Detecting Lies Content from the guide to life, the universe and everything

Detecting Lies

4 Conversations

Most lies are of the white kind, doing nobody harm and generally making life far easier than it would be if we all knew what was going on. Other lies are more important, actually having a distinct purpose. Criminals, adulterers and spies all need to lie regularly if they are to stay out of trouble, and some people are paid vast amounts of money to work out exactly who is telling the truth.

Lies are tricky things. They are difficult to pin down, having no odour, image or other form of physical manifestation. The usual way to uncover a lie is to collect evidence that points to the contrary. The police have to spend many long hours searching for clues, to piece together the real story in the face of six different versions from six different people. Private investigators can make a reasonably comfortable living sitting in cars outside houses which have their blinds mysteriously pulled down in the middle of the day waiting for a fleeting chance to take a conclusive picture.

Lie Spotting

Alas, no such form of foolproof detection exists. Contrary to popular beliefs, the famous lie detector is a myth. No expert or machine yet discovered can definitively spot a lie. Study after study has shown that lie detectors are a myth.

People who think they can spot a lie do no better than people who don't think so, who in turn do no better than chance dictates they should (although a study conducted entirely on mothers of teenagers may well yield different results). The only humans who do show lie detecting abilities are people with a type of aphasia (the inability to produce speech) which prevents them from understanding speech. This is thought to be because there are subtle changes in people's faces, between when they are sincerely expressing an emotion and attempting to replicate an emotion, which aren't noticed by most people because they are concentrating too hard on the spoken words. The only other group who are better than average at detecting lies are secret agents, which is unsurprising considering their lifestyle.

That is not to say that there is no way to detect lies. There are pointers which can be picked up if the observer knows what to look for. However, often these signs can be misinterpreted. Signs change from one individual liar to the next, and the motive behind a lie may affect how comfortable the teller is; and the more comfortable they are, the less likely they are to be caught out.

Types of Lie

Lies can be categorised in many different ways, but a widely held view is that there are four types of lie:

  • Pro-social - Lying to help someone else.

  • Self-enhancement - Lying to make yourself look better while not hurting another.

  • Selfish - Lying to personally benefit at the expense of another.

  • Anti-social - Lying to deliberately damage another.

Simple Signs of Lying

The type of lie, and whether it is in keeping with the liar's character, dictates how apparent the signs of lying are. Occasionally they are extremely obvious, especially when the liar is feeling guilty.

Obvious signs include:

  • Over denial - Repeating protests of innocence.

  • Stuttering - Stumbling over words without natural fluency.

  • Hand Wringing - Fiddling, rubbing, picking and playing with the fingers and hands while talking.

  • Eye Contact - Unwillingness to make or never breaking it.

However, the majority of people intending to tell a lie fabricate their story long beforehand and become comfortable with it, and so rarely get caught out so easily. Equally, many of the signs indicated may also simply be signs of nerves due to shyness or discomfort with a new situation.

Subtle Signs of Lying

There are of course more subtle signs which most people miss, and which can affect even word-perfect liars. These include:

  • Over formal speech - Use of long words, painfully correct grammar and the full forms of words or phrases that would normally be shortened, suggesting a scripted speech rather than natural conversation.

  • Very few gestures and no pointing - As physical movement illustrating something being described are a quite common and natural activity.

  • Justification - Attempting to justify every detail with lengthy explanations

  • Disparity - Mismatch between tone of voice and expression.

People who are used to detecting lies develop an instinct towards the more obscure signs, perhaps without even consciously noticing them. Most people, however, have a great deal of difficulty working out when someone is telling the truth. Which is why there are so many attempts to make a foolproof machine for catching lies.

Detecting Lies with Machines

The industry name for a lie detecting machine is a polygraph. 'Poly-' means many or lots and '-graph' refers to writing or the recording of information. This is essentially what a polygraph does, measuring and recording information from multiple sensory inputs that can be interrupted to show how comfortable a person is at a certain moment, or series of moments, in time.

Polygraphs work on the assumption that people telling lies are uncomfortable. Maybe not very much, but nevertheless they feel differently to when they're telling the truth, resulting in changes to heart rate, breathing, perspiration and so forth. A polygraph test assumes that these are involuntary changes to your body which always occur when you lie. This alone means that pathological liars often fool lie detector tests because they show no change when they tell a lie. However, taken together, it is assumed there is enough of a change in the readings between truth and lie to be able to make a definitive decision.

