Polygraph – Lie detector
A polygraph, also known as a lie detector, is a tool that simultaneously measures and records various physiological signals while an individual is asked a series of questions. The principle behind its operation is that deceptive responses will produce detectable physiological changes different from those associated with truthful responses.
The device consists of several components:
- Pneumographs: These monitor breathing by placing tubes around the chest and abdomen to measure the depth and rate of respiration.
- Cardio cuffs: These record heart rate and blood pressure, typically placed on the arm or finger.
- Galvanometers: They measure skin conductivity, reflecting changes in sweat gland activity. Electrodes attached to the fingers or palm capture these changes.
- Chart recorder or computer: Modern polygraphs often use computerized systems to record and analyze the physiological responses detected during the test.
The polygraph test involves several phases:
- Pre-Test Phase: The examiner explains the procedure and discusses the questions with the subject. Baseline physiological readings are established by asking neutral questions.
- Test Phase: Relevant, control, and irrelevant questions are asked. Relevant questions relate to the issue being investigated, control questions are designed to provoke a physiological response regardless of the truth, and irrelevant questions serve as neutral benchmarks.
- Data Collection: Physiological responses are measured and recorded during questioning, including changes in respiration, heart rate, blood pressure, and skin conductivity.
- Data Analysis: The examiner analyzes the collected data, comparing responses to relevant and control questions to detect patterns indicating deception.
- Interpretation: Based on the analysis, the examiner forms an opinion on the subject’s truthfulness. However, polygraph results can be influenced by various factors, including examiner skill and individual physiological reactivity.
- Post-Test Phase: Results may be discussed with the subject and relevant parties, potentially informing further investigation or decision-making.
In forensic applications, polygraphs are sometimes used as investigative tools to assist law enforcement agencies in evaluating the credibility of witnesses or suspects, particularly in cases where there’s a lack of concrete evidence. However, it’s important to note that polygraph results are not admissible as evidence in many courtrooms due to concerns about their reliability and accuracy.
Polygraph tests are most commonly used in:
- Criminal Investigations: They may be used to help law enforcement narrow down suspects or obtain confessions.
- Pre-Employment Screenings: Some government agencies and private companies use polygraph tests as part of their screening process, especially for positions involving security clearance or sensitive information.
- Probation and Parole: Polygraph examinations may be used as part of the monitoring process for individuals on probation or parole to assess compliance with conditions.
- Sex Offender Management: In some jurisdictions, polygraph tests are used as part of sex offender treatment and management programs to monitor behavior and adherence to treatment plans.