How Do Lie-Detector Polygraphs Actually Measure Deception?

A polygraph records several physiological signals simultaneously: respiratory rate via chest pneumograph tubes, cardiovascular activity through a blood-pressure cuff, and electrodermal activity using fingertip electrodes. The theory is that deceptive answers trigger sympathetic nervous system arousal, producing measurable spikes. However, the American Psychological Association notes that polygraph accuracy is debated, with estimates ranging from 70% to 90% depending on the study and technique. Countermeasures like controlled breathing can alter results, which is one reason polygraph evidence is inadmissible in courts in many countries.