This morning I read a new study which addressed the question of autism in siblings - how common is it?  The findings will be of vital interest to many; most especially young families with an autistic infant.

Earlier studies and "conventional wisdom" suggested the incidence of autism in siblings was in the 3-10% range.  This new study shows those numbers to be very far from the mark.

Overall, scientists found autism in 19 percent of the younger siblings.  However, the incidence is higher in families with two or mote autistic kids.  In that case, a new sibling's chances of being autistic rose to more than 32 percent. 

Being a boy makes a difference too.  "Only" 9% of girl siblings were autistic, as compared to 26% of boys.  I found this difference quite interesting because I often wonder if autism is under-diagnosed in females.  In this study, all the kids were screened with the gold-standard ADOS or ADIR tests prior to age three.  So even with top-notch screening, we still have more autistic boys.

Those are some strikingly high percentages.  As high as they are, and knowing autism is a spectrum condition, I have to wonder how many non-diagnosed siblings will eventually turn out to have less severe but still noticeable "differences."

There were a few more points I found interesting.  First of all, the IQ of the child did not predict anything.  Neither did severity of autism, as defined by the ADOS diagnostic scales.  So your odds of having a second autistic kid are higher, but those odds and knowledge of the first kid don't combine to give any insight into how a second kid might end up.

The conclusion is inescapable:  autism does run in families.  According to these findings, the more autistic kids you have, the more you are likely to keep having. 

We talk about autism having both genetic and environmental components.  This study, with 664 infants distributed all over the country, shows a very powerful genetic component.  That certainly does not diminish the role of environment, but it's sobering.

I predict the results of this study will have a profound impact on family planning, because it casts parents' chances of having a second or third child with autism in a strikingly different light that any previous study.

We already know (from other studies) that many parents stop having children when their child receives an ASD diagnosis.  This new finding may significantly reinforce that tendency.

Read the study yourself at this link

The study involved 664 infants from 12 U.S. and Canadian sites, evaluated as early as 6 months of age and followed until age 36 months.  Kids with previously identified autism-related genetic factors such as Fragile X were excluded from the study group

"It's important to recognize that these are estimates that are averaged across all of the families. So, for some families, the risk will be greater than 18.7 percent, and for other families it would be less than 18.7 percent," said Sally Ozonoff, Ph.D., professor of psychiatry and behavioral sciences at the MIND Institute and the study's lead author. "At the present time, unfortunately, we do not know how to estimate an individual family's actual risk."

This study was based on data from the Autism Speaks High Risk Baby Siblings Research Consortium (BSRC) and led by investigators from the UC Davis MIND Institute.

Your correspondent (John Elder Robison) is a member of the Science Board of Autism Speaks which supported the study.   

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