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Polygraph presentation

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Polygraph presentation

  1. 1. POLYGRAP H Guided By : Prof. Vidita Tilva Prepared By : Paxaj Shukla (12BIC056) Aasheesh Tandon (12BIC044)
  2. 2. WHAT IS A POLYGRAPH? • A polygraph, often called a “Lie Detector,” is a machine that measures human responses to questions, measuring a lot of physiological symptoms of anxiety or emotion to estimate if the subject is being truthful or not.
  3. 3. A BRIEF HISTORY • William Moulton Marston : creator of the systolic blood pressure test - one component of the modern polygraph. • John Augustus Larson : first American police officer to use polygraph in criminal investigations. • Leonarde Keeler : was the co-inventor of the polygraph. John Augustus LarsonWilliam Moulton Leonarde Keeler
  4. 4. HOW DOES IT WORK ? • Polygraph machine records multiple signals using 4 to 6 sensors attached to the patient’s body. The sensors usually record: • The person's breathing rate • The person's pulse • The person's blood pressure • The person's perspiration
  5. 5. HOW DOES IT WORK ?  The Nervous System reacts to different situations differently. For this, it uses different parts of it, namely : I. Somatic Nervous System II. Autonomic Nervous System a) Sympathetic Nervous System b) Parasympathetic Nervous System Arousal  Increased ANS activity  Lie
  6. 6. THE INSTRUMENTATION • Cardio – Sphygmograph : collects blood pressure and heart rate data • Pneumograph : measures respiratory patterns • Galvanograph : measures electro-dermal activity or sweat
  7. 7. ADMINISTRATION OF THE TEST • Familiarizing the subject with the test • Pre-test Interview • Formulating Questions • Analysis of the result
  8. 8. TYPICAL QUESTIONNAIRE The Relevant – Irrelevant Test Comparison Question (Control Question) Test Reid Comparison Question Test Zone Comparison Test Concealed Information Test Peak-of-tension Test
  9. 9. ACCURACY • U.S. Attorney General John Ashcroft concedes that polygraphs used in federal agencies have an error rate of about 15% . Some critics even assert that credible scientific research has found that commonly used polygraphs have error rates of 40% or more, only slightly better than flipping a coin to decide if a subject is lying . • A truth-teller might recognize that a question has significance in the investigation and exhibit the same increased cardiovascular and sweat activity as a liar. This innocent subject may be wrongly accused of lying, with very serious and unjust consequences. • Some people, such as spies, are trained to disguise their feelings, can lie very naturally, and can easily pass polygraph tests.

Notas del editor

  • A student in experimental psychology at Harvard University, William M. Marston invented the modern polygraph prior to 1921. His treatise The Lie Detector Test on understanding physiological responses related to deception was published in 1938. John A. Larson, a police officer in Berkeley, California, modified Marston's polygraph, developing a technique for continuous recording of physiological responses. One of Larson's colleagues, Leonarde Keeler, added the gavanograph component to the polygraph. He joined the faculty of Northwestern University School of Law in Chicago in 1930 and established the Keeler Polygraph Institute of Chicago.

    Lawyer, John E. Reid played an important role in the development of questioning techniques used during a polygraph test. In a 1947 paper, he described the use of control questions to evoke emotional responses. In collaboration with Cleve Backster's work, this idea eventually became the Control Question Test (CQT), which is used by the majority of forensic psychophysiologists today.


    During the 1960s and 1970s, the polygraph business grew rapidly.


    During the late 1970s and early 1980s, the use of the polygraph by the military and security agencies expanded drastically.
    In the 1980s the scientific validity of polygraphs was brought into question by psychologists. In 1988, the federal Polygraph Protection Act was passed, prohibiting employers from using polygraphs for employment screening.
  • The Nervous System reacts to different situations differently. For this, it uses different parts of it. Namely, somatic And autonomic nervous system.

    Somatic nervous system controls organs under voluntary control (mainly muscles) and the autonomic nervous system regulates individual organ function and homeostasis, and for the most part is not subject to voluntary control.

    Lying involves the Autonomic Nervous system. It's commonly believed that when someone lies, changes occur such as increased heart rate. It’s neater, for the physiological systems of the body, to tell the truth instead of lying. Lying, in normal and mentally healthy beings (not sociopathic), causes the body to develop “mental stress”.

