The lie detector or polygraph was developed in the US in 1921 by a police officer and a medical student. The device measures the small, unconscious stress reactions that would indicate that you are lying.
During the interrogation, the interrogator asks a suspect questions. He already knows the answer to some of them, such as the name and social status of the suspect, so that he can see how he responds when he speaks the truth.
In other cases, the interrogator does not know the answers, but will, for example, ask what the suspect did at the crime scene.
If he then responds in the same way as before, he probably tells the truth. But if he exhibits very different behavior - for example, if his wrist rises sharply - it is an indication that the suspect is telling a lie.
Lie detector sometimes lies
The use of a lie detector is controversial because the body reactions can have a cause other than lying. For example, an innocent may feel threatened when asked at the man: "Have you shot your girlfriend?" Then the lie detector turns out, even if, according to the truth, the suspect says "No!" says.
That is why the interrogator often asks more neutral questions to find out if the suspect knew about the crime, such as: "Was the victim shot with a 9mm gun?" But the test is not very reliable.
In the US, police, security and intelligence services often use the lie detector during investigations, but few states allow it to be used during the trial itself. The detector may not be used by the government in Europe.
If you tell a lie, your body is in sharp focus, causing various involuntary responses. By measuring the reactions of a suspect during an interrogation, the police can get an impression of whether they are lying or telling the truth.
Your body doesn't lie
The pulse and blood pressure rise when you lie.
Someone who is lying breathes faster and more superficially.
Your palms and fingertips are sweaty when you lie knowingly.
When you lie, your arms and legs start to shake slightly due to small, involuntary muscle movements.
Brain scanners become lie detectors
Various scientific studies show that the activity in the frontal lobe and the parietal lobe changes when someone consciously tells a lie.
By measuring brain activity during an interrogation you can use an fMRI brain scanner to find out whether the suspect is lying or not.