In the early 1900s, John Larson invented a polygraph commonly known as a lie detector.
John Augustus Larson was a Berkeley police officer and physiology student who developed his instrument in 1921 at the request of Berkeley police chief August Vollmer. Larson's machine recorded several physiological signals, including blood pressure, pulse, and respiration, on a continuous chart. The term "polygraph" refers to its ability to record multiple measurements at once.
The popular name "lie detector" became widely used because investigators hoped changes in a person's bodily responses could reveal deception. However, a polygraph does not directly detect lies; it records physiological reactions that an examiner interprets. Stress, fear, confusion, or the testing situation itself can affect those reactions.
Larson's work influenced later devices, especially the version developed by Leonarde Keeler, who improved the instrument and helped popularize its use in criminal investigations. Thus, "lie detector" is the standard everyday answer, while "polygraph" is the technical term.