You'd need the participants not to know they are part of a study. They'd need to be in a situation where they believe that the statement of their true beliefs

You'd need the participants not to know they are part of a study. They'd need to be in a situation where they believe that the statement of their true beliefs is critical. This isn't impossible (many studies do use primary sources which were not created for the purposes of a study, and where there was some incentive based on real-world circumstances for them to be truthful), but it is much more difficult and leaves gaps where insufficient primary sources materials addressing the questions of the study are available. Psychologists are well aware that the current model for many psychology studies, which consists of either asking an extremely small sample of participants to sit in a computer lab at a university for 20 minutes answering questions, or sending out very loosely-targeted emails soliciting participation, are both very inefficient. But they gotta get published, cause that's how you put food on the table.