An old idea (2008) from the drafts folder that I’m posting now. A related post back in 2008.
Two people are interviewed by a single person. During the interview the interviewer tells the subject a private piece of information about a third person, called X. Three conditions: shares information without comment, tells subject not to share information, pays subject small amount ($10-$20) to not share information.
Then the subject is interviewed by another person, perhaps at a later date. The second interviewer asks this person what they know about X. Who among the three conditions will share the information divulged by the first interviewer?
Other variables to throw in - X is a public/non-public figure, information shared about X is trivial/non-trivial, X is in a position of authority/weakness.