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Teachers: Citizenship: Scientific Development

Last Updated: Friday December 03 2004 15:46 GMT

Lie detectors

Citizenship 11-14/KS3/Levels E&F
James Brook
Science and ethics

This lesson idea comes from James Brook who is a PGCE Citizenship at Sheffield Hallam University.


Pinocchio's nose grew when he lied
A team of US scientists have found that a brain scan can tell if you're lying.

Students spot the liars in a game of Call My Bluff and discuss the ethics of using lie detectors.

Learning aims

  • The arguments for and againast using lie detectors
  • Different methods of lie detection
  • Lie detector tests and the law


    Magnetic scan of the brain

    Arguments for using lie detectors:
    They could help convict criminals.
    They could stop people lying about their experience in job interviews.

    Arguments against using lie detectors:
    Their accuracy is not proven.
    Anyone who is suspected of having broken the law has the right to remain silent in case what they say gets them in more trouble.

    Read out this story to the class.

    Ask students these comprehension questions:

  • What is the difference between a lying brain and a truth-telling brain? Different parts of the brain are used for lying and for telling the truth and there is more brain activity when you lie.
  • What is the brain scanned with? A strong magnet.
  • What kind of lie detectors are traditionally used? Polygraph machines.
  • Why do some people want to see them swapped for brain scanning equipment? They argue polygraphs can be fooled. They measure breathing rate, heart rate, blood pressure and perspiration, which can differ in individuals. Also, some say it's possible to learn how to cheat the test.
  • Why is it unlikely that brain scans will be used in every lie detecting case? They are expensive.

    Warm up

    Ask students to write down a statement describing their house which is either true or false.
    E.g. We have two TVs at home.

    A handful of students stand in front of the class and read out their statements. The rest of the class vote on whether they think the statements are true or false.

    Ask students: How did you decide whether they were telling the truth or lying?
    Prompt: Body langauge, tone of voice, eye movements, statement doesn't match prior knowledge.

    Make a class list of their suggestions.

    Main activity

    Call My Bluff

    Call My Bluff. Referee Bob Holness with team captains Sandi Toksvig and Alan Coren .
    In groups of three, students pick a word at random from a dictionary.

    One member of the group writes down the definition in their own words - so it doesn't sound too obvious this is the true meaning.

    The other two group members devise alternative definitions for the word.

    Tip: Look up words that start with the same prefix or end with the same suffix. Incorporate these meanings in your made-up definition. It will make it sound more convincing.

    E.g. Passerine is a word which describes birds with feet designed for gripping onto a perch.
    One meaning of pass is to overtake.
    One of your made up meanings could be: A passerine is a female motorist who likes to overtake male drivers on the road.

    Each group presents their definitions to the rest of the class, who vote on the definition they think is the original.

    A handful of students explain how they made their choice.
    Did they make a guess based purely on the definitions or did they look for clues in the students' body language, tone of voice, eye movements etc?

    Each group reveals the true definition.

    Extension activity

    Students research the history of lie detection and make a list of all the methods they can find. See Teachers' background below.


    Ask students:

  • In what circumstances should brain scanning equipment be used as lie detectors?
  • Polygraph tests are used in the US. Some people say they are 90 per cent accurate. Others say they are only 60 to 70 per cent accurate and that it's possible to learn how to cheat the test. In what circumstances do you think polygraph tests should be used?
  • Why do you think human rights organisations say that using lie detectors "infringe people's right to silence?"
  • Do you think people should be asked for their consent before taking a lie detecting test? In what circumstances do you think they should be forced?
  • At present, lie detector test results cannot be used as evidence in court. Do you think the law should be changed?

    Teachers' background


    Lie detectors, or polygraphs, have been in use for over 80 years but have always been controversial.

    Though scientists and psychologists have improved lie detectors over the years, there is still little hard evidence on how effective they are.

    Proponents say they are accurate in 90% of cases, but critics believe this figure is vastly inflated.

