Are researchers permitted to mislead subjects about the purpose of a trial? 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.