Now Reading:

Can a lie detector lie?


Can a lie detector lie?

In partnership with

Many of us have wondered if we are being lied to or told the truth. In some situations knowing the difference between lies and the truth is really important – such as police investigations, for example. Various methods, including body analysis and psychology, have been tried with various levels of success but none of them are foolproof. One of the best known methods is a “polygraph”, a much-loved feature of American detective shows.

Youri Schillinger, an examiner investigator and polygraph technician explained: “We interrogate people in judicial proceedings. Our job is to identify truthful people and to confuse liars. In the past polygraphs used ink needles and paper, but now they are computerised.”

During the test, the person sits in a special chair and various body sensors are attached which will measure breathing, both in the abdomen and the chest. And two small silver plates measure sweat production on the person’s fingers. Another sensor measures the percentage of arterial hemoglobin in the oxyhemoglobin configuration, in other words, blood oxygenation. Another sensor measures blood pressure and cardiac rhythm throughout the test.

Schillinger said: “When someone tells a lie, they put themselves in a dangerous situation. That’s why adrenaline will be released into the blood. Adrenaline is a natural stimulant and when it is released, the heart will beat faster and there will also be more blood flowing round the body. The blood carries oxygen and oxygen is what our muscles burn, so it creates a lot of activity inside the body.”

But adrenaline is not only produced when someone lies, it’s also produced when people are under stress. So to help people relax, the interrogator talks for some time in order to put the person at ease. Decreasing stress increases the reliability of the test. The machine has been tested on Francois, an actor and friend of Schillinger.

“For François lying is second nature. I know him very well and I know he can make anyone believe anything,” Schillinger said. “So if anyone can fool a lie detector, it’s him.”

The challenge was that Francois was asked to take a numbered card completely at random and then say ‘no’ regardless of question. The examiner would then see if the polygraph could detect the false answer.

When the examiner asked about card number 5, the polygraph showed a different curve: it successfully detected the lie. Although he denied it, Francois did in fact pick card number 5.

The test is not 100% reliable, however. Experts estimate that it is only 94% reliable, which is why in most EU countries results from a polygraph are not admissible evidence in court. In most cases, polygraph tests are voluntarily undertaken by a defendant wanting to substantiate his or her defence. So it seems as if there is still no way to know for sure if someone is lying or not…

Next Article