Do Lie Detectors Actually Work? Despite being commonly known as lie detectors, polygraphs actually measure stress, using blood pressure, heart rate, respiration, sweat, and other indicators. Because lying is stressful for many people, the idea is that a polygraph, used in conjunction with a series of specific types of questions, can help determine if the subject is being truthful or not. While this can work, polygraphs have proven to be wrong before, which is why several U.S. states have banned their results as evidence in legal proceedings.