The polygraphAnother interesting invention of the 1920s was the polygraph, or lie detector. It was invented in 1921 by a Californian policeman, John Larson. He used the ideas of other psychologists to make a machine that measured people’s heart rate, breathing and blood pressure while they were asked questions. The experts believed that sudden changes in these measurements showed that someone was lying. Although this invention is well-known, it can’t really detect lies. Marston tried to use measurements from his polygraph in a court case in 1923, but they weren’t accepted as evidence and never have been since then. However, polygraphs are still used by some police forces and the FBI because many people believe they work, so they tell the truth to avoid the machine. Some psychologists helped John Larson to build his invention.