twin girlsWhat causes autism spectrum disorders (ASD)? Many studies have suggested that genetics are the primary factor in autism risk, while others have suggested environmental factors are to blame. Research from the Karolinska Institutet in Stockholm finds that genetic and and environmental factors are equal causes of autism, with each accounting for about 50% of the risk. Although some scientists criticize the research model, the study’s comprehensive results on the relative risk of autism among family members may help families make informed decisions.

The researchers analyzed the recurrence of autism in families. Using a sample of over two million people born in Sweden between 1982 and 2006, the researchers tallied the instances of autism (14,516 diagnoses) and counted the number of familial relationships. The cohort included over 2.6 million full-sibling pairs, nearly a million half-siblings (some who shared a mother and some a father), 5.7 million cousin pairs and over 375,500 twin pairs.

Autism risk for an individual increases for each family member with an ASD. The risk of autism increases:

  • tenfold for siblings of a person with autism.

  • 153-fold for an identical twin of a person with autism, but only eightfold for a fraternal twin.

  • Threefold for half-siblings of a person with autism.

The researchers conclude that genetic factors inherited from each parent account for 50% of autism risk and non-shared environmental factors (for example, a mother having the flu during pregnancy for one child, but not another) account for the other 50% of risk.

Some scientists criticized the study for using a model that is inappropriate for evaluating autism risk. Critics also claimed that this study does not account for the fact that genetic and environmental factors interact—nor did the study specify what counts as “environmental.”

Despite the criticisms, the findings do resemble the findings of many other studies that examined the risk for ASD between family members. This study’s significantly larger sample sizes solidifies the relationship between autism and family.

This research is published in The Journal of the American Medical Association.

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