EXPLAINER: What is a lie detector test?

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Dismissed police officer Rodulfo Gracioso Jr. recently challenged Vice President Sara Duterte and former House Speaker Lord Allan Velasco to undergo a lie detector test at the National Bureau of Investigation (NBI).

Gracioso, a former security aide to Velasco, claimed he delivered suitcases containing over P2 billion in cash to VP Sara Duterte, former President Rodrigo Duterte, and Rep. Paolo Duterte on Velasco's instructions.

NBI director Melvin Matibag said Gracioso scored a 99% rating for truthfulness during a lie detector test administered by the bureau.

But what is a lie detector test?

A lie detector test, technically known as a polygraph test, is an instrument designed to record a person's physiological responses while answering a series of questions, according to Britannica.

It measures involuntary physical changes that may occur while a person responds to questions.

During a polygraph examination, sensors are attached to the person to monitor several physiological responses.

A pneumograph tube is placed around the chest to record changes in breathing. A blood pressure-pulse cuff is also attached to the arm to monitor blood pressure and pulse.

Polygraph records these physiological changes, traditionally through moving pens on a chart. In modern systems, these changes are recorded through computer-based recording.

The results are examined by a trained polygraph examiner, who analyzes changes in the person's physiological responses while the questions are being answered.

They use these patterns to assess whether the responses are consistent with possible deception.

However, a polygraph does not directly measure whether a person is telling the truth or lying. It measures physiological reactions that can have different causes.

The Supreme Court in People v. Adoviso held that lie detector test results are not admissible as evidence in court to prove the guilt or innocence of an accused in criminal cases.

This is due to lack of scientific acceptance.

“However, polygraph tests when offered in evidence for the purpose of establishing the guilt or innocence of one accused of a crime, whether the accused or the prosecution seeks its introduction, for the reason that polygraph has not as yet attained scientific acceptance as a reliable and accurate means of ascertaining truth or deception,” the SC said.

For the SC, lie detector test results are not conclusive.

“While the Court held that the results of a polygraph test cannot be offered in evidence to prove the guilt or innocence of an accused in a crime, it does not mean that it has no weight at all,” it said.

“The results of the polygraph test may be used in conjunction with other corroborative evidence to prove an allegation made by a party,” it added. —VAL, GMA News

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