Lie Detection and Interrogation
Polygraph - popularly referred to as lie detector. - measures and records several psychological indices such as blood pressure, pulse, respiration and skin conductivity while the subject is asked ans answers a series of questions.
- literally means "many writings".
- is derived from two Greek words "Poly"-
which means many and "Graphos"-
which means writing.
Personalities who contributed to the development of the polygraph machine.
Two types of present day polygraph instrumentation
1. Control Question - preliminary information question
2. Irrelevant Question -
3. Relevant Question
- In a polygraph test, the types of questions
alternates, the test is passed if the physiological
responses during the probable lie control questions
are larger than those during the relevant
Lie Detection - is the practice of determining whether someone is lying.
Three Basic Approaches To The Polygraph Test
1. The Control Question Test (CQT) - this test
compares the physiological response to relevant
questions about the crime with the response to
questions relating to possible prior misdeeds. This
test is often used to determine whether certain
criminal suspects should be prosecuted or classified
as uninvolved in the crime.
2. The Directed Lie Test (DLT) - this test tries to detect
lying by comparing physiological responses when
the subject is told to deliberately lie to responses
when they tell the truth.
3. The Guilty Knowledge Test (GKT) - this test
compares physiological responses to multiple choice
type questions about the crime, one choice of which
contains information only the crime investigators
and the criminal would know about.
3 Phases Of A Polygraph Examination
1. Pre-Test Phase - the examiner discusses with the
subject the test issue, review the test questions
that will be ask during the test and assess the
subjects emotional and physiological suitability to
undergo the polygraph test.
2. Testing Phase - subjects physiological responses
are recorded as the subject answers a set of
questions reviewed earlier.
3. Post Test Phase - examiner reviews test data
obtained and interprets the polygraph chart.
Conclusions That A Polygraph Examiner May Reach
1. Subject is telling the truth
2. Subject is not telling the truth
3. The result is inconclusive
* A polygraph test normally lasts between one and a
half hours to two and a half hours.
* Polygraph test result in the philippines is not
admissible in evidence as proof of the guilt of an
* Polygraph test is voluntary.
* The accuracy of polygraph test is about 90%
provided the examiner is competent and the
polygraph machine is in good working condition.
* Polygraph is an investigative tool.
* The principle behind a lie detection test is that when
the subject hears a questions which he or she
intends to lie, the brain interprets and triggers
automatic and uncontrollable physiological changes
captured by the polygraph.
Who Uses The Polygraph
1. Law Enforcement Agencies
2. Legal Community
3. Private Sector
Kinds of Errors in a Polygraph Test
1. False Positive - occurs when a truthful examinee
is reported as being deceptive.
2. False Negative - when a deceptive examinee is
reported as truthful.
Causes of Polygraph Errors
1. Failure of examiner to properly prepare the
examinee for the examination.
2. Misreading of the physiological data on the
3. Defective polygraph machine.
Reactions - changes in blood pressure, pulse rate,
breathing and sweat activity.