    The Autonomic Nervous System is most important in two situations, those emergency situations that cause stress and require us to "fight" or take "flight" (run away) and those non-emergency situations that allow us to "rest" and "digest".
    The autonomic nervous system is responsible for monitoring conditions in the internal environment and bringing about appropriate changes in them. There are two major components of the autonomic nervous system, the sympathetic and the parasympathetic systems.
    When the subject has an emotional response associated with the telling of a falsehood, the sympathetic NS tends to fire off all at once (flight or fight response) in such situations and all sorts of physiological functions are altered. For example, heart rate goes up, pattern of respiration changes, blood pressure goes up, and production of sweat by glands in the skin of the hand goes up. Thus Arousal  Increased ANS activity.
    Specific patterns of arousal during questioning could indicate guilt or lying.
  • Using the polygraph instrument, physiological data is collected from three major systems, including (a) the cardiovascular system – heart rate, blood pressure, and blood volume; (b) the respiratory system – movement of the chest cavity; and (c) the endocrine system – sweat gland activity. The polygraph instrument may also detect other responses like arm and leg movement 

    Respiratory patterns are measured by pneumographs, which are devices that record thoracic movements or volume change during respiration. One of the pneumograph tubes is strapped around the chest of the examinee and the other is placed around his or her abdomen. Each pneumograph is connected to the polygraph machine by an air-filled rubber tube. When the examinee breathes in and out, the air pressure in the tubing changes to reflect his or her breathing rate and frequency. The polygraph instrument records these responses. Respiratory responses that may point toward deception include shortness of breath and laboured breathing. 

    Cardio-sphygmograph component of the polygraph. An arm-encircling cuff that is placed around the upper arm collects blood pressure and heart rate data. The cuff is filled with air and connected to the polygraph machine by air-filled tubes. As blood pumps through the examinee’s arm, changes in blood pressure will alter the amount of air pressure in the cuff; these changes are recorded by the machine. Physiological responses such as a faster heart rate and an increase in blood pressure may suggest deception  

    Galvanograph. The measurement of electro-dermal activity – or sweat – is conducted by fingerplates that are attached to two of the examinee’s fingers. These plates measure the skin’s ability to conduct electricity. Dry skin is a poor conductor of electricity. If the examinee perspires, however, then the water and salt from the sweat will reduce resistance and allow a larger amount of electric current to travel along the surface of the skin. This increase in current reflects the amount of sweat produced in the examinee’s fingertips. Polygraph practitioners believe that an individual sweats more when placed under stress, and so sweaty fingertips may point toward deception
  • Familiarizing the subject with the test
    This includes familiarizing the subject with the examination room, polygraph instrument & attachments, giving instructions about the manner in which he has to sit through for examination, attaching the accessories, recording of normal physiological responses and instructing the examinee regarding the administration of the questionnaire.
    Pre-test Interview
    During the polygraph test, the examiner and the subject are alone in the questioning room. Before the test begins, the examiner spends about an hour talking with the subject. Most forensic psychophisiologists consider this pretest phase an extremely important part of the polygraph. The examiner obtains a baseline read on his or her emotional state and develops the questions that are asked during the actual test. Before the test begins, the examiner goes over each question with the subject so that he or she knows exactly what to expect. When they are ready start, the person administering the polygraph attaches the various components of the polygraph instrument to the examinee.
    Formulating Questions
    Different perspectives are employed using different sets of questions.
    Analysis of the result
    Employing the above mentioned knowledge, the results obtained from the polygraph are compared to a global scale. As a general rule, the frequency of the patterns obtained increases during ANS Shooting of the impulses, which might indicate a falsehood, on the part of the subject.
  • Relevant – Irrelevant test : the relevant-irrelevant test format compares examinee responses to relevant and irrelevant questions. A relevant question is one that deals with the real issue of concern to the investigation. These questions include asking whether the examinee perpetrated the target act or knows who did it and perhaps questions about particular pieces of evidence that would incriminate the guilty person. An irrelevant question is one designed to provoke no emotion (e.g., “Is today Friday?). Irrelevant questions are typically placed in the first position of a question list because the physiological responses that follow the presentation of the first question are presumed to have no diagnostic value; they are also placed at other points in the question sequence. Guilty examinees are expected to show stronger reactions to relevant than to irrelevant questions; innocent examinees are expected to react similarly to both question types.

