The polygraph works by recording several responses of the autonomic nervous system at once —breathing, cardiovascular activity and the galvanic skin response (sweating)— while the person answers a questionnaire prepared in advance. It does not detect lies directly, nor does it "read minds": it measures how the body reacts to different questions and compares the reactions to the relevant questions with those to the comparison questions. An accredited examiner interprets those charts to estimate the probability that the person is telling the truth. With validated techniques, the accuracy is around 85-90% according to the American Polygraph Association, and the National Academies review (2003) confirms they are right well above chance, though it is not infallible. The equipment alone decides nothing: what is decisive is how the questions are framed and who reads the records.
What exactly does the polygraph measure?
The polygraph captures three physiological channels at once through sensors placed on the body. Pneumograph bands on the chest and abdomen record the rhythm and depth of breathing; a pressure cuff measures cardiovascular activity (heart rate and pressure); and electrodes on the fingers pick up the galvanic skin response, that is, the changes in sweating produced by the nervous system. None of these signals "says" by itself whether someone is lying: what is analysed is the relative variation of the three when the person answers each question.
How does the test unfold, step by step?
The test has three phases. In the pre-test interview the examiner reviews every question with the person one by one —there are no surprise questions— and acclimatises them to the equipment. Then the sensors are fitted and the test is run, repeating each series of questions at least three times to check that the reactions are consistent and not a spike caused by nerves. Finally comes the analysis: the examiner scores the charts and expresses the result in terms of probability, not as an absolute certainty. The test itself lasts between 60 and 120 minutes and the full appointment between 2 and 3 hours.
Where does the polygraph come from? A little history
The instrument was born in 1921, created by John Larson, who continuously recorded changes in blood pressure. In the 1930s Leonard Keeler refined it by adding pulse and breathing measurement and a paper-based recording system, and in 1945 John Reid added muscular activity. The leap to digital came in 1992, when Stoelting brought the first computerised polygraph to market, showing the charts on screen instead of on paper. Today the equipment is digital, compact and more accurate, although the underlying logic —comparing physiological reactions to different questions— is still the same.
So, is it reliable?
The polygraph is a supporting tool, not an oracle. Properly administered it reaches 85-90% accuracy, but it can produce false positives and its result always depends on the examiner's training and the examinee's state. At El Polígrafo the test is carried out by Fernando, a polygraph examiner and forensic psychologist, with a Stoelting CPS Pro system and techniques recognised by the American Polygraph Association; his training in psychology is what makes it possible to frame the questions well and read the records correctly. If you want to go deeper, you can check whether a polygraph can detect lies by omission.