Perceived informativeness of facts.

There are many tasks in which people are called on to disregard information that they have already processed. Dealing with inadmissible evidence in a courtroom setting, second-guessing the past, and responding to experimental psychologists' debriefing instructions are three tasks of this type; in all these cases, people have been found to experience considerable difficulty. The present experiment investigates these difficulties in a general form, using almanactype questions. Subjects told the correct answers to such questions were found to overestimate both how much they would have known about the answer had they not been told and how much they actually did know about the answer before being told. Attempts to undo this knew-it-all-along effect by exhorting subjects to work harder or telling them about the bias failed. These results were discussed in terms of how the structure of one's knowledge is altered to accommodate new information.