"Modern social psychology is the study of undergraduate students half-assing things at universities that have social psychology departments" is such a thing that it's practically proverbial. And so obviously bad science, and yet people still do it! All the time! (It's cheap, everybody does it, it gets you results that *sound* interesting, and you gotta publish....) (Child psychology for a long time - in pretty much all the foundational studies that show up in undergrad textbooks - was "the study of children of university employees", although I think that one has improved slightly more over time.)

Every so often I stumble on a psych study that actually makes an effort to recruit from different populations and then compare the results, and it's hard to even read the rest of the paper through the sparkles of joy.