After telling people that I am a researcher studying autism, I am often asked the above question. Unfortunately, there is no straight answer for this question. If one goes back in the literature, it is true that for decades (several studies conducted from 1966-late 1990's) the prevalence estimates were on the order of 4-5:10,000 while studies conducted over the past decade have estimated the prevalence on the order of 2-6:1,000 with one study showing as high as 12:1,000. So, why this difference? Is it a real increase in the prevalence of the disorder or something else?