When was the subject of autism first dealt with in speculative fiction? Some clarification is needed as to the meaning of autism. The word itself dates to 1910, when psychiatrist Eugen Bleuler coined it, but the word was used for “morbid self-admiration” until Hans Asperger redefined it in its modern sense in 1938. The correct answer, then, would be a work that explored autism in its modern sense in science fiction or fantasy, whether or not the author used the term autism, but not one that merely used the word autism in Bleuler’s sense of “morbid self-admiration.” The Wikipedia definition of autism is “a developmental disorder characterized by difficulties with social interaction and communication, and by restricted and repetitive behavior.” (I am aware that there are some problems with this definition, but as a nonspecialist I will have to let it stand, as my main point is to avoid speculative fiction that uses the word in Bleuler’s sense, which is even farther off the mark.)
Autism has been gaining increasing attention in SF&F during recent years. Nicholas Whyte provides a list of SF&F stories that include autism as a plot element. In the ISFDB, only five stories had the tag “autism” when I searched for it on Feb. 17, 2020. At present, the oldest is “The Lost Language” by David H. Keller (1934) and it is available online. It does not use the word autism, but describes behavior that the tagger considered to be autistic. In it, a boy creates his own written language while rejecting all attempts to have him learn his parents’ tongue.
There was nothing wrong with the boy’s body. But he would not talk. That is the way they put it. He would not talk.
There did not seem to be much mental deficiency. He learned to take care of himself, to adjust himself to his environment, to dress, feed, and amuse himself. He was really a bright, adorable, loving child. Accepting life as he found it, he lived in the home and with his family without in any way being a burden. At five years he was a little man, but he did not talk.
Does this constitute autism? I doubt it, especially as the touted explanation ends up as “inherited memory” of a lost language, but evidently the tagger thought so. Presumably there are other stories that have a more definite description of autism.
Almost as old is a 1947 story by “Lewis Padgett” (pseudonym for Henry Kuttner and C. L. Moore), “Margin for Error”, which does use the word autistically, but in the Bleulerian sense of being self-centered, so this example must be rejected.
“Humans think autistically; they will always be convinced you want to rule their world. Their egotism will never let them admit the truth.”
The best answer will be one that uses quotations from the story as evidence that autism in its modern sense is described. However, the story need not contain the word autism if the description is convincing.