How Does a Lie Detector Work

What Is a Polygraph?

A polygraph (commonly known as a "lie detector") is a technical device that simultaneously records several physiological parameters of a person while they answer questions. The device does not directly "detect lies" — rather, it registers the body's involuntary reactions that typically accompany deception.

What Does the Polygraph Measure?

Modern computer polygraphs record multiple channels of physiological data simultaneously:

• Breathing patterns: Thoracic and abdominal breathing recorded using pneumograph sensors. Changes in breathing rate, depth, and rhythm may indicate stress or deception.
• Cardiovascular activity: Blood pressure, pulse rate, and blood volume changes detected using a cardio cuff and plethysmograph.
• Electrodermal activity (EDA): Skin conductivity measured through fingertip sensors. Sweat gland activity increases during emotional arousal.
• Muscular activity: A motion sensor placed under the chair detects deliberate or unconscious movements that may indicate countermeasure attempts.

Testing Conditions

For accurate results, the polygraph examination must be conducted under specific conditions:

• A quiet, isolated room free from external distractions
• The subject must be well-rested and in normal physical condition
• No influence of alcohol, drugs, or strong medications
• Voluntary participation — the subject has the right to refuse at any time
• All questions are discussed with the subject beforehand — there are no surprise questions

Key Stages of Polygraph Testing

1. Pre-Test Phase

The examiner conducts a detailed interview with the subject. During this phase, the purpose of the test is explained, all questions are reviewed, and the subject's informed consent is obtained. This stage typically takes 30–60 minutes.

2. Sensor Attachment

Non-invasive sensors are carefully placed on the subject's body. The procedure is painless and does not cause any discomfort. The sensors record physiological data throughout the test.

3. Testing Phase

The examiner asks the pre-agreed questions while the polygraph records the subject's physiological responses. Several rounds of questioning may be conducted to ensure reliability. This phase lasts 1–2 hours.

4. Data Analysis

The recorded data is analyzed using professional software. POLYSCORE® and OSS-3 systems provide computerized scoring that minimizes subjectivity and increases the accuracy of conclusions.

5. Report Preparation

Based on the analysis, the examiner prepares a detailed written report containing conclusions about the subject's truthfulness. The report is typically ready within 7–15 business days.

Can You Fool a Polygraph?

While various "countermeasure" techniques exist on the internet, in practice they are rarely effective against an experienced examiner using modern equipment. POLYSCORE® and OSS-3 systems are specifically designed to detect countermeasure attempts, and the polygraph's multi-channel recording makes it extremely difficult to simultaneously control all physiological responses.