The so-called lie detector , truth machine or polygraph is a particular type of measuring instrument used to record physiological responses. It generally detects variations in blood pressure, heart rate, respiratory rate, nerve stimuli, and galvanic response or skin conductance, which are generated by certain questions that are asked of the test subject. In Spain, the polygraph is not admitted as evidence of the charge that serves exclusively to support a conviction in the criminal sphere, although there is also no rule that prevents someone from voluntarily submitting to the truth machine. In this case, the result of the test can focus or direct the investigations in one direction or another, and serve to discard clues or to reinforce suspicions . The lie detector test would be framed within the so-called expert tests, which must be carried out by experts under appropriate conditions and which can be contradicted or reinforced by other tests, of the same kind or of another nature.
In any case, whoever has to submit to the machine must give their consent, as they have the right not to declare . On the other hand, if even sometimes a confession is considered insufficient to be considered as condemnatory, if it is not supported by other elements, details or evidence, insincerity in front of the machine as the only data to condemn does not fit in our justice system penal.
The polygraph has its widest use in intelligence and police agencies, especially in the United States and other countries in its area of influence, where it has been used to determine whether a person is lying or not. In this case, it is supported by the hypothesis that a person’s physiological responses change measurably when they lie. In the North American country, it has been used as evidence of defense in favor of the suspect. In Europe it is used rather in the allocation of some jobs by private companies and some security agencies. However, most psychiatrists, psychologists, neurologists, neurophysiologists, and other scientists agree that there is little basis for accepting the validity of polygraph tests.