    Comparison question tests (also called control question tests) compare examinees’ responses to relevant questions to their responses to other questions that are believed to elicit physiological reactions from innocent examinees. Relevant questions are defined as in the relevant-irrelevant test. Comparison questions ask about general undesirable acts, sometimes of the type of an event under investigation. For example, in a burglary investigation, one comparison question might be “Have you ever stolen anything?” In probable-lie comparison question tests, the instructions are designed to induce innocent people to answer in the negative, even though most are lying. Innocent examinees are expected to experience concern about these answers that shows in their physiological responses. In directed-lie tests, examinees are instructed to respond negatively and untruthfully to comparison questions (e.g., “During the first 20 years of your life, did you ever tell even one lie?”). In both forms of test, the expectation is that innocent examinees will react more strongly to the comparison questions, and guilty examinees will react more strongly to relevant questions.

    The Reid comparison question test, also known as the modified general question test, was the earliest form of comparison question test. It includes probable-lie comparison questions and is interpreted by the examiner’s global evaluation of the charts, combined with other observations made during the examination. Other characteristics of the test include a discussion of the examinee’s moral values during the test procedure and the use of a “stimulation” test between the first and second presentations of the questions.

    The zone comparison test is named for the three “zones” or blocks of time during the test: the relevant questions (called the red zone), the probable-lie comparison questions (the green zone), and other questions (the black zone). Black zone questions are included to uncover examinee concerns about an issue outside of the scope of the red and green zones, such as involvement in another crime. Each zone is presumed to be threatening to someone; however, depending on the examinee’s mental set, it is anticipated that one particular zone is more threatening than are the other two.

    Concealed information tests (more often called guilty knowledge or concealed knowledge tests) present examinees with sets of very similar items, much in the manner of stimulation tests, except that the similar items include one true and several (usually, four) false details of some aspect of an incident under investigation that has not been publicized, so that the true answer would be known only to the investigators and to those present at the incident. In a burglary, examinees might be asked about several possible points of entry into the house, one of which the burglar actually used. (For more detail about question construction and administration of concealed information tests, see Nakayama [2002].) When an examinee is asked whether he or she used each of these routes, the answer is expected to be negative regardless of the examinee’s innocence or guilt. Guilty examinees are expected to reveal their concealed knowledge by responding more strongly to the true item than to the others.
    Concealed information tests are applicable only under restricted conditions: when there is a specific incident, activity, or thing that can be the subject of questioning and when there are several relevant details that are known only to investigators and those present at the incident. Thus, these tests are not applicable in typical screening situations in which the only possible relevant questions concern generic events, such as unspecified acts of espionage that may or may not have occurred.

    The peak-of-tension test is similar in format to concealed information tests, but is distinct because questions are asked in an easily recognized order (e.g., “Was the amount of stolen money $1,000? $2,000? $3,000?” etc.). A guilty examinee is expected to show a pattern of responsiveness that increases as the correct alternative approaches in the question sequence and decreases when it has passed. Stimulation tests often have this format. In a known-solution peak-of-tension test, the examiner knows which alternative is the one truly connected to the incident and evaluates the examinee’s pattern of responses for evidence of involvement in the incident. It is also possible to use the peak-of-tension test in a searching mode when the examiner does not know which answer is connected to the event but wants to use the test for help in an investigation. It is assumed that the pattern of a guilty person’s autonomic responses will reveal the correct answer.




  • Despite the subjectivity and perhaps spurious assumptions associated with the polygraph, it is still widely used throughout the nation. The range of error in which the polygraph operates is at the root of the debate over whether polygraph tests should be administered at all. U.S. Attorney General John Ashcroft concedes that polygraphs used in federal agencies have an error rate of about 15% . Some critics even assert that credible scientific research has found that commonly used polygraphs have error rates of 40% or more, only slightly better than flipping a coin to decide if a subject is lying .
    These errors are not usually in the physical measurements made by the machine, but rather are embedded in the assumption that physiological conditions can indicate the psychological state of lying. Critics question the theory by which the polygraph operates, rather than the quantitative measurements it provides. It may be generally true that when people lie, they exhibit certain physiological phenomena. However, there may be other reasons for an honest subject to demonstrate these same physiological signs. For example, a truth-teller might recognize that a question has significance in the investigation and exhibit the same increased cardiovascular and sweat activity as a liar. This innocent subject may be wrongly accused of lying, with very serious and unjust consequences.
    Some people, such as spies, are trained to disguise their feelings, can lie very naturally, and can easily pass polygraph tests. Furthermore, since polygraphs rely heavily on interpretation by polygraph examiners, human error and bias can create inaccuracy in the results. Why, then, is the polygraph still used today? First, the willingness to undergo a polygraph test often provides proof that the subject has nothing to conceal. Second, the response to certain questions in the polygraph tests can sometimes lead investigators to new focuses . Finally, fear of the test can prompt the guilty to confess.

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