds. 

It’s long been known that certain rare genetic mutations can cause autism by themselves. However, the majority of the gene changes associated with autism appear to only increase risk to varying degrees. It’s in these cases that environmental influences are thought to play a significant role in autism’s development.

Strong support for ground-breaking Autism Speaks study
The new report is the latest in a string of studies that have profoundly shifted understanding of autism’s causes. Just ten years ago, the disorder was thought to be almost entirely genetic in nature.

In particular, the new findings confirm and extend those of a game-changing 2011 study that analyzed autism rates among twins and other siblings participating in the Autism Speaks Autism Genetic Resource Exchange (AGRE). That study provided the first clear evidence that, in the presence of genetic predisposition, prenatal influences likely play a significant role in autism’s development.

Such understanding has greatly increased interest in identifying the environmental influences that increase autism risk and understanding how these influences interact with genetic predisposition.

Autism Speaks Environmental Factors in Autism Initiative
Autism Speaks continues to invest in such research through its Environmental Factors in Autism Initiative. Descriptions of related Autism Speaks studies can be found here. This and all research funded by Autism Speaks is made possible by the generosity and hard work of its large community of families, volunteers and donors. 



Explore all the research Autism Speaks is funding using this website’s grant search.

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