Do you think that in the past depressives didn't get any treatment that's as good as placebo because they were told that their depression doesn't matter?
Yes.
But of course the framing wasn't "your depression doesn't matter" - the concept of "depression" wouldn't even come up. Even doctors would frequently not think to check for it. If it manifested psychosomatically, you'd get painkillers. If it manifested as difficulty sleeping, you'd get sleeping pills. And of course there was always self-medication with alcohol. All of these are worse than placebo in the long term. You could get a depression diagnosis, especially if you made a suicide attempt, but it was much rarer, and not a desirabl...
It's great to make people more aware of bad mental habits and encourage better ones, as many people have done on LessWrong. The way we deal with weak thinking is, however, like how people dealt with depression before the development of effective anti-depressants:
The only "anti-stupidity drugs" we have are nootropics. But the nootropics we have weren't developed as nootropics. Piracetam was, I think, developed to treat seizures. L-DOPA was developed to treat Parkinson's. No one knows who started using ginkgo biloba or what they used it for; it was used to treat asthma 5000 years ago. Adderall derives from drugs used to keep soldiers awake in World War 2.
And none of them are very good against stupidity. AFAIK, to date, not one drug has been developed by understanding and targeting the causes of different types of stupidity. We have the tools to do this--we could, for instance, sequence a lot of peoples' DNA, give them all IQ tests, and do a genome-wide association study, as a start.
We don't research these things because society doesn't want to research them. People don't conceive of stupidity as a disease that can be cured. We need, somehow, to promote thinking of stupidity as a mental illness. As something drug companies could make billions of dollars off of.