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Written by: Katherine To
Written on: December 6th, 2002
Tags: security & defense, biomedical engineering
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About the Author
During Fall 2002, Katherine was a junior majoring in Biomedical (Biochemical) Engineering and minoring in Business. In her spare time, she likes to get involved in campus activities. She also enjoys cooking and traveling.
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Volume V Issue I > Lie Detection: The Science and Development of the Polygraph
The idea of using a polygraph for lie detection emerged at the end of the nineteenth century. Despite major technological advances in the twentieth century, the polygraph has evolved little since its invention. The polygraph works under the assumption that lies can be detected by certain measurable physiological changes, a theory proposed more than 100 years ago. The three physiological vectors measured in polygraphs in the 1920s cardiovascular, respiratory and perspiratory activities are still measured by modern polygraphs. Although the polygraph is still widely used, critics question its accuracy because of the subjectivity involved in the test. Recent technological breakthroughs have created new machines for lie detection. However, since many of these new technologies still operate under questionable assumptions, they are still prone to error and misinterpretation.

Introduction

You are seated in a room, alone with an examiner. The examiner hooks wires and straps to your body. You are nervous and your heart begins to pound. He starts asking you questions. You try to stay calm, since you are innocent and have nothing to conceal. However, agonizingly, you feel your hands sweat, as you worry that the metal box and computer next to you will devastate your future.
This is one possible reaction that a test subject might have to the type of polygraph tests that are conducted throughout the nation every year. Although polygraph tests are inadmissible as evidence in most states, polygraphs are administered to many witnesses and suspects to streamline police investigations and project new directions of examinations. Over 20 federal agencies use polygraphs to screen job applicants and current employees [1]. Occasionally, polygraphs make their way to the news headlines and provoke heated debate. For example, when California Congressman Gary Condit was involved in the scandal of missing intern Chandra Levy, there was much debate about whether he should submit to a polygraph test. Under media pressure, he took the polygraph test and passed. A few months later, police linked Levy's murder to a criminal in the Washington, D.C., area. In this case, the polygraph provided accurate results, but serious doubts remain about validity of the test in all cases. The ability to critically evaluate the accuracy of the polygraph is predicated on an understanding of the fundamental theories behind the device's functionality.

The Birth of Polygraph

For centuries, humans have sought a reliable means to detect lies. In ancient Hindu and Chinese civilizations, authorities "detected" lies by asking the suspect to chew a grain of rice and spit it out. In China, a dry grain of rice would be indicative of the dry mouth of a liar [2]. In India, rice stuck to the mouth would be the sign of guilt [1]. Although these methods were primitive and non-scientific, they nonetheless highlight the fundamental assumption humans make in lie detection: the psychological state of lying can be detected by physiological signs.
Development of Underlying Assumptions in Modern Polygraphs
Of the three physiological signs recorded in modern polygraphs, cardiovascular activity was the first sign to be associated with lying. This association was first documented by an Italian criminologist named Lombroso in the late nineteenth century [3]. Lombroso noted an increase in a subject's blood pressure and heart rate when the subject was giving untruthful answers [3].
In the 1910s, some studies suggested that the ratio of inhaling and exhaling time during breathing changes when a person lies [3]. Using this theory along with Lombroso's findings, John Larson, a medical student working for the Berkeley Police Department, invented the first polygraph in 1921 [2]. This first polygraph simultaneously traced a subject's blood pressure and respiration [3]. Under Larson's assumptions, irregularities in blood pressure and breathing patterns would indicate lies.
The third physiological channel used in modern polygraphs "skin resistance" was added later in the 1920s by Leonarde Keeler [2]. Keeler assumed that a lying person would sweat more than a truthful person, which would decrease skin resistance due to a higher concentration of negatively charged chloride ions on the surface of the skin. By adding an extra metric for detecting lies, scientists had hoped to decrease the probability of errors in the polygraph test. Since this addition, there have been no new underlying assumptions introduced to improve the polygraph.

Recent Polygraph Designs

Technological breakthroughs in past decades have improved the retrieval and analysis of data obtained from a subject during a polygraph. However, the fundamental assumptions behind the polygraph and the parameters measured have not changed in over eighty years. For example, in the 1970s and 80s, the analog polygraph was used. This machine converted physiological responses into electrical signals which induced needles to move and record the physiological data on a scrolling roll of paper [4]. Although the analog polygraph incorporated advances in electrical engineering and helped examiners to conveniently record data, it still worked on the same principles as the first polygraphs.