SciTech

How Things Work: Lie Detectors

Credit: Molly Swartz/Staff Artist Credit: Molly Swartz/Staff Artist

One often hears of lie detectors being used to determine whether a suspect is providing an accurate account. The most people know is that the suspect is asked a series of questions and, by the end, the examiner studies the machine’s results and hypothesizes whether or not the answers were accurate. But what are the true mechanics of this machine?

Although it is a seemingly complex process for a machine to assess a complex array of human emotion and cognition based on a few responses, the mechanisms of a lie detector are relatively simple.

Lie detectors, also known as polygraphs, are most commonly used for criminal investigations or government-related job positions. The idea is to determine signs of lying by assessing the suspect’s physiological responses to a series of questions.

The examiner chooses from a variety of techniques to determine if the subject’s physiological reactions changed significantly or deviated from normal levels during the period of the test. The most common method examiners use is to ask suspects a series of multiple-choice questions, some of which only the criminal would know the answer to. During and after the test, examiners determine whether the suspect’s reactions changed significantly when asked these targeted questions.

Another method commonly used by examiners is assessing the suspect’s physiological reaction to a few simple, neutral questions at the beginning of the exam, and seeing whether their reactions to sensitive questions differ significantly. Similarly, examiners may ask the suspects to lie deliberately, so that the way they react may be compared to their reactions to questions asked later in the exam.

The last approach examiners use is alternating between asking the subject neutral as well as sensitive questions, assessing the fluctuation between their reactions.

To monitor physical reactions, sensors are attached to people taking the test to monitor their breathing rate, pulse, blood pressure, perspiration, and occasionally limb movements. Subjects’ reactions are recorded by the sensors and then graphed by needles on moving paper. In more modern versions of polygraphs, the same goal is achieved by advanced computer algorithms, which monitor these signs and then send the results to be graphed onto a digital screen.

While polygraphs are currently the most accessible means of determining whether or not a suspect is being truthful, the machine does have its drawbacks. It is important to recall that technology is deterministic — that is, it produces a certain result by going through a known series of steps.

Therefore, if a suspect is in fact a criminal and knows the workings of a lie detector, he or she can manipulate it so that it displays data to make answers seem truthful. Since increased sweat is often detected by the machine as an indicator of lying, one can use a strong antiperspirant to keep from sweating too much. One can also take a drug that lessens symptoms of stress, such as Valium, in order to give more measured responses.

On the other hand, blood pressure medication can have adverse affects on the polygraph test. If the person taking the test takes this type of medication, the results could show a number of lies, even if he or she was telling the truth. Errors may also be made on an examiner’s end, because human emotions are, after all, unpredictable and may not be accurately portrayed by technology.

Polygraphs are utilized in criminal thefts, national security matters, and even pre-employment screenings. So if you ever find yourself being tested by one, remember to stay calm, breathe, and answer honestly — whether it’s for your job or your alibi.