Sociopsychological effect whereby a subject will make more candid and sometimes more incriminating revelations about himself when he believes a device attached to him will reveal his true knowledge or attitudes. The BPL was first reported by Jones and Sigall, who convinced subjects that the electromyograph used in their study could measure internal feelings and found that their subjects were more willing to disclose socially-undesirable attitudes. Some critics of polygraphy contend that PDD is merely an elaborate BPL that has only the power to elicit confessions but not assess truthfulness or deception. See: Jones & Sigall (1971).