Reading a polygraph, often called a "lie detector," involves analyzing physiological responses recorded during a structured questioning process. It's not about literally "reading" truthful or deceptive statements, but rather interpreting patterns of change in a person's bodily functions thought to be associated with emotional arousal, which in turn is sometimes linked to deception.
Understanding the Polygraph Test
A polygraph test measures several physiological indicators simultaneously as a person answers a series of questions. The device typically records:
- Blood Pressure and Heart Rate: Using a blood pressure cuff or plethysmograph.
- Respiration: Measuring breathing rate and depth via pneumatic tubes placed across the chest and abdomen.
- Skin Conductivity (GSR - Galvanic Skin Response): Assessing perspiration levels through electrodes attached to the fingers.
These measurements are recorded on a chart or computer screen, creating distinct tracings for each physiological response.
The Questioning Process
The examiner uses different types of questions in a specific order:
- Irrelevant Questions: Basic questions with known answers (e.g., "Is your name John?"). These establish a baseline for normal physiological responses.
- Control Questions (Comparison Questions): Questions designed to provoke anxiety in a truthful person about past misdeeds (e.g., "Before age 20, did you ever lie to get out of trouble?"). These are used as a comparison point for the relevant questions.
- Relevant Questions: Direct questions about the specific matter being investigated (e.g., "Did you take the money from the office safe?").
Interpreting the Results
The core of reading a polygraph involves comparing the physiological responses to the relevant questions against the responses to the control questions. The theory is that a deceptive person will exhibit stronger physiological reactions (e.g., increased heart rate, erratic breathing, higher skin conductivity) when lying about the relevant question compared to their reaction to the control questions, whereas a truthful person will show a similar or stronger reaction to the control questions designed to evoke general anxiety.
Examiners analyze the charts, assigning numerical scores to the responses based on the magnitude and consistency of the physiological changes for each question type.
Scoring the Responses
According to scoring methods, responses to relevant and control questions are compared:
- Negative values indicate a higher reaction on the relevant questions.
- Positive values indicate a higher response on the control questions.
Scores are typically assigned to individual questions and then summed to get a grand total score.
| Grand Total Score | Interpretation |
|---|---|
| +6 and greater | Nondeception |
| -6 and less | Deception |
| Between -6 and +6 | Inconclusive |
As the reference states: "The negative values indicate higher reaction on the relevant questions and the positive values indicate higher response on the control questions. A grand total score of +6 and greater indicates nondeception, -6 and less deception, and anything in between is considered an inconclusive result."
A professional examiner analyzes these charts, considering factors like the subject's overall demeanor and any counter-measures attempted. However, interpreting polygraph results requires specific training and adherence to standardized scoring methods. The results are not considered foolproof and are often inadmissible as evidence in courts due to reliability concerns.
