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Old 9th September 2018, 02:51 AM   #308
Henri McPhee
Join Date: Dec 2012
Location: Bristol UK
Posts: 4,028
The matter of polygraphs in the MacDonald case was discussed during the McGinniss trial in 1987 with MacDonald defense lawyer Gary Bostwick. MacDonald was naturally reluctant to take a polygraph while he was still recovering from his collapsed lung. Some guilty people, like Mitchell and Fowler pass polygraphs while innocent people may fail them:


C R O S S - E X A M I N A T I O N

Q Mr. Backster, the polygraph instrument is not an infallible device for telling truth and falsity; is it?
A No, it is not.
Q And sometimes when people are telling the truth, it indicates that they are being deceptive; does it not?
A On issues that are not intense, I would say that there is a possibility certainly of that happening.
Q And in fact the accuracy rate in terms of whether a lie detector test can tell whether someone is lying or not varies between maybe 70 to 90 percent; isn't that correct?
A The figures you're mentioning relate to laboratory studies, not field studies.
Q But isn't it correct?
A That laboratory studies do show, or that it could be as low as 70 percent or as high as 90 percent, yes, that's correct.
Q And that you have no way, as a polygrapher, of telling when the error occurs or doesn't occur; isn't that correct also?
A We can work on probability, and there is a scale to tell you the probability of your accuracy, depending on the intensity of the case, the adequacy of your case information, and the distinctness of the issue that you're pursuing.
Q Isn't it true that the questionable accuracy of polygraph examinations is one reason that it's not admitted in criminal trials in federal courts?

MR. KORNSTEIN: Objection, your Honor. Calls for legal conclusion.

MR. BOSTWICK: Your Honor, I don't think --

THE COURT: Let's find out if he knows, first of all. Do you know about this subject?

THE WITNESS: Yes. I can talk about it.

THE COURT: All right. You may answer.

THE WITNESS: What was your question again, Counselor?

Q Isn't it true that the questionable accuracy of polygraph examinations is one of the reasons that it is not allowed as evidence in federal courts?
A I would state that it would relate more to the questionable qualifications of the polygraph examiners than it would the technique itself.

Last edited by Henri McPhee; 9th September 2018 at 02:54 AM.
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