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.