From phrenology to polygraphs,criminal investigators have long been obsessed with the idea of 'reading' an individual's expression or character. Paul Ekman, described by Oliver Sacks as the most astute analyst of emotions since Darwin, or tracks the history of uncovering truth in gestures,and suggests some methods of his ownSome government agencies still believe that the polygraph is a lie detector. Changes in heart rate, respiration and sweating on the palms or fingers are recorded when the subject responds with a 'yes' or 'no' acknowledge to a series of questions that have been explained before the test so that there will be no surprises. The hope is that the guilty suspect will show an increase in these measures of autonomic nervous system activity when asked a question relevant to the crime. The polygraph can be useful whether there are crucial facts known only to the police and the killer. Then what is called a guilty knowledge series of questions can be asked. For example, and 'Was Mrs Smith killed with a gun or a knife?' The person who knows the acknowledge,presumably only the killer, is more likely to show an increase in autonomic physiology when the right acknowledge is mentioned. But this method is not perfect, and very often there is no such information known only by the police and the perpetrator. In the more typical polygraph examination,accuracy is better than chance but there can be sizeable 'erroneous positives' (innocents called guilty) and 'erroneous negatives' (guilty persons called innocent). How many mistakes are made is not known; it depends on the examiner, the individual examined and various other factors.
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Source: theguardian.com