HOW LIE DETECTORS WORK
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How do lie detectors work?
Lie detectors, or polygraph machines as they’re more
officially known, are large contraptions that are filled with buttons, levers,
paper, and measuring devices. They have been used for years in law enforcement
to detect whether or not someone is actually lying to officials. The reason why
people would lie to law enforcement officials is obvious – they committed a
crime that they don’t want to get caught for. Lie detectors can help solve this
problem by asking a person who’s strapped to a lie detector several questions
and then analyzing the results. However, lie detectors still aren’t permissible
in court. This means that should a person fail a lie detector, they will not be
able to be proven as lying in a court of law. And likewise, a person will not be
able to use the results of a lie detector test to prove that they are telling
the truth. This is because lie detectors can’t actually detect if a person is
lying. They can only tell if the person is displaying deceptive behaviour. But
just how can these machines tell if a person is being deceptive with only some
paper and buttons? Just how do lie detectors work?
Even before noticing the buttons and the graphing
utensils, the first thing a person might notice about a lie detector the first
time they see one is the many straps, electronics, and monitors that are
attached to the person taking the lie detector test. These devices are medical
tools that measure things such as the person’s blood pressure, heart rate, and
the amount of perspiration that is present. It is fluctuations in these
measurements that examiners are looking for when they give the test. However,
the results given are wide open to interpretation by the examiner. It’s for this
reason that lie detector tests are so controversial and why any results taken
from a polygraph are not admissible in court. So why even use lie detectors at
all? They are used for much more than just solving crimes. Many government
agencies and other places of private employment use lie detector tests in their
Lie detectors work on a very simple principle. When
people lie, they all display most of the same physical attributes. Lying causes
blood pressure to rise, perspiration to increase, and heart rate to increase.
These symptoms of lying are caused by the stress that the body experiences when
someone is telling a lie. Lie detectors can pick up on these symptoms of
deception-induced stress and graph them in a way that the examiner can analyze.
The symptoms are also involuntarily and even with great focus, it’s very
difficult for someone to control these physical responses. This is why passing a
lie detector test when one is lying is very difficult.
When most people think of lie detectors, they
usually think of the large machines with scrolls of paper rolling through them
on which graphing utensils are furiously scribbling while the person taking the
test answers some simple questions. This is called an analogue polygraph and
while they were the machines that were first used as a lie detector, there are
now digital polygraphs. These machines use algorithms in place of the old
graphing tools and a computer monitor in place of the old scrolling paper.
Although these are certainly advanced technologies for polygraph machines, they
are no more accurate than the older analog versions. Now we’ll take a closer
look at the different measurements that are taken during a polygraph and how
each is measured using both an analog polygraph and a digital polygraph.
First, respiratory rate is measured. Two rubber
tubes called pneumographs are first strapped around the chest and around the
abdomen of the person being tested. These tubes are filled with air and measure
every time a person takes a breath. This is possible because when the person
breathes, the air in the tubes moves around and the polygraph is able to pick up
on this. With analog polygraphs, something called a bellows moves the air by
contracting when the tubes expand. The bellows is attached to an electronic arm
which moves a pen every time the person breathes. The pen then scribbles the
different movements to display the measurements. Digital polygraphs also use
pneumographs strapped around the person being tested but instead, they use
transducers to send the digital measurements to the computer.
For lie detectors to measure blood pressure, a
blood pressure cuff much like the ones used in doctor’s offices are placed
around the arm. Every time blood is pumped through the arm, it makes noise. This
noise moves the air in the tubes, much like the breathing of the person being
tested does. Just as the pneumographs are connected to a bellows, which is
connected to a pen that takes the actual measurements, so is the blood pressure
cuff in an analog polygraph. And again, digital polygraphs use transducers to
take electronic measurements.
The third measurement that lie detectors take is called the galvanic skin
resistance, also known as electro-dermal activity or, even more commonly known
as, sweating. Lie detectors use the sweat found on the fingertips to take
measurements of how much the person is perspiring. The fingertips are chosen
because this is one of the most porous spots on the body and so, a lot of
sweating happens there. This should be especially true in the case of someone
who is lying because people tend to sweat more when they are lying.
