Asked by: Lizzie Dawes, Braunton
The polygraph test, still used widely in the US, measures your heart rate, breathing and blood pressure as a way to tell how stressed you are feeling. The idea is that the interrogator asks you questions and when you lie, you get more stressed than when you tell the truth, and this difference is revealed in the physiological measures.
A simple way to cheat the polygraph is to deliberately distort your physiological readings when telling the truth, such as by biting your tongue, or imagining an embarrassing incident in the past. Similar problems afflict brain scan lie detectors, which follow the same principle of needing a reliable baseline against which to compare signs of lying.
Dr Christian Jarrett is a cognitive neuroscientist, science writer and author. He is the Deputy Editor of Psyche, the sister magazine to Aeon that illuminates the human condition through psychology, philosophy and the arts. Jarrett also created the British Psychological Society's Research Digest blog and was the first ever staff journalist on the Society's magazine, The Psychologist. He is author of Great Myths of The Brain and Be Who You Want: Unlocking the Science of Personality Change.