Exactly. It was virtually unknown. I do remember a few families where there was a child with Down’s Syndrome. There is a book about autism that was written in the 1950s that effectively disproves the genetic theory. I recommend it, a fascinating book: “Pathways to Madness”, by Jules Henry.
The book did not disprove genetic theory. It explored social interaction and behavioral conditioning and its effects on mental illness and published in the mid 60’s.