Polygraphs Getting Smarter, but Beware of Liars Bearing Tacks

Polygraph machines can be fooled.

What the equipment does is measure changes in pulse, blood pressure, fingertip perspiration and respiration--body functions that may be altered by the stress of deceit. The trouble is, a variety of tricks can produce the same effects.

Trying to beat a polygraph involves increasing stress during innocent questions, perhaps one confirming the test taker’s name, and relaxing while answering tough questions or telling lies. The intent is to have an inconclusive test in which all questions seem to cause stress.

Test subjects sometimes put tacks in their shoes before facing a polygraph and press a toe down on the tack’s point during innocent questions, said Chris L. Fausett, polygraph product manager at Lafayette Instrument Co., the nation’s largest manufacturer of the machines. Other tactics that can briefly raise blood pressure include curling toes and pressing feet down inside shoes, he said. “Any movements at all that the examiners don’t see can affect the chart.”

Polygraph examiners are trained to detect these ploys, however. In addition, the more expensive machines now include highly sensitive motion sensors that are supposed to do the same. A top-of-the-line machine with such sensors can cost $5,300, compared to $2,800 for the most basic model that Lafayette sells, Fausett said.