Risk of autism: how much is inherited and how much is from environmental causes?
Autism was first reported in the medical literature 70 years ago. In 1943 a child psychologist named Leo Kanner described a child with social difficulties and repetitive, stereotypic movements; the following year Hans Asperger described four such children. Since then we have gradually learned more about what we now call autism spectrum disorders (ASD), although we still do not know what causes it. From early on it was apparent that the risk for developing autism was higher among children who had other family members with the disorder. But how much higher? Twice as high? Ten times as high? Recently a very good study from Sweden has given us solid information about genetic risk for ASD. The study’s title is “The Familial Risk of Autism“.
Sweden has a very centralized and complete health care and medical record system. This allows tracking of relatives beyond the immediate family — cousins and grandparents — as well as siblings and half-siblings. One thing to note, however, is that Swedish society is much more homogeneous than ours, something that we should keep in mind when we interpret the results. The researchers also made no attempt to understand what environmental factors could be playing a role in ASD. The power of the study is that the authors asked a simple question: What is the risk for an individual child for having ASD and how is that risk affected by having other family members with the disorder?
The authors studied just over two million Swedish children born between 1982 and 2006. That is a huge study group, much higher than earlier ASD studies. What they measured was the relative recurrence risk for autism. This is the risk for the disorder among children with family members who have it compared to the risk for children with no family members with ASD. If the risk is the same for both groups, there is no genetic component. The extent to which the risk is greater for children who have affected family members is the contribution of genetic factors. This risk should get higher the closer the relationship — a sibling should cause a higher risk than, say, a first cousin. There is an important caveat to keep in mind here. Close family members often share the same environment as well as the same genes. A standard way to get around this problem is to study closely related individuals, especially identical twins, raised in separate environments. The authors weren’t able to do that, but it is still a very powerful and compelling study simply because of its huge size.
So what did they find? The risk for ASD in a given child was 10-fold higher if there was an affected full sibling, 3-fold higher if there was an affected half-sibling, and 2-fold higher if there was an affected first cousin. Plugging these numbers into a series of calculations, the authors determined that, overall, the contribution of genetic factors to ASD was 50%; that of environmental factors was 50%. So, half and half.
Autism is a complicated disorder. Like other neurological problems with both genetic and environmental components, such as schizophrenia, it is most likely a complex interplay of environmental influences on a genetically susceptible brain. There is an enormous amount of ongoing research to identify what is happening and what we can do about it, and I fully expect useful answers and treatments within the next few years. Studies such as this one are crucial to figuring it out.