A polygraph or lie detector test measures physiological changes such as blood pressure, pulse, respiration, and skin conductivity while asking questions. The premise is that lying causes subtle physiological changes to be detected by the polygraph machine.
Polygraphs measure changes in physiological activity that are assumed to correspond with lying or evasion. The main parameters measured are:
- Blood pressure – A blood pressure cuff monitors changes in blood pressure. Lying is thought to increase blood pressure.
- Pulse rate – Pulse or heart rate is measured by a photoplethysmograph on the finger. Increased pulse rate signal lying.
- Respiration – Breathing rates and depth are measured by pneumographs around the chest and abdomen. Irregular breathing may indicate lying.
- Skin conductivity – Electrodermal activity on the skin changes with sweating, which is caused by nervousness from lying.
lie detector test follows a structured format, starting with a pre-test interview where questions are reviewed. The examiner calibrates the baseline readings for each physiological measure. During the actual test, the subject answers a series of questions, some irrelevant, some relevant, and some control questions. Measurements during relevant questions are compared to the baseline and control questions to determine if lying is occurring.
While polygraphs remain popular, especially with law enforcement, their reliability remains highly controversial. Critics point to several issues:
- The polygraph only detects general stress levels, not specific lies. All the questions seem stressful to an innocent subject.
- Simple countermeasures like pressing toes to the floor alter readings and produce incorrect results.
- The examiner interprets the results subjectively, leaving room for bias and error. Results depend heavily on the examiner’s training and integrity.
- Sociopaths, hardened criminals, and spies remain calm under stress and fool the test.
- Conversely, innocent people fail due to being anxious or having medical conditions that alter results.
Studies on polygraph accuracy find correct lie detection ranging widely from around 50% to over 90%, depending on factors like the use of countermeasures. The lack of a consensus on accuracy raises doubts about its suitability as evidence.
Federal standards &and uses
US federal agencies use polygraphs for security screening of employees and applicants. Standards for polygraph testing were set in 1978 by the Employee Polygraph Protection Act to limit abuse. They include:
- Right to Refuse – Polygraph subjects cannot be forced and cannot be punished for refusing.
- Certified Examiners – An accredited school must provide at least 6 months of training.
- Limited Use – Private sector employers generally cannot use polygraphs for pre-employment screening or random checks. They are mostly limited to the investigation of economic loss.
- Question Restriction – Only relevant questions important to occupational objectives are asked.
Despite reliability concerns, polygraphs remain in use for government background checks, especially in intelligence and law enforcement agencies. Subjects who fail polygraphs may not be prosecuted but denied employment or security clearance.
