A press release from Blackwell Publishing (28 Nov) highlights a new study coming out in the next issue of the journal Psychophysiology.
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. A new study published in Psychophysiology finds that the use of a written test, known as Symptom Validity Testing (SVT), in conjunction with polygraph testing may improve the accuracy of results.
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. SVT has previously been shown as effective in detecting post-traumatic stress disorder, amnesia and other perceptual deficits for specific events.
The study finds that SVT is also 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 preexisting but relatively uncommon concealed information polygraph test (CIT), test accuracy is 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,” says lead author Ewout 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.”
- Ewout H. Meijer, Fren T. Y. Smulders, James E. Johnston, Harald L.G. J. Merckelbach (2007). Combining skin conductance and forced choice in the detection of concealed information. Psychophysiology 44 (5), 814–822.
Abstract below the fold
An advantage of the concealed information polygraph test (CIT) is that its false positive rate is determined on statistical grounds, and can be set a priori at arbitrary low levels (i.e., few innocents declared guilty). This criterion, however, inevitably leads to a loss of sensitivity (i.e., more guilty suspects declared innocent). We explored whether the sensitivity of a CIT procedure could be increased by adding an independent measure that is based on an entirely different psychological mechanism. In two experiments, we explored whether the accuracy of a CIT procedure could be increased by adding Symptom Validity Testing (SVT), a relatively simple, forced-choice, self-report procedure that has previously been used to detect malingering in various contexts. Results of a feigned amnesia experiment but not from a mock crime experiment showed that a combination measure of both tests yielded better detection than either test alone. [Abstract Copyright © 2007, Blackwell Publishing Ltd.]