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April 17, 2007 > TechKnow Talk

TechKnow Talk

Do Lie Detectors Really Work?

The accused spy sits in front of a bright light, soaked in perspiration, wires trailing from his body, his interrogator demanding a confession. That's the way it works in movies, anyway. But what in reality, what is a lie detector, and how it is used? Does it even work at all?

First, let's stop calling it a lie detector. This is an unfortunate appellation; though catchy, it is not at all descriptive. The proper term is polygraph, which simply means a device that produces several graphs. A polygraph is not intended to "detect lies" per se. There is no machine that can do that. It does measure physiological changes which research indicates are associated with deceptive behavior.

More specifically, a polygraph measures heartbeat, blood pressure, respiration, body movement, and galvanic skin resistance. Heartbeat and blood pressure are measured with a blood pressure cuff, just like the one in a doctor's office. Respiration is monitored with two expandable air-filled tubes, one wrapped around the chest and the other around the diaphragm. Body movement is detected using a motion-sensitive pad in the seat of the examination chair. Small electrodes (metal plates) are attached to fingertips to measure galvanic skin resistance. The electrodes measure electrical conductivity, which is affected by both the amount of moisture (perspiration) and its chemical composition. The output from all these sensors is plotted on a paper graph for an analog polygraph, or analyzed with special software and displayed on a computer monitor in the case of the more modern digital polygraph.

Polygraphs are routinely used in the U.S. by law enforcement in crime investigation, by employers for pre-employment screening, by banks and similar institutions in internal investigations, and by various government agencies and organizations such as the NSA, CIA, and the FBI.

The examination itself consists of several phases, and may last many hours. First, the examiner will talk with the subject to establish a rapport and to derive some first impressions of the individual's psychological state. The subject may also be asked to divulge information that may affect the polygraph results, such as how much he or she slept the night before or if any alcohol or medications were consumed in the prior 24 hours.

After connecting the sensors, the examiner will ask questions to establish "baseline" responses. For example, the subject may be asked to respond truthfully to questions such as "Are you 26 years old?" or "Do you ever drink water?" and then lie to similar questions. This provides an indication of how the subject may react during and immediately following truthful and deceptive responses. Questions designed to trigger a deceptive or evasive response in most people may also be asked, such as, "Have you ever told a lie?" This is intended to cause some anxiety reaction, regardless of how it is answered.

Following the baseline questions, the examination itself will begin. A series of questions will be developed by the examiner and reviewed with the subject prior to beginning. Some will be "control" questions, unrelated to the matters of interest. Sprinkled among the control questions will be those relevant to the matter at hand. These will be very direct and unambiguous. For example, in the case of a criminal investigation, questions such as "Were you in San Jose on the evening of March 27th, 2007?" or "Did you physically attack John Smith?" are likely to be used.

The most invasive type of examination is the "lifestyle" polygraph, which may include questions on sexual activity, drug and alcohol use, gambling habits, and financial matters. Government agencies may wish to conduct this type of examination on persons entrusted with classified information.

Finally, the examiner will compare the responses recorded from the relevant questions to the baseline and control questions, and determine whether the subject is exhibiting signs of deception related to one or more specific topics. If so, or if there is doubt, the series of questions will be repeated again. The examiner will compare these results with those previously gathered to determine if there is a consistent pattern and if the anxiety level is decreasing or increasing. In some cases, the process may be repeated many times over several hours. The theory is that anxiety will grow with repetition of a deceptive response, while the person with nothing to hide will relax and become more comfortable with the question over time.

Interpretation of polygraph results is in part a subjective technique, making the examiner a key component of the process. Some trained examiners prefer to be called forensic psychophysiologists, a title that acknowledges the need to place the data in the context of the subject's overall performance and psychological make-up, as well as the potential effects of factors such as lack of sleep, medications, and the stress of the polygraph experience itself.

The requirements for becoming a polygraph examiner vary greatly from state to state. There are certification courses leading to a license, and only licensed examiners are utilized by government agencies as well as by many law enforcement entities. But there is no government body that controls the licensing process. In many states, anyone can buy polygraph equipment, print business cards, and open a business as an examiner, with no prior experience and no license or other proof of competence.

The notion that certain physical changes accompany deceptive behavior is ancient. But the use of medical technology to measure these changes was not attempted until the early 20th century. Though digital polygraphs are now in use, the basic techniques of polygraphy have not changed much since the 1930s. The level of acceptance of the results has not changed much either. It was controversial then, and it remains controversial today.

Is the polygraph really able to determine guilt and innocence relative to a specific activity? The debate continues. The scientific evidence is spotty and inconsistent. Most research indicates polygraph examinations are 70-90% accurate, with the majority of inaccuracies due to honest people failing.

We all know intuitively that we undergo physiological changes when we are anxious, but the ability of the polygraph to quantify these changes accurately for all subjects has not been demonstrated. Detractors argue the interpretation of results is far too subjective and in fact, a skilled psychologist can make similar judgments without the assistance of technology. Mothers and wives can be pretty good at this too!

There is no doubt that a very skilled examiner is more likely to achieve consistently accurate results. However, most state courts do not permit polygraph results to be admitted as evidence; their accuracy has not been sufficiently proven. Courts do sometimes allow examiners to testify or results to be entered with the consent of both the prosecution and the defense. But no defendant can be compelled to submit to a polygraph.

The legal battle over the admissibility of polygraph evidence at trial continues in the federal court system. It is interesting to note that the United States is one of the few countries that use the polygraph in any widespread fashion. It is rarely used in other countries around the world.

Assuming the polygraph does have some utility and can accurately detect an attempt at deception, can it be defeated? The Internet is rife with books and CDs on ways to deceive a polygraph, including various mental gymnastics, taking depressive drugs, corrupting the baseline data by telling the truth when asked to lie, and even placing a tack in a shoe or biting a cheek, to elicit an emotion-masking pain response to every question.

There is no hard evidence that any of these techniques are effective, though they can certainly disrupt the process and make the examiner's job more difficult. An experienced examiner is likely to catch the subject trying any of these tricks, which may lead to a great deal of trouble.

The TechKnow Guy's advice: never volunteer for a polygraph examination. It is better at identifying deceitful behavior than exonerating the innocent, so you might fail even if you have nothing to hide. If your job requires you to take a polygraph, be patient, polite, truthful, and as relaxed as possible. Never make an admission of wrongdoing or volunteer any information beyond the answers to the questions. This could lead to new and potentially unpleasant lines of inquiry!

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