The expression psychological set was introduced in PDD by Cleve Backster who initially attributed it to a psychological writer Floyd L. Ruch (Matte & Grove, 2001) but later claimed to have made up the expression himself (Senter, Weatherman, Krapohl, & Horvath (2010). Backster has made the concept central to his Zone Comparison Technique and has tethered the concept to the emotion of fear. According to Backster’s PDD hypothesis, examinees are expected to attend more to the category of question that presents the greater threat to their interests, either the relevant or comparison questions. Subjects who are lying to the relevant issues consider these questions more threatening than the others, which, in turn, draw more attention to the relevant questions, and more physiological arousal. Similarly, innocent subjects find the probable-lie comparison questions more disconcerting, and the greater attention paid to them generates the larger arousals. The expression psychological set, together with its underlying assumptions, have long been questioned by scientists on both sides of the polygraph debate. Competing concepts include “Differential Salience (Senter, Weatherman, Krapohl & Horvath, 2010) and “Relevant Issue Gravity” (Ginton, 2009.) See: Krapohl (2001); Matte & Grove (2001).