Sometimes a polygraph ("lie detector") examination report is helpful in the defense of a criminal case. Such reports are not generally admissible at trial, so that the jury will probably never see the report. But it might be a useful tool in convincing either the prosecutor or the grand jury not to proceed with the case. . . .Mark is absolutely right. Never, never, never take a polygraph examination without first consulting with a highly qualified criminal defense lawyer. If the lawyer suggests you take a polygraph administered by the police, fire that lawyer and hire another. The only polygraph you should take is one given by an expert hired by your lawyer, which protects the results under the attorney/client work product privilege.
Government polygraph examiners use the polygraph as a tool to get confessions. Being told that the machine says you're lying provides tremendous motivation to change -- or at least explain -- your story. Sometimes a police polygraph examiner will tell the accused that he has failed the polygraph regardless of the real result.
Thanks Mark for bringing up the polygraph issue. I've had many unfortunate people contact me on the phone and explain they had submitted to a police polygraph and failed. My next question is always: "What did you tell them after that?" Rarely is the answer a good one for the accused. People need to know the dangers of taking polygraph examinations administered by the police. They are used as a tool to get confessions, pure and simple. My advice is "Just don't do it."