In ancient times, people used different methods to detect the lies of a person. In some countries accused person were subjected to do water test. The people who sank were considered innocent and those who float were thought guilty.
In India, the accused person mouth was filled with some dry rice. Then they were asked to spit it out. If rice got stuck in their mouth they were thought guilty.
Same some witchcraft methods were also implied which was not right for the people. Some innocent people got hurt and punished for the crimes that they did not commit.
So people developed more humane methods. With the passage of time and advancement in science the lie detection techniques have grown better and better. Polygraph is a modern lie detecting technique. Poly means multiples or many and graph means to write.
The system records physiological responses perspiration, heart rate, breath rate and blood pressure and graphs them out visually for an examiner to interpret..
The little elevation in any of these can be considered as a lie. The polygraph technique can be carried out by two methods. First one involves the Controlled Question Technique.
In this, the examiner asks some relevant and irrelevant question and check the graph of physiological responses at the same time. The lie can be detected through stress of lying or heat beat elevation.
Sometimes a polygraph will also record things like arm and leg movement. Second one is Guilty Knowledge test. In this the examiner not only tests the guilt but also the lie of the person.
Examiner asks person a question about the crime. To notice if there is any change in his statement. This will show us if he was present at the scene or related to the crime or not. More quick and detailed answer can show us he was present.
Polygraph can give false positive results some times which can result in making some innocent person a criminal. Polygraph can be fooled by a trained individual who just need to control his physiological responses.
But with trained examiner taking the test there is better chance of getting the result right. And it still is better technique then other methods and still new advancements are being made making it more and more reliable.