Psychology is a field notorious for being derided against as a soft science particularly because of how little data exists with poor sample sizes and ideology / purposeful tampering coming into the way like you've said.

Any meaningful evidence for a conclusion requires a much better understanding of the brain both biologically and also as a function of general cognition. This will require many more breakthroughs within the fields of biology and computer science. We're definitely getting closer and closer everyday on that front, but as of right now, the tools that we have access to are far too crude in my honest opinion.

Psychology in that respect is akin to the alchemy that was a precursor to chemistry. I'm not saying no real science was done by the alchemists of course, just that it was far and away from what the field of chemistry would ultimately become.