    Outside the United States, the machines are not widely used.

    The concept of lie detecting machines is based on the fact that when a person lies, there are certain changes in physiology which can be measured.

    The modern polygraph works by measuring several basic changes, which can include breathing rate, the electrical conductivity of the skin, heart rate, and blood pressure.

    While the subject is wired up to the machine, he or she is given a set of questions carefully designed by psychologists.

    Use of polygraphs peaked in the USA in the 1980s, when government agencies administered over 20,000 tests.

    They were mostly used on suspected criminals, though job applicants were also subjected to testing.

    A review by the government's Office of Technology Assessment in 1983 concluded that an accurate measure of polygraph accuracy was impossible.

    The report said that polygraphs cannot be objective, because the examiners, the questions asked, and the type of people being tested, vary too widely.

    In the aftermath of September 11th, polygraphs have gained a higher profile in the USA.

    Americans working at laboratories with stocks of anthrax have been tested. Sceptics would point out that the FBI has yet to make an arrest.

    The fall of the Twin Towers and the anthrax investigations have also focussed attention on newer ideas for lie detection which could be more reliable.

    A brief history of detecting lies

    In English medieval courts, truth was tested by ordeals of fire and water, on the basis a truthful person would be protected by God.

    Someone suspected of lying would have to carry a red-hot iron bar for nine paces. Alternatively he could opt to walk across nine red-hot ploughshares.

    Either way, if the suspect was burned then this was proof that he was lying and so could be promptly hung.

    Other courts went in for trial by water. In the ultimate "no-win" situation, the person accused of lying was put into a sack and thrown into a pond.

    If the accused sank this showed he was innocent, but he might well drown anyway. If he floated this was taken proof that he was lying and he would be hanged.

    Such practices were ended in 1215 by edict of the Latern Council.

    By the 1600s the idea arose that the truth of any statement could be arrived at by the means of detailed questioning and the application of scientific and logical reasoning to what was being said.

    Modern legal conventions of cross-examination and the presumption that somebody is telling the truth unless it can be proved otherwise "beyond reasonable doubt" date from this time.

    The philosopher Descartes wrote that "the power of distinguishing the true from the false, which is properly speaking what is called good sense or reason, is by nature equal in all men".

    Trying to work out whether somebody was lying was a matter of questioning, debate and the clash between different points of view based on the gathering and analysis of evidence.

    The 19th Century saw a reversion to ideas of truthfulness and lying as moral conditions embedded in the unique personality of the accused person.

    The new "sciences" of phrenology - measurement of "bumps" on a person's skull - and psychology - led to the idea that lies could be detected by looking at physical symptoms.

    It was eventually argued by phrenologists that pathological liars and "criminal personality" could be determined by measuring the shape of a person's skull. Psychologists concentrated on of past personal life, "personality type" and even the content of someone's dreams.

    Phrenological "evidence" of criminal personality type was presented in Victorian murder trials and used to secure conviction of "guilty types" whose evidence could not be relied upon.

    The search for "scientific" ways of spotting liars moved from bumps on the head to brain chemistry, with the search for a "truth serum" drug.

    Barbiturates including scopolamine, sodium amytal and sodium pentothal were given to suspects in the hope that drugs could somehow rewire the brain, making it incapable of telling a deliberate lie.

    The "medicine" had the intended effect of causing the victim to lose control over what he was saying. But the result was normally an endless stream of drug-addled gibberish rather than "the truth".

    In 1963 the US Supreme Court said "serum-induced confession" was in effect a form of torture and the practice was ruled unconstitutional.

    Recent ideas include a heat-sensing camera which can measure tiny temperature fluctuations on people's faces, and magnetic resonance scanners which observe the brain's inner workings.

    Neither technique is ready for use and civil liberty campaigners believe that both would bring new infringements to the rights of the citizen.

    For hundreds more news-based lessons, click on Teachers on the left hand side of this page.

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