The Legitimacy of Using Lie Detectors in the Criminal Truth-Seeking Process with Emphasis on Privacy and the Principle of Fair Trial

Abstract

Technological advancement in the field of criminal sciences has rendered the use of modern instruments in the process of criminal truth discovery inevitable. The lie detector, or polygraph, as one of these instruments, seeks to identify honesty or deception in individuals’ responses to targeted questions through the recording and analysis of physiological reactions such as blood pressure, heart rate, respiratory rhythm, and galvanic skin response. The application of this technology in criminal proceedings has generated a serious tension between the necessity of discovering the truth on the one hand, and the protection of the accused’s fundamental rights—particularly privacy and the requirements of a fair trial—on the other. The principal question of this study is whether the use of polygraph examinations in criminal proceedings is legally legitimate, or whether privacy, the right to silence, and the privilege against self-incrimination constitute serious barriers to the use of this instrument. Using a descriptive-analytical method and a comparative approach, this study demonstrates that the legitimacy of employing polygraph examinations is neither absolute nor unconditional, but rather conditional and relative. The use of such an instrument is compatible with the principles of fair trial only when free and informed consent is obtained, the scope of its application is restricted, and its results are considered merely as corroborative indications alongside other forms of evidence. The existing legislative gap within the Iranian legal system highlights the necessity of enacting explicit regulations in this field.