How polygraph works?
Published on: Mar 4, 2016
Transcripts - Polygraph presentation
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
• John Augustus Larson : first American police officer
to use polygraph in criminal investigations.
• Leonarde Keeler : was the co-inventor of the
John Augustus LarsonWilliam Moulton 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
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
• 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
The Relevant – Irrelevant Test
Comparison Question (Control Question) Test
Reid Comparison Question Test
Zone Comparison Test
Concealed Information Test
• 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