In my career, I was involved in over 3,000 polygraphs.
Generally speaking, I noticed that there are trends. One thing is that there is a subset of people who can lie and completely pass. This is a small number, and in my experience we ultimately were able to pick most of them out through the use of other tools we had.
Another group was people who thought they could beat it. Some of them tried different methods, like breath control, muscle tension, etc. Most didn’t succeed, mainly because we had other information and the confrontation with it would often result in truth coming out.
A subset of people confessed openly, usually immediately prior to the test. This would be the "parking lot" or "lobby" confession.
But most people who took the test and wanted to pass were open and honest. Most were nervous, but eager to pass.
And most did pass. The failure rate wasn’t high, but there were definitely failures among our subjects.
When people failed, they usually blamed "nerves", or said things about the polygrapher being biased, or bad, or cheating.
But of the many, many tests performed, not a single person who failed ever came back to "prove" themselves honest, nor did any of them sue our entity over the results.
(I am not able to disclose the entity I speak of, sorry.)