Galvanometers, which are metal plates placed on either side of the finger, are
placed on either side of the two fingers that are used for this part of the
test. As the person is asked questions, the galvanometer will determine how much
electricity can be conducted on the fingertips. The more moist the fingertips,
the better chance they have to conduct electricity.
So just who is qualified enough to analyze these
results to make accurate judgements? These are the examiners that give the
tests, and now many of them are given the title of forensic psychophysiologists.
The role of the examiner is very important. Not only does the examiner need to
be able to accurately analyze the results of the test, but they also need to
remain impartial throughout the test. In addition to this, the examiner also
needs to be very careful to display neutral and non-intimidating behaviour. This
is because the stress of taking the test alone can be enough to give false
results and an intimidating examiner will only worsen this problem. The examiner
is the only other person in the room with the person that’s being tested so it’s
very important that they keep a cool and calm demeanour. The examiner will also
need to set up the machine, prepare the person to be tested, and of course, ask
the pertinent questions.
Another important job of the examiner is to
complete a profile of the person being tested. This is very important because
the examiner must know the background and some basic information about the
person they are testing. This is because some questions may not be appropriate,
or can even be offensive, on religious and cultural grounds. For this reason,
should one of these questions be asked, a person’s blood pressure may start to
climb, their heart rate may start to increase, and they may begin to sweat quite
a bit more than usual. All of these symptoms will give a false positive, and say
that the person is lying when in fact they might not be. Once the polygraph and
the person being tested are all set up and ready to begin the test, it can take
anywhere from just a few minutes to several hours, depending on the extent of
the questions and the person’s responses. Because the test takes so long, it’s
divided up into several different parts.
The pretest is the first part of the polygraph
test. This part of the test is made up of many things. Firstly, the examiner and
the person being tested must get to know each other. This is so the examiner can
profile the person being tested and so that the person can get to know the
examiner some before the actual test begins. During the pretest, the examiner
will also ask a few general and basic questions completely unrelated to the
reason for the polygraph. This is so that the examiner can get a good feel for
how the person answers questions and processes information. Once the pretest is
completed, design questions are then asked. These questions have to do
specifically with the issue in question.
The third part of a polygraph test is called the
in-test. During this part of the test, the examiner will only ask about ten or
eleven questions. Of these, only four or five will have to do with the actual
issue at hand, while the others will be control questions. Control questions are
questions that almost everyone will have to honestly answer “yes” or “no” to.
For instance, the examiner might ask if the person being tested has ever stolen
something before in their life. If the person responds with a “no”, the examiner
will have a good idea that the person is prone to lying. After the in-test,
there’s the post-test. During the post-test, the examiner will review the test
results to determine how many fluctuations are present in the test. Many
fluctuations show that the person is most likely lying, while a steady test will
show that the person is most likely telling the truth. Examiners can especially
tell that someone is lying if they have many fluctuations when being asked the
same questions over and over again.
So now that you know how polygraph tests work, want
to find out how they don’t work, or how they can be fooled? Many people have
shown interest in beating a polygraph, whether it’s out of simple curiosity or
because they really do want to trick the polygraph into thinking that they’re
telling the truth. But in a test that seems to be completely objective, and
based on involuntary responses, how can you beat it? It’s difficult and most
often does not work. But many people have tried a few simple tactics.
Using sedatives, applying deodorant to the fingertips, and biting on the tongue,
cheek, or lip are all very common ways to try and beat the polygraph. Another
way that people try to beat the polygraph is to do something after every
question that will result in a physical response. This can be something such as
placing a tack inside their shoe and stepping on it with their toe after every
question is asked. The mentality behind this is that the body will have the same
response to this stimulus every time and so, the responses on the test will be
Still, unless you really want a particular job,
failing a polygraph will have very little consequence. The results from
polygraph tests are generally not admissible in court unless both sides of the
case agree to it. And in these cases, there is usually nothing incriminating
found in the polygraph test.