Folktales from Europe describe children with autism, suggesting autism is not a new phenomena, according to this report in the Archives of Diseases of Childhood:
Fairytales from the British Isles, Germany, and Scandinavia include stories about changelings. They describe a child who exhibits remarkable and sudden changes in behaviour and/or appearance, explaining that supernatural folk steal normal children and replace them with one of their own, or some other substitute. The new child—the changeling—is characterised by unresponsiveness, resistance to physical affection, obstreperousness, inability to expressemotion, and unexplained crying and physical changes suchas rigidity and deformity. Some are unable to speak.Of course this describes autism pretty accurately. So it doesn't seem that autism is brought on by some aspect of modern society, but represent underlying biological predisposition.