Reference: Polygraphs—Introduction at Trial https://www.justice.gov/usam/criminal-resource-manual-262-polygraphs-introduction-trial Functional MRI-based lie detection: scientific and societal challenges https://www.nature.com/articles/nrn3665 In a typical college exam, if you get 10% of the quiz wrong, you are happy because you pass the exam. But how about a 10% error rate of the judgement in court? It means in every 100 cases, 10 of them are wrong. So if a lie detector with the error rate of 10% is used in deciding who is the killer, in each 100 such cases, there will be 10 that are wrongly identified. Scientists have already found out the error rates of all existing lie detectors, and currently, no method of lie detection has the accuracy that is high enough to be admissible in trials. That is why the courts are still not using lie detectors to decide cases, even when they are so popular nowadays. But how about brain scans? Surely they sound more accurate and reliable than observations of someone's facial expressions, behavior changes, or even the measurement of heart rate. In fact, functional magnetic resonance imaging, has been widely studied in the use of lie detection. Scientists have carried out numerous experiments trying to find the associations between brain scan images and lying. Well, they did find something. They found out that lying is mentally taxing, which means lying causes certain regions of the brain to show more activities than other regions under brain scans. The problem is in the standards of determining if one is lying or telling the truth given the results of brain scans. As research was conducted more deeply, scientists found that the regions associated with lying in brain scans are also associated with executive function, attention and memory processes. For example, if a person is trying to recall something that happened a long time ago, this technique could mistaken the activities of certain brain regions as the marks of lies, while in fact these regions are active because of memory processes. This is like the case in which you know that you need an umbrella when it rains, but you can't simply say that it is going to rain just because you happen to take your umbrella. This is why even brain scans haven't been used in court trials for lie detection yet. We certainly hope one day there can be an effective technique to tell the lies from the truth, but this obviously still has a long way to go.