I think much research would not be very successful if the subjects knew the purpose of the research. When researchers gather human subjects for a trial, can they provide the subjects with misleading information about the purpose of the trial?
Standard 8: Research and Publication of the "Ethical Principles of Psychologists and Code of Conduct" of the American Psychological Association pertains. Specifically:
8.07 Deception in Research
(a) Psychologists do not conduct a study involving deception unless they have determined that the use of deceptive techniques is justified by the study's significant prospective scientific, educational or applied value and that effective nondeceptive alternative procedures are not feasible.
(b) Psychologists do not deceive prospective participants about research that is reasonably expected to cause physical pain or severe emotional distress.
(c) Psychologists explain any deception that is an integral feature of the design and conduct of an experiment to participants as early as is feasible, preferably at the conclusion of their participation, but no later than at the conclusion of the data collection, and permit participants to withdraw their data. (See also Standard 8.08, Debriefing.)
Whether or not a planned study satisfies these requirements is for the Institutional Review Board to decide, which must review every proposed study before it starts.
I recently participated as a volunteer subject in a psychological experiment, where it turned out that the experimenter had deceived me about the main purpose. At the end of the experiment, she told me about the deception, and offered to show me the paperwork from the ethical review board that had approved it. This was in the UK, and I presume that it was all in accordance with the standard UK rules. For me the experience was quite interesting, but it could have been upsetting for people with a particular (and uncommon) set of life experiences. However, it seemed clear that that kind of issue had been carefully considered by the review board.