Inside the polygraph room – What happens during a lie detector test?

Public imagination has long been captivated by polygraphs, commonly called lie detector tests. Polygraphs have been used as all-knowing truth arbitrators in TV shows and movies for decades. The polygraph works, but how? What are the results of polygraph tests?

It helps to understand what exactly the polygraph machine is measuring. The polygraph works by tracking changes in a person’s physiological response patterns when they are asked questions. It records several bodily functions simultaneously, including:

  • Blood pressure
  • Pulse
  • Respiration
  • Skin conductivity (sweat gland activity)

Small pads connected to the machine are placed on the person’s body to measure these functions. The idea is that when someone lies, their body betrays subtle signs of stress or anxiety. Their heart rate increases, their breathing quickens, and their skin becomes clammy. The polygraph aims to capture these physiological changes.

The key to ‘reading’ the results lies in asking control questions. The lie detector test administrator asks wide-ranging questions, some irrelevant, some pertinent to the issue at hand. The responses to benign questions establish a baseline. When relevant questions are asked, physiological changes from the baseline indicate deception.

Pre test protocol

Before strapping sensors to someone’s body, a polygraph examiner follows certain protocols to prepare the subject and ensure accurate results.

  • Pre-test interview: The examiner will discuss the test process and have the subject sign consent forms. They also talk through the issue under investigation to frame relevant questions.
  • Establish a baseline: They will engage in small talk to gauge the subject’s normal physiological responses and ask broad questions with predictable answers like “Are you over 21 years old?”
  • Review questions: The examiner will review the exact questions to be asked and allow the subject to offer explanations or clarify responses.
  • Instruct on procedures: They will provide instructions on sitting still, limiting movement, and remaining silent except when responding to questions.

Polygraph test step by step

Once the preliminary protocols are complete, it’s time for the actual polygraph test. The procedures unfold in a precise sequence:

  • Sensor placement: The examiner attaches blood pressure cuff, fingertip sensors, and respiratory belts in proper locations.
  • Baseline phase: The test begins with neutral questions unrelated to the investigation. This establishes a comparative baseline for the subject’s physiological patterns.
  • Relevant questions: The examiner moves to pertinent questions about the investigation, scattered among other benign questions. Responses are compared to the baseline.
  • Break: The test pauses for a few minutes before continuing.
  • Stimulation phase: Additional questions are asked, some relevant, some not. Reactions indicate deception.
  • Second baseline: Neutral questions reestablish a baseline at the end. This assesses changes as the test progresses.
  • Post-interview: The examiner conducts an interview to review responses and allow the subject to explain reactions.

The entire polygraph test takes about 1 to 2 hours. The examiner makes careful notes of responses and changes in vital signs at every stage.