How does a lie detector work?

The modern polygraph was developed in 1921 by John Larson, a police officer and physiologist in Berkeley, California. Sensors record blood pressure, pulse, breathing rate and skin conductance, which rises as the skin sweats, while an examiner asks a series of questions. The theory is that deception produces measurable stress. The problem is that nervousness, fear or anger can cause the same changes, and trained people can learn countermeasures. A 2003 report by the US National Academy of Sciences found the scientific basis for polygraph testing weak, and results are generally not admissible in US courts. The 1988 Employee Polygraph Protection Act bans most private employers from using them.