In a blog entry titled “Leakscape,” retired CIA officer Melissa Mahle discusses, among other things, the unreliability of the polygraph:
The polygraph is a very blunt instrument and not accurate. If an officer registers any discomfort on a question, the polygrapher will hone in on the issue. After being asked the same question 100 times, the poor person on the box is feeling so beat up that the emotional reaction gets stronger and stronger as a function of the test, not the issue. Once there is suspicion, it is impossible to make it go away. The CIA just does not accept exculpatory information, even if it comes from a CIA counterintelligence investigation. Suspicion equals guilt. Putting this in the context of Ms. McCarthy, it is easy to imagine that an admission of contacts with the press becomes a presumption of leaks to the press. How do you prove a negative?