Written test can improve accuracy of polygraph results

A new study has found that the use of a written test, known as Symptom Validity Testing (SVT), in conjunction with polygraph testing may improve the accuracy of results.

Currently, in order to prevent false positive results in polygraph examinations, testing is set to err on the side of caution. This protects the innocent, but increases the chances that a guilty suspect will go unidentified.

SVT is an independent measure that tests an entirely different psychological mechanism than polygraph examinations. It is based on the rationale that, when presented with both real and plausible-but-unrelated crime information, innocent suspects will show a random pattern of results when asked questions about the crime.

“The written test, SVT, has previously been shown as effective in detecting post-traumatic stress disorder, amnesia and other perceptual deficits for specific events,” said project leader Doctor Ewout Meijer from the Faculty of Psychology at Maastricht University.

The classical lie detection asks directly: ‘did you do it, were you there?’

“In this form, the question is if certain information is present in someone’s head,” said Dr Meijer. “Therefore, it should really be called a memory detector. If you can prove that someone has knowledge of certain details such as the used weapon or the type of get-away-car, you can at least assume complicity in the crime.

“Besides the traditional perspiration measurement in the palms of the hand, registration of brain activity is also advancing. We have been investigating if the combination of perspiration and brain activity is more accurate in detecting lies as well as testing the technique on specific groups of individuals such as psychopaths.”

Dr Meijer’s research team found that SVT is an easy and cost-effective method for determining whether or not a suspect is concealing information. In simulated cases of mock crime questioning and feigned amnesia, it accurately detected when a participant was lying.

Furthermore, when used in combination with the pre-existing but relatively uncommon concealed information polygraph test (CIT), test accuracy was found to be higher than when either technique is used alone.

“We showed that the accuracy of a concealed information test can be increased by adding a simple pencil and paper test,” said Dr Meijer of Maastricht University.

“When ‘guilty’ participants were forced to choose one answer for each question, a substantial proportion did not succeed in producing the random pattern that can be expected from ‘innocent’ participants.”