How accurate is a lie detector test?

lie detector test

The American Polygraph Association, and leading international body on the subject of polygraphy, officially estimates lie detector testing to be 87.5% accurate. They have reached this figure through the undertaking of numerous, vigorous studies and research compiled in the field of lie detector testing. 

Many providers of lie detector services claim their polygraph testing to exceed this figure in terms of accuracy but it is unlikely that they have carried out the independent and objective studies and research to make such a claim valid and reliable; think about it, how would they actually know how accurate their testing is?

Subjects are unlikely to call the examiner after a test for research purposes to let them know “by the way, you know that lie detector test you said I passed? Well, I was actually lying…”! Testing on friends and family for research purposes using non-consequential examinations do not work to produce reliable estimates of accuracy as they lack the significance and saliency of a real-life lie detector test and, therefore, the genuine perception of threat and resultant bodily responses that lie detector testing relies on to detect deception.

Despite the general figure of 87.5% lie detector test accuracy being recommended by the American Polygraph Association, there are many other factors that can influence the accuracy of polygraph test results and findings. At the end of the day, the test involves humans, human responses and bodily changes and the correct recording and analysis of these. 

Humans differ from individual to individual and responses and reactions can be different from one person to the next. Whilst there are scientifically proven and evidenced standards of bodily response in all human beings, there are subtle changes and nuances from person to person and all kinds of factors can cause atypical responses in people on occasions. There is also the matter of human error and, whilst, testing employs technical and scientific instrumentation and computerised recording devices, the potential for error remains and this can impact on the end result.

The following factors can have a significant impact on the accuracy achieved with8in a lie detector test:

The Examiner:

The examiner plays a crucial role in the lie detector test process and can definitely directly affect whether a lie detector test is accurate, ethically undertaken and even remotely valid. From the formulation of the test order and questions used, to the creation of a suitable environment for testing, to the correct application of sensors and to the analysis of data, the examiner is responsible for ensuring that all stages of the test are undertaken correctly to gain an accurate result.

The majority of people undertaking lie detector testing are nervous; the test is important to them and often will have significant consequences for them in their personal or professional lives. The failure by the examiner to obtain baseline readings or undertake an acquaintance typeset can cause these nerves and anxieties to provide a false indication of deception or hide deception that is present.

Incorrectly worded questions can cause confusion and ambiguity, sometimes causing the subject to respond in ways that indicate deception when in fact they are just unsure and panicking. It is important that the examiner word questions clearly and without any room for confusion to provide an accurate result.

Polygraph machines and software do not produce a ready formulated decision and data needs to be analysed by the examiner to achieve the result. If the examiner does not have sufficient knowledge and understanding around this or is not thorough in their application of scoring this will reduce or prevent accuracy.

The Environment:

The environment the examiner creates for the test to take place is important to achieving accuracy; the area needs to be free of other influences, distractions or pressures such as other people, TVs or recording devices other than those being used for the purpose of the lie detector test itself. The environment also needs to allow for confidentiality and be sufficiently private so that the subject can speak honestly and openly. The environment also needs to be a suitable temperature so as not to affect the bodily changes being recorded to detect deception.

Language Barriers:

If English is not a person’s first language or their understanding and use of English is limited or compromised in some way and the questioning is being provided in English, this could impact on their responses due to confusion or incorrect interpretation. In such situations, an interpreter can be used as a third party intermediary within the test but this does complicate the analysis of data and the timing of responses, as well as potentially changing the end wording used to communicate questions. Any language barrier is likely to impact on the accuracy of a lie detector test.


Any impairment to being able to hear and understand the questions asked within a lie detector test, or being able to understand the issues at the heart of the test, will render the result inaccurate. It is important that any physical or cognitive difficulty or disability is recognised and either adaptation made to prevent the result being inaccurate or careful consideration given to whether it is ethical and correct to undertake a lie detector test with a particular individual.

Substance Use:

Alcohol and illicit substances such as cannabis, cocaine and amphetamine affect the brain and bodily responses which can cause incorrect or inaccurate results in a lie detector test. It is important that you are not under the direct influence of any substance when you take part in a lie detector test and that you make the examiner aware of you have recently used alcohol or any drug or if you are a chronic user of alcohol or any drug.

Prescribed Medication:

Medication prescribed to manage depression and/or anxiety can cause changes in bodily responses. Whilst this does not mean that you cannot sit a lie detector test, it is important that you make the examiner aware that you are currently taking this medication so that they can take this into account when the data is analysed to ensure a valid and accurate result.

Medication prescribed for heart or blood flow issues (hypertension medication) can also impact on the bodily responses that an individual exhibits in response to threat or fear. Once again, this does not mean that you cannot sit a lie detector test but it is essential that you let the examiner know what medication you are on and what symptom the medication is prescribed in response to so that they can take this into account when analysing data to reach an accurate result.

Equipment Used:

The sensors attached to the body of the subject and the software used to collect and analyse data obviously needs to be in full working condition and properly maintained to achieve accurate lie detector test results.

Essentially, whilst there is an evidence-based universally accepted figure of the accuracy of lie detector testing of 87.5%, this is dependent on correct methods and principles of lie detector testing being employed. It is vital that your test is undertaken by an examiner who is knowledgeable, experienced and ethical and who is able to be robust and committed in ensuring the best conditions possible to obtain an accurate result, and that you as the subject are co-operative and communicate any information requested of you by the examiner to the best of your knowledge.

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