Environment Trumps Genetics in Autism

According to a new study by UCSF and Stanford, environmental factors play a much more important role in causing autism than previously thought, and have an even greater influence than genetics.

According to the San Francisco Chronicle, the study was published in Monday's issue of the Archives of General Psychiatry, and examined 192 pairs of twins in California. The researchers found that genetics accounted for about 38 percent of the risk of autism, and environmental factors accounted for about 62 percent. This runs counter to previous studies which indicated that autism was 90% inheritable.

The Chronicle notes that for much of the 20th century autism was linked mostly to environmental factors, with the blame falling on mothers for things like “poor parenting.” With with autism rates skyrocketing in the '80s and '90s (nearly 1% of all U.S. children are now thought to have Autism Spectrum Disorder—ASD), scientists and patient advocates have stopped blaming families. This is good because blaming parents who may already be struggling with guilt or stress from all the extra medical examinations and treatments is just plain cruel and unlikely to have any effect on reducing ASD.

The study did not indicate what the environmental factors might be, which makes it difficult for parents to do anything proactive to reduce the risk of autism. However, there is one factor that is known to increase autism risk: age of parents. During roughly the same time that autism rates were skyrocketing, so was the average age of parenting. Between 1995 and 2007, there was a 70% increase children being born to mothers over the age of 40 and a 50% increase for mothers between 35 and 39. This was not due to an overall increase in birth rates, either. Women under the age of 30 had a declining birthrate in that time period. What was happening was that women were choosing to delay having families longer, while they completed college and developed their careers.

It should be pointed out that, contrary to common belief, just because twins grow up in the same family does not mean they are exposed to exactly the same environmental influences. Identical twins share 100% of their genes; therefore, if ASD was purely genetic, there should be no difference between two identical twins: either they both have it or neither has it. The environmental factor could be something as ephemeral as a momentary post-natal exposure to an environmental chemical or a slightly elevated exposure to a hormone in utero.