The technical problem in the LX4000 polygraph causes the amount of sweat produced by the person undergoing examination to be measured incorrectly, McClatchy newspapers reported Monday as part of a series of investigations about the use of polygraphs by police departments and federal agencies.
Polygraphs interpret the truthfulness of a person by measuring their blood pressure, respiration and the amount of sweat they produce during the examination. The glitch means that some innocent people may be considered suspects while criminals may be cleared.
The manufacturer, Lafayette Instrument Co., has acknowledged the problem, describing it as "occasional" and "minor." However, the extent or source of the problem has not been independently studied.
Polygraph results are not admissible in most U.S. courts, but the machines have been used to test the trustworthiness of tens of thousands of Americans.
Supporters say the machines are 85 to 95 percent accurate, but polygraphs don't have to undergo any independent testing to verify their results.
Polygraph machines have "a very weak scientific foundation," said William Iacono, a University of Minnesota psychology professor who has researched the procedure. With the information about the machines' technical issues, he said, "now, they might as well be flipping a coin."