Customer Reviews

The Polygraph and Lie Detection

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The National Research Council published this book as a summary of what is known scientifically about polygraphs. While they are famous to the general public, this is also accompanied by extensive misunderstandings that have been part of the cultural folklore. (Mythology might be a stronger word.)

Reading the book will give you an appreciation of the scientific foundations and limitations of a polygraph. You might well find that some chapters leave you confused, because there is necessarily no conclusive summary. But if so, it reflects fairly the current level of knowledge. It also explains why several American states, like California, ban polygraph evidence as unreliable.

A very interesting passage is on the countermeasures that astute subjects can take. These can largely nullify the value of any measurements on them.

There are a lot of myths about lie detectors. This book explores them scientifically and points out their limitations.

I will get polygraph personnel say they get 95% or 99% success rates. However, twenty-four studies found correct detection of guilt ranging from 35% to 100%. Overall, 83% of guilty subjects were diagnose as "deceptive," as were 43% of innocent subjects.


The earliest lie detector was from China where suspects were given rice to chew and then spit out. If the rice was dry they were too nervous to form saliva and were guilty. The problem is about half the innocent people were too scared to get the rice wet and some brazen criminals weren't scared. The same problem exists today.

Lie detectors are especially unreliable for truthful people. Many more innocent people test as "deceptive" than guilty people test as "innocent." Those who run a special risk include people who get upset if someone accuses them of something they didn't do, people with short tempers, people who tend to feel guilty anyway, and people not accustomed to having their word questioned. All of these feelings can change heart rate, breathing, and perspiration and their heightened feelings are easily confused with guilt.

According to one researcher, one prison inmate, who became the jail-house polygraph expert after studying the literature, trained twenty-seven fellow inmates in the seat techniques; twenty-three beat the polygraph tests used to investigate violations of prison rules.

It takes more time to learn to be a barber than it does to get a polygraph license. Federal agents are better trained but as of 2006 this has resulted in agencies not recognizing other agencies results and an underground Washington business for hypnotists and pharmacists for ways to beat the polygraph.

This study after 9/11 considered a test with a theoretical accuracy index of 0.90, if used to detect 80 percent of major security risks, would be expected to falsely judge about 200 innocent people as deceptive for each security risk correctly identified. Unfortunately, polygraph performance in field screening situations is highly unlikely to achieve an accuracy index of 0.90; consequently, the ratio of false positives to true positives is likely to be even higher than 200 when this level of sensitivity is used. Even if the test is set to a somewhat lower level of sensitivity, it is reasonable to expect that each spy or terrorist that might be correctly identified as deceptive by a polygraph test of the accuracy actually achieved in the field would be accompanied by at least hundreds of non-deceptive examinees mislabeled as deceptive.

As the study goes into there are trade-offs you can make - a test that picks up half of the guilty parties may only falsely accuse 20-40 suspects for 1.

It also goes into new methods of lie detection like voice analysis and is a handy primer on the current state of the art.

I made extensive use of this text on cross-examination of the State's "expert" on polygraphs in a murder prosecution in Ohio. The failure of that expert to recognize the limitations of the procedure , especially in the lack of verifiable results, made her testimony that my client had failed a "stipulated" polygraph inadmissible in the trial of that murder.
This NAS text is invaluable examination of polygraph experts

The authors of this academic (boring) book complied a variety of research, but it would have been more helpful if they had conducted some experiments instead of just compiling the research of others. It really seems to be useless.