To most experts in autism and autism epidemiology, the biggest factors accounting for the boost in autism prevalence are the shifting definitions and increased awareness about the disorder. Several decades after the introduction of autism as a diagnosis, researchers have reported that professionals are still engaging in “diagnostic substitution”: moving people from one diagnostic category, such as “mental retardation” or “language impairment,” to the autism category. For instance, in one recent study, researchers at UCLA re-examined a population of 489 children who’d been living in Utah in the 1980s. Their first results, reported in 1990, identified 108 kids in the study population who received a classification of “challenged” (what we consider today to be “intellectually disabled”) but who were not diagnosed as autistic. When the investigators went back and applied today’s autism diagnostic criteria to the same 108 children, they found that 64 of them would have received an autism diagnosis today, along with their diagnosis of intellectual disability.
Further evidence of this shift comes from developmental neuropsychologist Dorothy Bishop and colleagues, who completed a study involving re-evaluation of adults who’d been identified in childhood as having a developmental language disorder rather than autism. Using two diagnostic tools to evaluate them today, Bishops’ group found that a fifth of these adults met the criteria for an autism spectrum diagnosis when they previously had not been recognized as autistic.
Another strong argument against the specter of an emergent autism epidemic is that prevalence of the disorder is notably similar from country to country and between generations. A 2011 UK study of a large adult population found a consistent prevalence of 1% among adults, “similar to that found in (UK) children” and about where the rates are now among US children. In other words, they found as many adults as there were children walking around with autism, suggesting stable rates across generations—at least, when people bother to look at adults. And back in 1996, Lorna Wing (the autism expert who’d translated Asperger’s seminal paper) tentatively estimated an autism spectrum disorder prevalence of 0.91% [PDF] based on studies of children born between 1956 and 1983, close to the 1% that keeps popping up in studies today.
It appears that quite a few people that were warehoused in insane asylums in previous centuries would now be diagnosed more accurately. Read a lot more about the rates of autism at The Crux. Link
(Image credit: Flickr user dierk schaefer)