generalizability at too great an expense relative to internal validity. In the end, there were many threats to internal validity in this study, and so it is impossible to conclude (as the authors did) from the published report that the “treatment” had actually caused the improvement in behavior that was noted.
Turning to a line of work that we regard as scientifically more successful, in a series of four randomized experiments, Baumeister, Bratslavsky, Muraven, and Tice (1998) tested three competing theories of “will power” (or, more technically, “self-regulation”)—the psychological characteristic that is posited to be related to persistence with difficult tasks such as studying or working on homework assignments. One hypothesis was that will power is a developed skill that would remain roughly constant across repeated trials. The second theory posited a self-control schema “that makes use of information about how to alter one’s own response” (p. 1254) so that once activated on one trial, it would be expected to increase will power on a second trial. The third theory, anticipated by Freud’s notion of the ego exerting energy to control the id and superego, posits that will power is a depletable resource—it requires the use of “psychic energy” so that performance from trial 1 to trial 2 would decrease if a great deal of will power was called for on trial 1. In one experiment, 67 introductory psychology students were randomly assigned to a condition in which either no food was present or both radishes and freshly baked chocolate chip cookies were present, and the participants were instructed either to eat two or three radishes (resisting the cookies) or two or three cookies (resisting the radishes). Immediately following this situation, all participants were asked to work on two puzzles that unbeknownst to them, were unsolvable, and their persistence (time) in working on the puzzles was measured. The experimental manipulation was checked for every individual participating by researchers observing their behavior through a one-way window. The researchers found that puzzle persistence was the same in the control and cookie conditions and about 2.5 times as long, on average, as in the radish condition, lending support to the psychic energy theory—arguably, resisting the temptation to eat the cookies evidently had depleted the reserve of self-control, leading to poor performance on the second task. Later experiments extended the findings supporting the energy theory to situations involving choice, maladaptive performance, and decision making.