A polygraph interview will usually take several hours, and takes place in a room where only the interviewers, who like to be called forensic psychophysiologists1, and the subject are present, although others can be watching from behind one-way mirrors. First the examiner and the subject have a basic chat, where the examiner calibrates the equipment, and tries to put the subject at ease. The actual test comes later when the examiner asks several relevant questions and also some control questions to check that the responses are still the same. Analysis of the questions takes place afterwards.

There are two types of wrong polygraph readings. False positive is when a subject has tested positive for a lie, although they are in fact telling the truth and is the commonest type of error. False negative is when a subject has been acquitted of lying, even though they are guilty.


Polygraph readings, however, are not solid evidence for anything. The very act of having to defend their character may agitate a perfectly innocent person to the extent where the machine records that they are lying. There are also various ways to cheat the polygraph:

  • The examinee may bite their tongue, dig their nails into the palm or do something else that causes pain and significant discomfort just before they answer every question. The pain should prompt a greater physiological response than a lie would, so it would drown out the response to the lie, and would mean that the response to the test would be the same every time.

  • The examinee may use deodorant on their fingertips if they know they will be measured for sweatiness. This absorbs the sweat and so fixes the results.

  • The subject may use a sedative, to dull their body's response to the point where the difference between the reactions to truth and lies is so small as to make the test inconclusive.


Polygraph tests are an ambiguous area of the law. Depending on where you are, there are laws regulating their use. Many courts will not accept evidence from a polygraph test, because there is too big an area of uncertainty - although lawyers are allowed to mention them in defence of their client if they wish. Some places consider them an infringement of civil rights and ban people from making others take them. If they are to be used by companies, there are laws relating to the questions that can be asked, and no employee can be forced to take one, although the company is free to draw whatever conclusions they wish from that refusal. Polygraphs are very widely used in the United States, with the federal government being the largest user of them, while the police use them to assist in their investigations, even though they are not always used as actual evidence.

Many people are opposed to polygraph tests, and there are many organisations which oppose them, such as They do this because they claim they are fundamentally inaccurate. Statistics show a range of 70% to 90% accuracy depending on who delivers the statistics and whether they count 'inconclusive' results as wrong results, or merely discard them since they gave no evidence one way or the other. There are scientists who point out that polygraph tests do not in fact detect lies, merely register the physical state of the subject at the time, which is open to myriad outside influences. There are also those in the legal industry who object to the fact that polygraphs can be used as evidence by a defendant, but not the prosecution, while some believe that polygraph tests are immoral and a violation of human rights.

Defenders of polygraphs, like the American Polygraph Association argue that people with nothing to hide wouldn't be so opposed to them, and point out that they are very rarely used on their own - they are usually presented in conjunction with other evidence. There is also the fact that they are improving all the time with computer programmes which can analyse the voice for tremors, which is far less intimidating than being attached to wires. In addition, new research suggests that brainwaves change when lies are told, paving the way for even more accurate technological lie detectors of the future.

Detecting Lies with Chemicals

Truth inducing drugs are a staple part in the plot of many Hollywood espionage movies. They offer a very convenient means to ease the plot on its way. But is there any credibility behind the romantic ideal?

Well, thiopental sodium, which is trademarked under the name 'Sodium Pentothal' by Abbott Laboratories is an anaesthetic with side-effects similar to truth serums of the movies. Allegedly, when a small enough dose is administered the patient does not lose consciousness, instead entering a very relaxed state. Individuals in this state are far more open to suggestion than normal and lose many of their inhibitions. This in turn makes them more likely to voice what they are thinking without considering their words first. However, they keep their self-control, and would not do or say anything against their nature. This would make it a poor tool in detecting deliberate lies, since if the subject is unwilling to tell the truth they will not be forced to do so by the effects of the drug.


Life would be far easier for some people if there were a foolproof method for spotting lies. However, whatever method we use there is always the possibility that our assumptions or estimations will be incorrect. Clearly, it would be far easier still if people simply didn't lie in the first place. But then how would you keep children from learning the truth about Santa Claus and the Tooth Fairy?

1But it'll never catch on.

Bookmark on your Personal Space

Edited Entry


Infinite Improbability Drive

Infinite Improbability Drive

Read a random Edited Entry

Categorised In:

Written by


h2g2 Entries

External Links

Not Panicking Ltd is not responsible for the content of external internet sites

Write an Entry

"The Hitchhiker's Guide to the Galaxy is a wholly remarkable book. It has been compiled and recompiled many times and under many different editorships. It contains contributions from countless numbers of travellers and researchers."

Write an entry
Read more