THE POLYGRAPH FOR LIE DETECTION is a ''placebo'' technology whose ability to sort truth from falsehood depends essentially on the subject's belief in the efficacy of the instrument (as opposed to any ''real'' efficacy). In this sense, the lie detector is a technology that belongs to the realm of the social imaginary. As such, it offers an ideal probe into the shifting grounds of trust and mistrust in twentieth-century America. Originally designed to rein in corrupt municipal police forces—by substituting scientific interrogation for brute force beatings—the lie detector by midcentury was being used two million times each year to interrogate criminal suspects, corporate employees, and the denizens of the national-security state. Yet all the while polygraph evidence was formally banned from the courtroom. This article demonstrates that the historical process that led to this curious state of affairs was the outcome of two interdependent strategies for marketing and validating expert knowledge. Polygraph inventors and operators thrived by passing themselves off as the purveyors of an objective method of ferreting out the truth, transforming their subjects' bodies into a piece of counterfeit-proof circumstantial evidence. This appealed to the American public's preference for mechanical rules as the most certain guarantee of just and equal treatment. In fact, the inventors and operators succeeded in attracting clients among police, prosecutors, corporate managers, and the chieftains of the national-security state largely by virtue of the vast discretion that undergirded their methods of interrogation, and the confessions they could wring in the process. These methods (logically akin to judicial torture) document the degree to which American justice actually operates according to informal rules that favor the already powerful. This dual history illuminates the shifting grounds of the laity's trust in expert science, the blurred line between the rule of law and the informal procedures of American justice, and the evolving demands of loyalty within the institutions of corporate capitalism and the national-security state. Given the renewed attention to national security in the past few years—and the renewed assurances that the lie detector will help uncover spies, traitors, and terrorists—this history has new urgency.