There is some question about the validity of polygraph testing. In one case with which we are familiar, from the days when polygraphs could be used as part of a pre-employment test, an early teenager was denied employment because the test showed that he had been in prison (he hadn’t), and was lying by denying it. In another equally-familiar case, a polygraph confirmed that someone was not having an affair (he was).
There are a number of sources of information, pro and con, about polygraphy. On the pro side is the American Polygraph Association (http://www.polygraph.org/). They say that research indicates the accuracy of polygraphy, if you have “a combination of a properly trained examiner, a polygraph instrument that records as a minimum cardiovascular, respiratory, and electrodermal activity, and the proper administration of an accepted testing procedure and scoring system” to be between 80% and 98% (http://www.polygraph.org/validityresearch.htm).
On the other side is antipolygraph.org (http://antipolygraph.org/). This group has a somewhat more jaundiced view. In fact, they consider relying on polygraphs to be a potential risk, and note an article on defeating polygraphs on an al-Qaeda Web site at http://www.tawhed.ws/r?i=3165.
While we have no enthusiasm for using polygraphs (or voice stress analyzers, for that matter) to determine if someone is lying, they can be useful in investigations when you are trying to get information from someone willing to take one.
In investigative use, you have three things going for you. First, because the person is willing to take a polygraph at all, you are assured of at least some level of cooperation, even if they are doing this to serve their own interests. Second, they have no idea what exactly you (and others who will look at what is said) actually know, and are likely to be a little more forthcoming in the information they provide, even if they are editing it to make themselves look better. Finally, they may well want to give the impression of not lying, and of being cooperative, and so will be more willing to supply you with information that they feel to be true, but that will not make them look bad.
This can work to the investigator’s advantage because it generally gives previously unknown details that can be used to piece together the actions of the players, and clarify things not previously understood, and help move the case in the right direction.