According to the CDC, autism prevalence more than doubled from 2000 to 2010 as rates went from 1 in 150 children in 2000 to 1 in 68 children in 2010. These data have led scientists to frantically search for the cause of this dramatic increase in autism. Nothing is safe from accusation. Vaccines, pesticides, organic food consumption, c-sections, maternal age, pollution, etc. have all been implicated by fearmongerers, correlative data dredgers or both. However, what if the answer was something more simple: doctors are using this diagnosis more?
This is not a new question: many people have suggested that the rise in autism is largely a factor of over diagnosis. In fact I spoke about this topic in May when two studies came out that presented data that suggested that autism phenotype (as opposed to an autism diagnosis) was not increasing in prevalence despite the increase in diagnosis. These two studies showed that reporting practices changed but prevalence of the actual signs and symptoms of autism have not. But exactly what practices are changing has not been thoroughly investigated. Are parents more likely to push for the diagnosis to get their child extra attention? Is misdiagnosis rampant? Are school boards seeking increased funding for special ed partly to blame?
A new study from researchers at Penn State that was published in The American Journal of Medical Genetics may have an answer to this question. These researchers found that the number of children in special education classes has remained steady over the 11 year period from 2000-2010. This reinforces the data from the aforementioned studies that autism phenotype has not increased in prevalence. They also found that children diagnosed with intellectual disability has been on the decline during this period while autism has increased. Furthermore, this reported decline in children diagnosed with intellectual disability almost completely offsets the increase in children diagnosed with autism. In other words, over the period from 2000-2010 autism increased because physicians transitioned away from using intellectual disability as a diagnosis, preferring to use autism instead. Incidence is not changing, the label is.
Data from study showing concurrent decline in intellectual disability diagnosis and increase in autism diagnosis. Photo credit: Penn State University
As I said back in May, the real problem with chasing down every suspected autism lead is that it diverts resources away from legitimate science. In studies of identical twins, the concordance for autism is as high as 95 percent, and parents who have one child with autism have as high as an 18 percent chance of having a second child with autism. These data point to autism having a very strong genetic component and therefore resources need to be focused at identifying these factors.