John Larson, a University of California medical student, invented the modern lie detector (polygraph) in 1921. Used in police interrogation and investigation since 1924, the lie detector is still controversial among psychologists, and is not always judicially acceptable. The name polygraph comes from the fact that the machine records several different body responses simultaneously as the individual is questioned.

The theory is that when a person lies, the lying causes a certain amount of stress that produces changes in several involuntary physiological reactions. A series of different sensors are attached to the body, and as the polygraph measures changes in breathing, blood pressure, pulse and perspiration, pens record the data on graph paper. During a lie detector test, the operator asks a series of control questions that set the pattern of how an individual responds when giving true and false answers. Then the actual questions are asked, mixed in with filler questions. The examination lasts about 2 hours, after which the expert interprets the data.

So, how accurate are the scans? In simple lab experiments, they can detect lies around 78 to 85% of the time. “We’re not that close to a perfect lie detector,” says giorgio ganis from the University of Plymouth, who uses fMRI to study deception. “There’s also a 15-20% chance of an innocent person being wrongly determined to be a liar.”

Lie detection depends on a persons memories, which are subjective. If people are convinced of their lies or if they have simply forgotten crucial information, scans wont pick up on that. There are also different types of lies. If you have been pulled over for speeding you have to some up with a lie instantly giving you very little time to think where as if your on trial for a crime you would have more time to rehearse a story. It was said that these brands of of lies produce different patterns of brain activity. Rehearsed ones are accompanied by a weaker buzz in so called action areas and a stronger one in memory centres.

Finding a sure-fire method for revealing lies is, of course, an age-old dream. All cultures have had their own traditions on how to identify the perpetrators. The polygraph, which is an American invention dating back to 1913, is not used much in Europe. In many countries, it is not deemed to provide reliable evidence. In the US, on the other hand, it is used frequently by defense attorneys, prosecutors and police and, according to a Supreme Court decision, it is up to the individual judge whether polygraph data may be used as evidence in a case. The lie detector, however, is under intense attack as ineffectual. There are even organizations – for example – that are campaigning to scrap the apparatus altogether because of its lack of scientific grounding.