A lie-detector machine, also known as a polygraph, measures involuntary physiological markers such as pulse, blood pressure and respiratory rate.
More recent tests might also measure pupillary changes or even brain activity with an MRI. When asked a series of questions, the change in these readings is supposed to indicate whether you are lying or not.
But in practice, polygraphs are notoriously unreliable and today many scientists regard them as pseudoscience.
The problem is that people are all very different and their polygraph readings are dependent on many different factors, many unrelated to telling a lie.
For some people, just the stress of being given the test or being asked questions that they feel might be incriminating is more than enough to spike the readings: pulse rates rise, sweaty palms ensue.
For others, lying is a normal behaviour, and they can calmly say what they like without disturbing the machine. This is a big problem: an innocent person is more likely to be stressed by accusations than a hardened criminal.
You can consciously beat a polygraph by trying to manipulate the baseline readings taken to establish your normal measurements. Some people deliberately breathe faster or bite their tongues.
Then, during the primary questions, some distract themselves with mental puzzles or relaxing daydreams. The result is a set of difficult-to-interpret readings.
But the most successful guilty people who beat the polygraph did so because they felt no remorse at all. Killers such as Gary Ridgway and Lewis Randolph Williamson were able to beat multiple polygraph tests.
DNA evidence decades later finally brought them to justice.
This article is an answer to the question (asked by Dominick Jones, via email) 'Can you beat a lie detector test?'
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Read more:
- How to spot a lie: brain scans outperform lie detectors
- How to actually catch a liar, according to the new science of lie detection
- How to crack the subtle body language of liars
