Given the generally poor rates of lie detection accuracy shown by people, and also by some law
enforcement groups, can we turn to machines to help us improve our accuracy?
When most people think of lie-detecting machines, they probably think of the polygraph, which
is also sometimes simply called a lie detector. You've probably seen a polygraph in various
movies, like ‘Meet the Parents’, and on various TV shows.
The polygraph is a machine that simultaneously records a number of physiological responses
from sensors placed on the body. The name ‘polygraph’ actually mean exactly this:
'poly' meaning 'many', and 'grapho' means 'to write'.
The polygraph output can be displayed either on paper or on a screen. That output can show
galvanic skin response, which is an indirect way of measuring sweating, blood pressure,
heart rate, and respiration, which is breathing.
The assumption is that lying is accompanied by identifiable and predictable physiological
activity in the liar's body. This is actually a pretty old assumption.
Quite some time ago, Chinese investigators used to force suspected liars to fill their
cheeks with rice powder. If the powder remained dry, the person was identified as a liar.
So when is the polygraph actually used?
It’s been used for criminal and forensic investigations in a number of countries such
as Canada, Israel, Japan, South Korea, Mexico, Pakistan, the Philippines, Taiwan, Thailand,
and the United States of America. There have also been some trials of
the polygraph in the United Kingdom.
The use of a variation of the lie detector test based only on voice was actually recently
abandoned in the United Kingdom in 2010 due to low levels of reliability. The UK government
had hoped that the test would allow people who were cheating the social welfare system
to be identified when they called benefit offices.
They are however used in prisons.
There has also been some use for employment screening, although this is limited in the
United States of America by the Polygraph Protection Act of 1988. Their use in courts
is also somewhat restricted, as polygraphs aren't accepted as evidence in many jurisdictions
So how do they work? And, perhaps more importantly, do they work?
Well, there are actually different ways of using a polygraph. The most commonly used,
and also most commonly disputed test, is the control question test.
This test compares responses to deception-relevant questions with responses to control questions.
Generally there might be ten of these questions all up.
The relevant questions might be specific questions about a crime. For example:
"On May 17, did you stab Frank Redmond?"
Control questions refer to acts that are related to the crime but do not actually refer to
the crime in question. These will be more general, vague, and cover long periods of
time.
The idea with these questions is to embarrass the suspect so that they are forced to lie.
This has the effect of increasing their level of physiological arousal.
For example, you might ask a suspect: "Have you ever tried to hurt someone to get
revenge?"
The suspect will be told that answering ‘yes’ to this question will lead the investigator
to assume that they are the type of person who would commit the crime in question.
So how does this test tell us whether someone is lying or not?
Well, it makes two assumptions.
The first is that an innocent suspect will show higher levels of arousal for control
questions compared to relevant questions. This is because they have to respond deceptively
to the control questions, but they respond truthfully to the relevant questions.
The second assumption is that a guilty suspect will show higher levels of arousal to relevant
questions compared to the control questions. Although they will respond deceptively to
both types of questions, the relevant questions pose a more immediate threat to the suspect
and so they should elicit a stronger physiological response.
As an aside, if the guilty suspect doesn't respond deceptively, then they have essentially
confessed to the crime and we don’t need to do a polygraph.
To work out whether the suspect is innocent or not, the polygraph tester will look at
the output recorded by the machine and then rate the differences in physiological responses
to each of the question types. The differences are generally rated from minus three to positive
three.
If, on the basis of these ratings, the suspect fails the test, they are pressured into a
confession in a post-test interview.
The second type of polygraph test is called the guilty knowledge test.
This test aims to determine whether the suspect has knowledge about a particular crime that
they do not want to reveal.
For example, imagine the following scenario:
A suspect has killed a person with a knife. They deny this during an interview
and then the polygraph examiner then shows the suspect a series of knives, including the
one used in the killing. For each knife, the examiner asks the suspect if he or she recognises
it.
Now, both an innocent and a guilty suspect will deny recognising the knife. The difference
however, is that the guilty suspect should show a heightened physiological response when
shown the correct knife that was actually used in the crime. For the innocent suspect,
all of the knives produce the same reaction because none of them have been seen before.
Now obviously you can only use this version of the test if you have a piece of evidence,
like a knife, that you can show the suspect, and that hasn't been reported in the media.
So how reliable are these tests?
Well, despite some of the claims made by companies that offer polygraph services, the accuracy
rate varies quite a bit.
As you can see, while the accuracy rates seem a bit better than those for relying on nonverbal
behaviour, they don't always perform at the levels that some people think, and are quite
a way off being one-hundred percent accurate. You can also see that the accuracy depends
on whether you are trying to detect a truth or detect a lie, and for the crucial role
of lie detection, accuracy can fall to levels not much higher than chance for the guilty
knowledge test.
When we look at polygraph accuracy across a large number of studies, we can see that
there are about 78% of guilty suspects are classified accurately as being guilty. Almost
70% of innocent suspects are classified correctly as being innocent. Now although these figures
seem reasonably reassuring, just over 15% of innocent people are incorrectly classified
as being guilty, and about 10% of guilty people are incorrectly classified as being innocent.
