POLYGRAPH

Guided By :

Prof. Vidita Tilva

Prepared By :

Paxaj Shukla (12BIC056)

Aasheesh Tandon (12BIC044)

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.

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 Larson William Moulton Marston

Leonarde Keeler

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

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

THE INSTRUMENTATION

• Cardio – Sphygmograph : collects blood pressure and

heart rate data

• Pneumograph : measures respiratory patterns

• Galvanograph : measures electro-dermal activity or sweat

ADMINISTRATION OF THE TEST

• Familiarizing the subject with the test

• Pre-test Interview

• Formulating Questions

• Analysis of the result

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

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.