Polygraph
Definition
An instrument that simultaneously records multiple physiological channels including blood pressure, respiration rate, galvanic skin response, and sometimes heart rate while the subject answers a series of questions. Deception is inferred from physiological changes assumed to accompany lying. Accuracy estimates vary widely and the technique is not generally accepted as scientifically reliable in most legal systems.
- Also called
- Lie detector test
- Channels recorded
- Blood pressure, respiration, skin response, sometimes heart rate
- Basis
- Physiological arousal inferred to accompany deception
- Legal status
- Not generally accepted as scientifically reliable
- Common use
- Investigative lead, rarely admissible as trial evidence
Common questions
Why do most courts refuse to admit polygraph results as evidence of guilt?+
The underlying assumption that lying produces a distinct and reliably measurable physiological signature has not held up under rigorous scientific testing, with error rates and countermeasure vulnerability that fall well short of the reliability standards courts apply to forensic evidence.
If polygraph results are usually inadmissible, why do investigators still use the test?+
Investigators use it as an interview tool to generate leads or prompt admissions, since the process of undergoing the test and discussing the results with an examiner sometimes leads a subject to confess or reveal inconsistencies, independent of whether the chart itself would hold up in court.
Can a subject learn to beat a polygraph test?+
Countermeasures such as controlled breathing, muscle tension, or mental distraction during control questions have been shown in research to reduce detection accuracy, which is one of the core scientific criticisms used to challenge the technique's reliability.
Related terms
- Narco-Analysis
- A technique in which a subject is administered a barbiturate or benzodiazepine drug (commonly sodium pentothal or midazolam) to induce a hypnotic...
- Article 20(3) (Indian Constitution)
- The constitutional provision that states no person accused of any offence shall be compelled to be a witness against themselves. India's Supreme...
- BEOS
- Brain Electrical Oscillation Signature profiling, developed by Champadi Raman Mukundan and validated 2003 at NIMHANS Bangalore; uses 32-channel EEG to classify probes...
- Brain Electrical Activation Profile (BEAP / P300 Test)
- A method that uses electroencephalography (EEG) to detect the P300 event-related potential, a positive-going brainwave that occurs approximately 300 milliseconds after a...
- CQT
- Control Question Test (Reid/Keeler technique): compares physiological response to relevant questions with response to control (probable-lie) questions; the dominant US polygraph format.
- Criminal Profiling
- The inference of an unknown offender's likely characteristics from crime scene behaviour, victimology, and case details. Also called offender profiling, criminal investigative...
- FBI BAU
- Behavioral Analysis Unit at the FBI Academy, Quantico, evolved from the Behavioral Science Unit set up by Howard Teten and Pat Mullany...
- GKT / CIT
- Guilty Knowledge Test, also called Concealed Information Test (Lykken, 1959): tests whether the subject recognises crime-specific details only the perpetrator would know;...
- Modus Operandi
- How the offender commits the crime: the functional behaviour needed to complete it; evolves with experience and feedback.
- Organised vs Disorganised
- The BSU dichotomy proposed by Ressler and Douglas, classifying offenders by scene presentation. Organised scenes show planning; disorganised scenes show chaos. The...
- P300
- Event-related brain potential occurring about 300 ms after a meaningful stimulus; Lawrence Farwell's 'Brain Fingerprinting' uses P300 amplitude to flag concealed recognition...
- Section 23, Bharatiya Sakshya Adhiniyam 2023 (Discovery Rule)
- The provision (successor to Section 27 of the Indian Evidence Act 1872) under which information given by a person in police custody...
Explained in these topics
- Criminal Profiling, Polygraphy, Narco-Analysis and Brain MappingMulti-channel physiological recorder measuring blood pressure, pulse, respiration (thoracic and abdominal) and galvanic skin response, used to infer deception...
- Narco-Analysis, Polygraph and Brain Mapping in Law