Lie Detectors

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Lie Detector Tests

A lie detector test (or polygraph) is a device which measures change in a number of physiological variables such as blood pressure, pulse rate, respiration, electrodermal activity when a person is asked a series of questions relating to an issue under investigation. A chart is produced which is read by a polygraph examiner.

Admissibility in Court

Lie detector tests are admissible in court in a number of countries such as USA and Japan.


In USA a person must agree to the admission of a lie detector test under the terms of a stipulation before the test, in order for this evidence to be relied on in court. The case of Frye v United States (293 F 1013 (D.C. Cir. 1923) established that scientific evidence, including lie detector tests, was admissible on the condition that, “the thing from which the deduction is made must be sufficiently established to have gained general acceptance in the particular field in which it belongs”. Now evidence must meet the ‘Daubert standard’ and by produced from sound ‘scientific methodology’. In case of United States v Scheffer the Supreme Court allowed individual jurisdictions to decide on the admissibility of lie detector tests. Different states rely on the two different standards (Frye and Daubert), although many states still do not allow lie detectors to be used as evidence.

The states which do use lie detector tests are: Alabama, Arizona, Arkansas, California, Delaware, Georgia, Idaho, Indiana, Iowa, Kansas, Nevada, New Jersey, New Mexico, North Dakota, Ohio, Utah, Washington, Wyoming. In California, Arizona, Nevada, Georgia, and Florida, they can be used but both parties must agree. In California, they may be used as evidence as long as both parties agree to their use in advance, in this case, the jury is presented with the lie detector results; in Georgia defendants who suffer due to false results may sue the operator for damages and attorneys fees, and in Florida, people can be submitted to take the test but this will not be used in court, but instead for their therapy. In Michigan appellate courts, in a motion for a new trial lie detector results of a witness may be used to show that a prosecution witness committed perjury or to suppress evidence in a pre-trial motion where the defendant believes the police officer acted illegally to obtain the evidence.

In New York, Texas, Illinois, Pennsylvania, and the District of Columbia, lie detector tests are inadmissible even if both parties agree. In Massechussettes, they can be used as ‘supporting probably cause’.

It was stated in the case of United States v. Posado that “… we have today opened the door to the possibility of polygraph evidence in certain circumstances.” The per se rule excluding unstipulated lie detector evidence has been overturned.


In Europe, lie detector tests are not used as they are not regarded as reliable evidence. The Federal Court of Justice of Germany has ruled that such evidence is inadmissible. In the United Kingdom lie detector tests are not used in criminal courts due to concerns over reliability, the use of the test in the future would require legislation.


The Indian Constitution states, “"No person accused of any offence shall be compelled to be a witness against himself” and in 2010 the Supreme Court ruled that nobody (accused or suspected of a crime) will undergo a lie detector test unless their consent is obtained. Therefore lie detector tests may only be used if the defendant agrees.


In the case of Menora Insurance v Jacob Sdovnik in the Israel High Court (Civil Appeal 551/89) it was held that lie detectors may not be used as evidence in civil cases due to issues regarding the reliability of the tests. Lie detector tests are also inadmissible in criminal cases.


In the case of R v Béland in Canada the use of lie detector tests was held to be inadmissible although it is believed that they are still used today.


In 1982 the New South Wales District Court ruled that lie detector tests are inadmissible. The Lie Detectors Act 1983 states, “(1)Subject to subsection (2), anything that is, or purports to be: (a) output from an instrument or apparatus when used in the commission of an offence against this Act referred to in section 5, or (b) an analysis of, or opinion as to the effect of, any such output, is inadmissible as evidence before any court or any person or body of persons authorised by law or by consent of parties to receive and examine evidence”.


Amongst, the possible polygraphs test can be carried out are the Control Questions Test (CQT) and the Guilty Knowledge Test (GKT). The Control Questions Test is the most common used test and two types of questions will be asked, Control Questions and Relevant Questions. Control Questions are those which are related to the suspect’s crime, and relevant questions are specific to the person’s involvement. Both of these sets of questions will then be compared. The Guilty Knowledge Test aims to find out the level of the knowledge the person has of the crime, and questions about any aspect of the crime can be asked. It is believed that the Guilty Knowledge Test is the most accurate test.


A pre-test will be carried out where the examiner explains the procedure and may ask the person to tell a lie to show the reaction of this on the test. During this stage, information is gathered regarding personal information from the present and the past and the examiner aims to ensure the person feels that the test is being carried out fairly. It is believed that the more person knows about the procedure, the better the test. The examiner will have gone over information relating to the situation with the person being able to tell their side of the story also. It is emphasised that this an interview not interrogation.


There are four different types of questions that will be asked, irrelevant, relevant, controlled (these aim to create doubt in the person’s mind in relation to truthfulness) and symptomatic.


The answer and response measured are then evaluated (including a comparison of the questions asked) and a computer algorithm and numerical scoring system are often used. The examiner will then give an opinion of truthful, deceptive or inconclusive. An interrogation can be used if the opinion is deception. Any confession made by the person can be used in criminal proceedings. The examiner will present their opinion orally and in report form.


Whilst some private companies claim lie detector tests are over 90% effective, no consensus on their accuracy has been reached. The validity of the lie detector test is questioned as innocent people can also ‘experience nervousness, fear and emotional upset’. Some believe that the term ‘lie detector’ test is incorrect as it does not confirm that a person is lying but measures the change in their reaction to particular questions.