Ekman, Friesen, and Scherer (1976) tested whether lying influenced one's voice quality. Participants were randomly assigned to view either a pleasant film or an unpleasant film, but all of the participants were asked to describe the film they saw as being pleasant. (Thus, the sub-jects who watched the unpleasant film had to lie about what they saw.) An analysis of voice quality showed that participants used significantly higher voices when they were describing the unpleasant film rather than the pleasant film. Why can't the authors conclude that lying produced the differences in voice quality?