Lie detection commonly involves the polygraph, and is used to test both styles of deception. It detects autonomic reactions, such as micro-expressions, breathing rate, skin conductivity, and heart rate.Forensic Psychology. BBC Science & Nature. Micro-expressions are the brief and incomplete nonverbal changes in expression while the rest show an activation of the nervous system. These changes in body functions are not easily controlled by the conscious mind. They also may consider respiration rate, blood pressure, capillary dilation, and muscular movement. While taking a polygraph test the subject wears a blood pressure device to measure blood pressure fluctuations. Respiration is measured by wearing pneumographs around the chest, and finally electrodes are placed on the subject's fingers to measure skin conductivity. To determine truth it is assumed the subject will show more signs of fear when answering the control questions, known to the examiner, compared with the relevant questions, where the answers are not known. Polygraphs focus more on the exams predictive value of guilt by comparing the responses of the participant to control questions, irrelevant questions, and relevant questions to gauge arousal, which is then interpreted as a display of fear and deception is assumed. If a person is showing a deception there will be changes in the autonomic arousal responses to the relevant questions. Results are considered inconclusive if there is no fluctuation in any of the questions.
