Lie detectors are just not accurate.
In a secure basement room of the US Capitol building, senators are reading a secretive FBI report into allegations of sexual misconduct made against Supreme Court nominee Brett Kavanaugh.
The contents of the report are not meant to be revealed, and there has been some criticism that the scope of the investigation has not been wide enough.
Throughout the investigation senior Democrats have called for Mr Kavanaugh to take a lie detector test. One of his accusers, Christine Blasey Ford, has already done so.
But how accurate are these tests? And how do they work? Let’s start with the basics…
What is a polygraph test?
In short, polygraph tests record a number of different bodily responses which can then be used to determine whether someone is telling the truth.
They usually measure things like blood pressure, changes in a person’s breathing, and sweating on the palms.
“The polygraph, like any other lie detection technique, measures an indirect effect of lying,” says Dr Sophie van der Zee, who has expertise in forensic psychology and has researched deception for many years.
“There’s no human equivalent of Pinocchio’s nose,” she says. “But lying can increase stress… and with lie detection techniques you can measure the behavioural and physiological changes that occur when you feel stress.”
So polygraph tests do not measure deception or lying directly, but rather possible signs that a person could be deceiving the interviewer.
This information is then used in conjunction with everything else that is known about the person to form a clearer picture of whether or not they are being truthful.