Imagine how arrogant it is to assume people fifty years ago were too stupid to recognize autism. I have nine cousins. My Dad's family had two kids, my Mom's family had three. So I have three aunts and three uncles. That is three more families that we were directly connected to. Between the four families we had over 100 close relatives. Family reunions could easily be 50 to a hundred people. Weddings, funerals, all the same. If we had autistic people we would have known. Anyone who has an autistic brother or sister, that makes a huge impact on your life.
I was in the boy scouts. I was in two different patrols of eight kids each. Each week you go to the home of one of the kids, that is sixteen homes, about three kids each, so that is 48 kids. I was also a patrol leader for three different patrols. Again, that is about 72 kids in 24 homes. Between my family, my family reunions, weddings, funerals and the boyscouts I saw at least three hundred kids. No one was autistic. I did see the occasional down syndrome kid, but no one that was autistic. Rarely you would see some kid in a wheelchair, or who was blind or deaf, but never someone who was autistic.
I also went to church, probably five hundred in the congregation. If someone had an autistic kid I would have seen.