Lie Detector and Polygraph Buyers Guide

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Lie Detector and Polygraph Buyer's Guide

A lie detector machine, or polygraph, has been used since the early 1900s to test people’s truth telling while being questioned when it comes to crimes and criminal activity. It is a police tool that measures the subject’s body responses such as heart rate while being questioned in regards to a criminal situation. While these machines have been controversial over the years, there are circumstances where they might be acceptable. The logic behind the machine is that when someone is lying, their body changes and causes certain things to occur that aren’t natural if someone is telling the truth, such as the heart rate speeding up, sweating, or breathing more rapidly.

Since the technology still has not been perfected even after many tries and many years, these machines and their results generally are not admissible in courts to be used as evidence against a defendant. Since the machines are not precise, such as other scientific methods of collecting evidence like DNA or fingerprint collection, they are not held as valuable as those methods which are definitive, unlike the polygraph machine which someone very skilled could possibly pass even if he or she was actually lying.

A History of Polygraph Machines

The polygraph machine was invented in 1921, by a medical student whose name was John Larson, though William Marston liked to claim the credit himself, being as he used his own type of device to measure lying. He used a series of instruments that he rigged together, including a blood pressure machine, to measure a subject’s heart rate to see if it went up when he was lying. Although Marston’s invention was interesting, judges in the 1920s would not allow the results from the lie detector to be admissible in court due to the number of variables that could not be accounted for when the subject was given the test.

Larson, whose early polygraph was used by the FBI and the police until it was ruled inadmissible in courts in the 1930s, created the actual prototype for the lie detector test we see today.

How A Polygraph Machine Works

The polygraph machine essentially measures the body’s human responses to stimulus, specifically questions that are being asked of the person being interrogated. The machine itself, which is made up of sensors, is attached to responders that record the subject’s body responses on graph paper with attached pens.

The subject is hooked up to the machine with up to six sensors placed on the fingers, chest, arms, legs, or head. The subject is then asked a series of questions that he or she must answer truthfully, such as his or her name, to provide a control for the other questions in the series. The polygraph will then record results that occur when the subject is asked more intense questions, and the responses will indicate whether or not the subject is sweating, shaking, has a faster heartbeat, or if his or her arms and legs are moving during the interrogation, which could indicate that the subject is lying. This is what worries judges and police officers and is why these tests are not completely conclusive: a very calm person could sit through an entire test and convince his body he is telling the truth, which would then in turn not trigger the sensors to indicate he is lying. The machines can be fooled.

Newer polygraph machines may have more additional features than older models had; machines can now measure body temperature, and the amount of stress in someone’s voice when they speak. Muscle tension can be measured, as well. Since the lie detector machine is simply studying physiological responses, it is not actually testing whether or not the subject is telling the truth, hence the controversiality of the machine.

Basic Types of Polygraph Machines

There are now two main types of lie detectors, or polygraph machines, that have been used and are still being used today. Older polygraphs use analogue technology when coming up with results, but newer technology has now allowed for polygraphs to be created that measure responses using digital technology, which is supposedly more accurate, and quicker with findings. Digital technology can also measure additional responses.


The analogue lie detectors are the ones that most people see in the movies, which are hooked up to the paper that scribbles the lines with the sharp pens that look like needles. The paper rolls while it records the results, which can then be torn off and read. This is old technology, which has been used since lie detectors first came onto the scene in the early 20th century.


Digital polygraph machines use computer software programmes to record results, and can offer more options when it comes to measuring body responses, such as electrodermal activity, which is the moisture and temperature of the skin. Digital polygraph machines are said to be more accurate than analogue machines. Although these machines may not be used frequently by police officers, they are used by the government in some situations.

What Lie Detectors Look For

Lie detectors measure a number of the body's basic responses that are supposed to change whenever the person is lying. Tracking these responses should be a good indicator of whether or not a subject is telling the truth or lying.

 Body Part

 Response Studied


Facial expression

Facial temperature


Forehead perspiration


Blood pressure changes









Breathing rate acceleration

Sensors are attached to the subject at four to six spots and data is recorded either by graph paper or by computer as the subject is asked questions. The responses are evaluated by a professional lie detector administrator to determine whether the subject is lying or telling the truth based on the above physiological responses.

Newer Methods of Lie Detecting

When it comes to lie detecting, it is a science that has confounded biologists, physiologists, and scientists for years. They are consistently trying to find new ways to tell when or how people are lying. One of the newest methods, and supposedly the most accurate, of being able to tell someone is lying is by using Magnetic Resonance Imaging, or an MRI machine. An MRI machine measures the areas of the brain that are used when a person is lying, and during an MRI test when a subject is asked certain and specific questions, it can trigger a response in those areas.

The blood flow to these areas may shift, which could prove that the subject is lying. While these MRI tests are still being tested and perfected and are not completely accurate yet, their success rate may be higher than standard polygraph tests, and one day they may be used with some degree of success when it comes to the area of lie detection. An MRI exam may not be as easy to fool as a traditional lie detector test using a polygraph machine.

How to Buy a Lie Detector or Polygraph Machine on eBay

Although professional polygraph machines can’t be purchased for civilians, there are some that can be played for simple amusement and be purchased for an affordable price on eBay. For fun, a polygraph machine can be purchased as part of a couple of different board games. One is called Shocking Liar, and one is called Saint or Sinner. Search lie detector to bring up some of these games. These games offer a rudimentary form of polygraph machine that the game players can rest their hands on and answer questions. Liars may receive an electric shock. Purchase a fun lie detection machine on eBay from a merchant that's a Top-Rated Seller.

Other fun lie detectors that can be enjoyable for consumers to play with are part of spy kits for consumers, such as Spy Gear Lie Detector. This is a basic lie detector that has fingertip sensors, which people can use to find out if a subject is lying based on the simple responses. It’s probably as close as most consumers will ever get to the real thing. Consumers can find a few different types of fun lie detectors for purchase on eBay, but they are in no way accurate; they are simply for amusement.


Though polygraph machines have been around for a long time, they are not really taken any more seriously than when they first came onto the scene in the 1920s. Since the right person can fool a machine easily, they are not allowed as evidence in court, though they are used sometimes by government agencies to screen employees, though they are considered widely unreliable when it comes to police or forensic use. Polygraphs are fascinating, and scientists continue to explore alternate methods that will help law enforcement and government agencies see who is being deceitful.

Other methods of lie detection, such as MRI imaging, facial expression analysis, and thermal imaging, which uses high tech thermal cameras to measure the body’s change in temperature, are constantly being developed and tested to provide a more accurate way to measure lying. While the measurements of truth telling will probably never be an exact science, scientists hope to get closer to a method that will be more accurate than the controversial polygraph machines of yesterday and today.


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