Police investigating the death of actress Nida “Tangmo” Patcharaveerapong have resorted to using a lie detector after witness accounts became suspiciously murky.

Polygraph testing was deemed necessary following conflicting accounts given by the five people who were with her on the speedboat when Tangmo fell into the Chao Phraya River on the night of February 24.

The question investigators are seeking to answer is whether her death was an accident or the result of foul play.

In a bid to find out who told the truth and who lied to them, they have turned to polygraph testing.

Polygraphs are a well-established but controversial investigative technique. Thai police have used them in several high-profile cases where suspects and witnesses have offered conflicting testimony.

Many researchers and experts insist that polygraph tests are still far from perfect as tools to determine if someone is lying. They also criticize the term “lie detector” to describe polygraphs as a complete misnomer.

Question of reliability

Courts in many countries – including the Thai Supreme Court – have repeatedly rejected the use of polygraph evidence because of its inherent unreliability.

The subject of a polygraph test answers questions while attached to sensors that record their pulse, breathing rate, blood pressure, perspiration, limb movement, and sometimes also pupil dilation. Typically, the tester will ask a mix of “relevant” questions and “control” (or comparison) questions.

The relevant questions relate to the alleged crime, such as “Did you shoot your wife?”. The control or comparison questions are neutral and might include something like, “Did you have breakfast this morning?”.

Meanwhile, the subject’s physiological reactions are recorded on the polygraph and then examined to see whether they changed significantly during answers given to any of the questions. In general, significant changes such as a faster heart rate, higher blood pressure, increased perspiration are taken as signs that the subject is not telling the truth.

Well-trained polygraph examiners claim they can detect lying with high accuracy. However, critics say polygraphs are ineffective because they cannot tell the difference between physiological changes caused by lying and changes caused by other forms of anxiety. In other words, innocent people can fail polygraph tests out of pure nervousness.

There are also cases in which criminals have fooled polygraph examiners by tricks such as taking medications that alter their arousal patterns.

In 2010, the Thai Supreme Court rejected a polygraph result as proof of guilt in a murder case, citing the subjective nature of the test.

“A lie detector is just a scientific tool that relies on the analysis of the defendant’s answers to determine if he or she told the truth or lied. That is just an academic comment and cannot be used to clearly prove that the defendant committed the offence. The lie detector and the comment from the polygraph expert are insufficient to prove that the defendant committed the murder,” the court ruled.

The case was dismissed over insufficient evidence.

What lies beneath

Relying on lie detector

Polygraph tests have also been conducted on suspects and witnesses in other high-profile Thai cases.

These include the 2020 death of a three-year-old girl in Mukdahan province in which her uncle was a suspect. The 44-year-old man, Chaipol Wipha, his wife, the girl’s parents, and other relatives were all subjected to polygraph tests. The saga finally ended a year later when police charged Chaipol with the girl’s murder.

In 2017, polygraphs were used to determine ownership of a winning set of lottery tickets with Bt30 million at stake. A retired policeman was finally declared the rightful winner of the prize, despite a teacher’s claim that he had bought the tickets at a market.

Two other high-profile cases have seen investigators resort to polygraph testing in an attempt to nail their suspects.

Those suspects were medicine student Serm Sakhonrat, who was found guilty in the 1998 murder of his girlfriend, and taxi driver Sompong Luedthahan, who was sentenced to three years in jail for fraud and public deception.

In 1997, the taxi driver had earned public praise and over Bt200,000 in cash rewards after claiming that he had returned Bt20 million left in his cab by a foreign passenger he was taking to the airport. Sompong was found to have lied.

By Thai PBS World’s General Desk