Learn About Polygraphs

Lie detector tests, or polygraphs, have been in use for nearly 80 years. The exams, which have not changed much in basic design from early prototypes, remain a controversial tool for investigators.

How accurate is a lie detector test?

Frank Horvath of the American Polygraph Association says critics contend the test is about 70 percent accurate, while proponents claim it's 90 percent accurate.

How does a lie detector test detect lies?

A polygraph simultaneously measures several physiological activities such as respiratory rate, sweat gland activity and cardiovascular activity as a person is asked questions. The theory is when a person lies, it produces stress and this stress is reflected in changes in breathing, heart rate and perspiration.

How does a polygraph test monitor changes?

Rubber tubes are placed over a subject's chest and abdominal area to measure respiratory activity. Small metal plates attached to the fingers record sweat gland activity and a blood pressure cuff or similar instrument monitors the cardiovascular system.

Conventional machines use moving paper feeders and styluses that record the simultaneous input from the three physiological responses. Computerized polygraphs generate chart analyses from the data and display the results on a computer screen.

What kinds of questions are asked during a test?

A polygraph test consists of only "yes" or "no" questions. Experts say the kinds of questions asked during an exam play a significant role in the accuracy of the exam's results, therefore it's important that the examiner is properly trained in the practice.

What are the stages of a polygraph test?

According to Nate Gordon, founder of the Philadelphia-based Academy for Scientific Investigative Training, the test has three phases of questions: a pre-test, a chart-collection phase and then test data analysis. During the pre-test phase, the examiner helps familiarize the subject with the testing procedures and ensures the subject is mentally and physically capable of undergoing the test. They may also ask the subject to lie to see if the test reveals signs that they're lying.

During the chart collection phase, the examiner asks questions relevant to the investigation and then analyzes the charts. When appropriate, the examiner will also give the subject a chance to explain the physiological responses.

During analysis, changes in physiological activity are analyzed for signs of lying.

Can taking the test cause nervousness that may lead to false results?

Examiners say they can recognize differences between test anxiety and deception anxiety. In deception anxiety, certain questions cause specific physiological reactions while reactions from test anxiety remain consistent. However, Gordon says research indicates that false positives are more common than false negatives, perhaps because of anxiety over the exam.

Is it possible to deceive a polygraph test?

Polygraph experts says it's unlikely, although not impossible, to create false results.

Is testimony based on polygraph tests allowed in court?

Some states and federal courts permit the use of lie-detector results, but only if the judge and both sides agree to admit them. This has effectively excluded polygraph tests from trials.

How long have polygraph tests been used?

John Larson, a University of California medical student, invented the lie detector in 1921. It has been used in police interrogation since 1924.

Are there other technologies used to detect lying?

Researchers are experimenting with computerized brain scans that reveal the amount of blood flow to different sections of the brain. Early tests suggest people use more sections of their brain when they lie than when they tell the truth and this is evident in increased blood flow throughout the brain.

Some law enforcement agencies use a device that measures changes in the voice. Computer voice stress analyzers are designed to detect lies by monitoring voice frequencies. However, there is a movement in the United States to ban the use of these devices, claiming they're inaccurate and flawed.

ABCNEWS' Bryan Robinson and Amanda Onion contributed to this report.