Monday, April 19, 2010

I cannot tell a lie...

We all know from television (that great oracle, that font of all wisdom) that the results from a lie detector test are not admissible in court. You knew that, right?

But did you know that they are still very much a tool for law enforcement? In major cases detectives sometimes ask those close to the investigation, friends and family members of a murder victim, say, to take polygraph tests to "rule them out" as suspects.

I was interviewing an FBI profiler for a book and he told me that this guy basically confessed after taking a polygraph, because he knew it would show he'd been lying. (An aside: I also interviewed Nenno himself, four days before his execution. A very weird experience.)

What I didn't realize was that polygraph tests are used to monitor compliance in sex offender cases. So, after a conviction a sex offender can be required to take a test to make sure he's not been using the Internet inappropriately (or at all), to ask him whether or not he's been lurking near schools. It makes sense, I guess, assuming the tests have a (large) degree of accuracy to them.

But do they? A quick google search will answer that for you, though opinions do vary. I note that a Texas court recently said that even the results from these sex offender check-ups can't be used in court. For example, if an offender is on parole with certain restrictions and when he's tested he comes back as dishonest and in violation of those conditions, his parole can be revoked after a hearing. However, the test results can't be used as evidence in that hearing.

Sound contradictory? A little, maybe. But until polygraphs are deemed reliable enough to be used in court it seems like a fair decision. And, as the appellate court pointed out, investigators can use the results to kick off or direct further, more traditional, investigative techniques and evidence-gathering methods.


  1. But, you know, if they're not reliable, then any information obtained from them is also potentially unreliable and could send an investigator down the wrong road - potentially excluding guilty suspects or included innocent ones.

    If it gets somebody to confess all on their own, that's one thing, but the results themselves (too unreliable to be scientifically valid) are potentially problematic.

  2. Lie detector tests fascinate me. Very interesting post.


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