To the best of my knowledge (and I've looked) there is not a single scientific long-term randomized study showing the effectiveness of any type of treatment for autism.

Why isn't there? There would seem to have been more than enough time & funding for at least one. Is there some more subtle problem here?

(I'm thinking a scenario like "parents of autistic kids are constantly trying new approaches both quack and genuine, and would refuse to stop this, thereby making the results worthless; and this is foreseeable in advance by any would-be experimenters.")