Now while some people might be willing to accept these accuracy rates, even though they
are less than perfect, the issue is actually a bit more complicated than that. To get a
full understanding of how reliable polygraphs are, we need to take into account the underlying
base rate of deception when we use a polygraph. And this can actually be pretty difficult
to know.
So let's take the accuracy percentages we just talked about, and apply those to a context
in which the rate of deception is 50%, which might be typical in an experiment testing
the validity of a polygraph. So imagine we have 500 liars and 500 truth tellers.
When we do this, we can generate a table like the one you see now. This table combines the
percentages of correctly and incorrectly identified liars and truth tellers and applies that to
our 500 liars and 500 truth tellers.
Now the numbers on the right, just outside of the table, tell us how many people were
accused of being liars (next to the positive row) and how many people were identified as
truth tellers (next to the negative row).
So there were 471 people identified as liars and 398 people identified as truth tellers.
We can work out the chance that a person identified as a liar is actually a liar, and the chance
that a person who is identified as a truth teller is actually a truth teller.
So for each of the 471 people accused of lying on the basis of this test, there is a 83.44%
chance that he or she is actually lying.
And for each of the 398 people certified as being honest, there is an 87.69% chance that
he or she is actually honest.
Now that doesn't look so bad, still not perfect, but perfection is difficult to achieve.
What happens when the base rate of deception decreases however? Is it really the case that half
the people who might take a polygraph are being deceptive?
Let's rework those numbers but this time assume a base rate for deception of 5%. So we have
50 liars in a group of 1000 people.
When we do this, we can see that the numbers actually change quite a bit. So let's work
out the chance that a person who fails the test is actually lying. There are now 186
people accused of lying, but for each one of these, there is only a 20.97% chance that
he or she is actually lying because the test now identifies a large number of people as
liars even though they are actually truth tellers.
For each of the 667 people classified as being honest, there is a 99.25% chance that he or
she is actually honest. This looks more reassuring.
So in sum, although the polygraph can discriminate between liars and truth tellers, the number
of false positives is rather high. False positives are those people identified by the test as
liars, but who are actually telling the truth.
So perhaps the best use of the polygraph is to identify when people are telling a truth,
rather than when they are telling a lie. In other words, use the test to rule people out
of an investigation, rather than rule them in as a suspect.
So why don't polygraphs perform better? What are the problems with polygraphs?
Well, one of the problems we have already discussed are potentially low base rates of
deception. It’s difficult to get an accurate estimate of these base rates in real life
as we’ve already said.
Some people are also relatively emotionally non-reactive, and so they don't produce the
physiological responses we expect to see when lying.
It's also possible that innocent people may react strongly to questions about whether
they committed a crime or not, because they worry that nobody will believe they are innocent.
The polygraph examiner also has to convince the suspect that the polygraph is a flawless
detector of lies for it to be most effective and most accurate. This necessarily involves
lying to the suspect, which is not be allowed in many jurisdictions.
A problem specific to the guilty knowledge test is that a suspect may well admit to having
guilty knowledge but deny that they were the person who committed the crime. For example,
it's possible they know which knife was used because the actual criminal told them or showed
it to them. The test can't differentiate between this type of knowledge and guilty knowledge
due to actually committing the crime.
Also, as we mentioned before, the guilty knowledge test can only be used when there is sufficient
specific details about a crime known to the investigators, and these details haven't been
released to the public.
More generally, polygraphs suffer from a lack of standardisation. The scoring is subjective,
and the questions will differ from test to test.
Another problem is that suspects may employ countermeasures in an attempt to defeat the
polygraph. For example, they might bite their tongue or press their toes on the floor. According
to research by Honts and colleagues in 1994, these types of counter measures can actually
reduce the detection of lies by 50%.
Another reason why we should be cautious about claims made about the accuracy of polygraphs
is that it's very difficult to actually test the accuracy of polygraphs, mainly for two
reasons. In the lab, studies don't involve the same severity of consequences for participants.
It simply would not be ethical to put research participants in the same context that a real
suspect experiences. And while field studies might seem like a solution to this, unfortunately
we can never know for certain who is lying once we test the polygraph outside of a lab
setting.
So what about other machines that might help us detect lies?
Well, some researchers such as Kozel and colleagues suggest that activity in particular brain
regions, such as the right anterior cingulate gyrus, might be a better test of whether someone
is lying or not.
One way we can measure activity in the brain is by using fMRI. fMRI stands for functional
magnetic resonance imagining, and it is a machine that is used to image how brain activation
changes over time in response to different stimuli.
It does this by using a very large magnet to detect changes in blood oxygenation in
the brain, and it is assumed that increases in blood oxygenation are associated with increased
brain activity in those regions.
Langelben and colleagues
suggest that fMRI has high deception classification accuracy, and they suggest
that such a test could be feasible for practical use. However, one issue is that fMRI machines
are very expensive to purchase and run, and not particularly portable. And they also require
highly trained staff to run them and interpret the scans. It's also not clear whether the
courts would accept evidence based on an fMRI scan.
So, the research leaves us with a somewhat difficult conclusion. Non-verbal behaviour
is not a great marker of deception, and polygraphs are not fantastically reliable either. It
is worth knowing this however, because it is important that people, such as investigators
and jurors, don't rely on unreliable indicators of deception and truth when making important
decisions. And perhaps it’s best if people focus on the strength of the evidence when
trying to decide if someone is being deceptive.