Merci à Étienne qui m'a envoyé un lien vers cette étude:
Prior studies generally support the contention that autism rates are rising due to changes in diagnosis and surveillance. (...) For example, Taylor, after reviewing the evidence, wrote in 2006: "The recorded prevalence of autism has increased considerably in recent years. This reflects greater recognition, with changes in diagnostic practice associated with more trained diagnosticians; broadening of diagnostic criteria to include a spectrum of disorder; a greater willingness by parents and educationalists to accept the label (in part because of entitlement to services); and better recording systems, among other factors."
Also, in an excellent review of this question, Gernbacher, Dawson, and Goldsmith pointed out that the DSM-III, the diagnostic manual for mental disorders published in 1980, required six specific criteria for the diagnosis of autism, while the 1994 DSM-IV had 16 less-specific criteria, out of which any 8 would qualify for the diagnosis.
(...) I should also mention that at present the majority opinion is that the epidemiological evidence does not demonstrate a true rise in autism rates, but a small true rise could be hiding in the data and has not been ruled out.