Here’s an example. A professor was asked to send weekly e-mail messages to students who had done poorly on their first exam for the class. These students were divided into three groups. The mail to one group of students included only a review question. A second group received the review question plus some advice–along the lines of “study harder.” The third group received (in addition to the review question) a classic message for self-esteembuilding–“You’re too smart to get a D,” or words to that effect.
A blog entry from 2003, that I came across recently via Comrade Snowball.