Question: How do lie detectors work?

Rebecca Dewey answered on 15 Jun 2015:

When someone has a lie detector test, they will be asked to say some things that are definitely true (what is your name? are you 29 years old? were you born in Reading?) or false (is it correct that you are a monkey? are carrots green? do you live on the moon?) and the computer will record what they brain or their body does. Then when they answer the *real* questions, (the ones that we want to know whether they are lying or not), we compare their brain or their body response to see whether it looks like the true answers or the false ones. The recordings can be done by voice stress test (where the sounds in their voice are analysed to see if they are slightly higher pitch or just sound slightly different when they lie) or using polygraph (where the body’s responses such as heart rate, breathing rate, blood pressure, size of blood vessels, and muscle movement). However, none of these are really perfect in everyone, and so aren’t considered totally relaible. The technique I use to look at the brain, near-infrared spectroscopy, is currently being tested for its suitability for lie detection by a group I’ve worked with in Japan – so maybe that will be better!!