Editorial note: The conclusion reached in these these stories is contentious/disputed. Autism is not "swallowing up" other diagnoses as is suggested. Other information is available elsewhere, for example here, here and here.
The way autism is diagnosed has led to an apparent tripling in cases in recent years that does not reflect reality, researchers say.
Rather, more youths with intellectual or developmental disabilities are being reclassified as autistic, the United States-based study published in American Journal of Medical Genetics found.
University of Western Australia Telethon Kids Institute's Andrew Whitehouse said the trend was "almost certainly the same in Australia".
The prevalence of autism in the US was just one in 5,000 in 1975.
It leapt to one in 150 in 2002, and reached one in 68 in 2012, according to the US Centres for Disease Control and Prevention.
"This new research provides the first direct evidence that much of the increase may be attributable merely to a reclassification of individuals with related neurological disorders rather than to an actual increase in the rate of new cases of autism," the study authors wrote.