Experts still debate the effectiveness of the lie detector
The earliest version of the polygraph was developed in 1921, when previously developed measures of respiration, heart rate, and blood pressure, which individually showed promise as measures of deception, were combined. Technological developments continued, and the modern polygraph is now an integrated, modern computerized system that continuously monitors blood pressure, heart rate, respiration, and perspiration.
The theory behind the polygraph is that when people lie, they experience a different emotional state than when they tell the truth. Specifically, it is believed that during deception, especially in high-stakes scenarios such as police interrogations, subjects become anxious or fearful of being caught lying. When guilty subjects are asked questions that reveal their guilt (e.g., 'Where were you last Tuesday?') and they lie, the fear of detection causes increased activation of their sympathetic nervous system. This activation leads to increases in heart rate, blood pressure, respiration, and perspiration.
These changes are part of the fight-or-flight system that is triggered by the body whenever a person is frightened. You have probably felt your heart pound or your palms sweat when faced with danger, whether it be a vicious dog, an angry boss, or an upcoming exam.
The polygraph is designed to detect these subtle changes in a person's physiological response when they lie. The general idea is that when a person is honest, their physiological responses remain stable during questioning, while the guilty person's heart beats faster. Anyone interested can learn all the intricacies of how the polygraph works today.
One of the most common procedures on the polygraph is called the comparison question test (also called the control question test). The subject is asked relatively mild questions, such as 'Where do you live?' They are also asked questions unrelated to the crime but likely to elicit an emotional response. For example: 'Have you ever told a lie?' Then they are asked questions about the alleged crime, such as 'Did you steal the documents?'
The premise of the comparison question test is that the guilty person will have a much stronger physiological response to the crime question, while the innocent person will not.
The polygraph is often used in criminal investigations, although it is generally not admissible as evidence in court. It is also used as a screening tool for pre-employment and continued employment for many employees in sensitive positions. However, today private businesses cannot force their employees to undergo a polygraph test.