A polygraph, also known as a lie detector test, records various physiological signals from a person that usually include:
- their breathing rate
- their pulse
- their perspiration
- their blood pressure
Generally, a rise in any of these things is considered suggestive of lying (but are really just suggestive of stress – the presumption is that lying = more stressful, but being anxious for any other reason can also get you pinged for “lying”).
There are three main sensors attached to examinees in order to collect these readings:
- The respiratory sensor, which consists of two tubes, one that goes around the examinee’s abdomen and another that goes around the chest (“just below the breast line” for a male, and just above for a female).
- Fingerplates on the non-dominant hand, usually on the index and ring fingers, to measure electrodermal activity (i.e. sweat). In a room that is too cold or too hot, readings will not be very accurate.
- A cardio cuff, placed on the opposite arm from the EDA sensor, to measure cardiovascular activity. What they want is for the cuff to be placed over the brachial artery, but also to minimise any other sources of pressure on the cuff (e.g. from the person’s chest or the chair).
Polygraph tests are mainly just used in the USA, and very occasionally in countries closely allied with the US, like Canada, Israel and Japan. In the US, they are often given to emergency service workers, or to applicants for security clearances, and passing a polygraph is a condition of employment for many federal jobs (like with the FBI).