The prevalence of ASD seems to have increased significantly over the last thirty years, and scientists (and others, unfortunately) have sought some explanation, the absence of which has opened the field for speculation of the vilest sort. Everything from mercury in vaccines, the vaccines themselves, to pesticides and other chemicals have been blamed none of which have been supported by scientific research. Last year, a study done by researchers in Denmark provided some explanation for at least 60 percent of the increase in prevalence. In 1994, changes were made to the diagnostic criteria which defined autism as a range of disorders, allowing milder cases to be defined along the spectrum. In 1995, the national health registries of Denmark included diagnoses made outside of hospitals.
Although it's not clear how this relates to prevalence in the United States, according to Dr. Glen Elliott, chief psychiatrist and medical director of Children s Health Council in Palo Alto, California, there have been significant changes in the way autism is defined in the United States. Just last year the definition was updated in the Diagnostic and Statistical Manual of Mental Disorders Fifth Edition (DSM-5).
The idea that the increase in autism rates is due to changes in diagnostic criteria is often ignored by those who subscribe to pseudoscience, says science journalist Tara Haelle in her article in Forbes. However, they do point to the fact that some of the increase in prevalence is still unexplained 40 percent according to the Denmark study.
So, asks Haelle, if there is a real increase again, it s not clear do we have any idea what might account for it? She responds, Well, we know for darn sure it isn t vaccines. (Seriously, there IS NO LINK.) It s possible a small amount of the increase may relate to the increase in the average age of parents. Both women and men are becoming parents later now than at any other time in history, and autism spectrum disorders appear linked to older parents, though the data on older fathers is more solid than that on older mothers. And then there is a long, long, ridiculous list of other things linked to autism (car exhaust? labor induction? really?) that have little to no solid evidence.
The take-away message according to Dr. Elliot is this: At least a significant amount of the perceived increase of autism spectrum disorders is due to changes in criteria we use to make the diagnosis and the increased awareness about autism spectrum disorders that has led both to earlier diagnoses and greater likelihood of detection.