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Practice Exams

Module 7.14: Polygraphy & Lie Detection

I. The Theory of Polygraphy

The use of the polygraph is based on the Psycho-physiological Principle: a person's fear of being detected in a lie will produce an involuntary, uncontrollable physiological response.

  • The "Fight or Flight" Response: When a person perceives a threat (in this case, the threat of their lie being discovered), their autonomic nervous system triggers a "fight or flight" response.
  • Physiological Changes: This response causes a number of measurable changes in the body, including:
    • Increased heart rate and blood pressure.
    • Faster and shallower breathing.
    • Increased sweating.
  • The Polygraph's Role: The polygraph instrument does not detect lies. It simply measures and records these physiological changes. It is the polygraph examiner who interprets the charts and forms an opinion as to whether the person's responses are indicative of deception.

Board Exam Tip: This is a critical distinction. The polygraph is not a "lie detector." It is a recorder of physiological arousal. The human examiner is the "detector."

III. The Polygraph Examination Process

A professional polygraph examination is a lengthy and systematic process that consists of four main phases.

Phase 1: Pre-Test Interview

  • This is the longest and arguably most important part of the examination.
  • The examiner meets with the subject, explains the process, and discusses the case in detail.
  • The examiner explains the purpose of the test and advises the subject of their constitutional rights.
  • Crucially, the examiner formulates the exact questions that will be asked during the test and reviews every single question with the subject before the test begins. There should be no surprise questions.

Phase 2: The Examination (Chart Collection)

  • The subject is attached to the polygraph instrument.
  • The examiner conducts an "acquaintance test" to familiarize the subject with the process.
  • The examiner then asks the prepared set of questions 2 to 3 times, with a short break in between each run-through.

Phase 3: Post-Test Interview / Interrogation

  • If the charts indicate deception, the examiner will often conduct a post-test interview to give the subject an opportunity to explain their responses.
  • This can be a very effective tool for eliciting a confession. The subject, believing the "machine" has caught them, may decide to confess.

Phase 4: Chart Analysis and Diagnosis

  • The examiner analyzes the physiological data recorded on the charts, looking for significant, timely, and consistent responses to the relevant questions.
  • Based on this analysis, the examiner will render one of three opinions:
    • No Deception Indicated (NDI): The subject appears to be truthful.
    • Deception Indicated (DI): The subject appears to be deceptive.
    • Inconclusive (INC): The results are unclear and no opinion can be formed.

V. Legal Status and Validity

  • Not Admissible in Court: In the Philippines, the results of a polygraph examination are not admissible in court as evidence of guilt or innocence. The scientific community has not universally accepted its reliability, and it is considered to fall short of the standard for scientific evidence.
  • Investigative Aid: Despite its inadmissibility, law enforcement agencies (like the PNP and NBI) use it extensively as an investigative tool.
    • It can help narrow down a list of suspects.
    • It can be very effective in inducing confessions.
    • It can help verify the statements of witnesses.
  • Pre-Employment Screening: It is also widely used for pre-employment screening for sensitive positions in law enforcement and intelligence agencies.

Issues of Accuracy

  • False Negatives: A deceptive person who feels no fear or guilt (e.g., a psychopath) or who has been trained in countermeasures (e.g., a spy) might be able to "beat" the test.
  • False Positives: A truthful person who is extremely nervous might react strongly to the relevant questions, leading to a false indication of deception.

Introduction

The quest for a reliable method of detecting deception is as old as crime itself. The most well-known, and most controversial, of these methods is Polygraphy. The polygraph, popularly known as the "lie detector," is not actually a lie detector. It is an instrument that measures and records several physiological responses from the human body on the theory that a person's fear of detection will cause these responses to change when they are being deceptive. This module will explain the theory behind polygraphy, the components of the instrument, the examination process, and its limited role in the criminal justice system.

Key Learning Objectives:

  • Explain the psycho-physiological theory behind the polygraph.
  • Identify the four major components of a modern polygraph instrument.
  • Describe the stages of a polygraph examination.
  • Differentiate between relevant, irrelevant, and control questions.
  • Understand the legal status of polygraph results in Philippine courts.

II. The Polygraph Instrument

The term "polygraph" literally means "many writings." A modern polygraph instrument simultaneously measures and records at least four physiological responses.

  1. Cardio-sphygmograph (Cardio) Component:

    • Measures: Changes in blood pressure and heart rate.
    • How it Works: A standard blood pressure cuff is attached to the subject's upper arm.

  2. Pneumograph Component:

    • Measures: Changes in breathing (respiration rate and depth).
    • How it Works: Two convoluted rubber tubes are placed around the subject's chest and abdomen.

  3. Galvanograph (Galvanic Skin Response - GSR) Component:

    • Measures: Changes in the skin's electrical resistance, which is related to sweating.
    • How it Works: Two small electrodes are attached to the fingers of one hand. As the sweat glands open, the skin's resistance to the small electrical current decreases.

  4. Kymograph Component:

    • Measures: This is the recording part of the instrument. In older analog polygraphs, it was a motorized chart recorder that pulled paper under a set of ink pens. In modern computerized polygraphs, this is the computer screen that displays the tracings.

IV. Question Formulation

The "art" of polygraphy lies in the proper formulation and sequence of questions. There are three main types of questions used.

  1. Irrelevant Questions:

    • Purpose: To establish a baseline for the subject's normal physiological response.
    • Nature: Neutral, non-threatening questions to which the answer is known and of no consequence.
    • Example: "Are you now in the Philippines?" "Is today Wednesday?"

  2. Relevant Questions:

    • Purpose: To get a reaction from a deceptive subject.
    • Nature: Direct questions related to the specific crime being investigated.
    • Example: "Did you take that missing P50,000?" "Did you stab John Doe?"

  3. Control Questions (Probable Lie Questions):

    • Purpose: To serve as a basis of comparison. They are designed to provoke a stronger response from a truthful person than the relevant questions.
    • Nature: Broad, general questions about past misdeeds that almost everyone has committed but will be tempted to lie about.
    • Example: "Besides what you told me, have you ever stolen anything in your life?" "Have you ever lied to get out of trouble?"

The Theory of Comparison:

  • A truthful person's biggest concern is the control question (they are worried the examiner will find out about their past). They will react more strongly to the control question than to the relevant question (which they know they are innocent of).
  • A deceptive person's biggest concern is the relevant question (they are afraid of being caught for the crime). They will react more strongly to the relevant question than to the control question.

Conclusion

The polygraph is one of the most fascinating and misunderstood tools in criminalistics. It is not a magical lie detector, but rather an instrument that records the body's stress responses. In the hands of a skilled examiner, it can be a powerful investigative aid for focusing an investigation and eliciting confessions. However, its lack of universal scientific acceptance and its potential for error mean that its results must be treated with caution. It remains a tool for investigation, not for adjudication, and its findings cannot replace the need for solid, physical evidence and thorough investigative work.