Updated on February 5th, 2024 at 08:51 pm

A polygraph is a lie detector test used to determine whether a person is telling the truth based on changes in various physiological variables such as respiration, pulse rate, electrodermal activity, and blood pressure. These results are measured when the defendant is asked various questions about the issue under investigation.

Lie detector test being performed on a woman in Milwaukee, Wisconsin

Are Lie Detector tests admissible in court in Wisconsin?

According to Marquette Law Review, the results of a Polygraph may be held up in court provided that it’s used in combination with other evidence. However, the defendant must agree to the legal terms of a stipulation prior to the lie detector test.

Let’s discuss when a polygraph can be used and whether you have the right to deny the test when asked to do so.

Under Wisconsin law, the lie detector test results can only be accepted as evidence in limited situations.

This can happen only when both the defense and the prosecution agree that the results are admissible. When the results are “admitted into evidence,” the results of the polygraphs are shown to the judge or jury.

However, before the results can be admitted into evidence, the following agreement must take place:

  • The witness or defendant must have agreed to take the polygraph test
  • The defendant must have completed the polygraph test
  • The opinion of the lie detector test examiner
  • The fact that the witness or accused failed or refused to take a polygraph test

Can an employer mandate their employees to take a lie detector test?

If you’re a witness or an accused individual, it’s important to understand your rights as an employee. The law of Wisconsin states that a private employer doesn’t have the power to require their employees or job candidates to take the polygraph test.

This cannot be administered for the person to obtain employment or have continued employment. However, the employer must ask the employee and provide the employee with information about his or her right before administering the polygraph test.

If the polygraph test is performed, the employee must agree to take the test. Civil service laws prevent government employees from being subjected to taking polygraph tests.

Should you take a polygraph if asked to do so by law enforcement?

A person should always consult with a criminal defense attorney prior to taking any lie detector test. The reason is that these polygraph tests aren’t always accurate. Since they can be used as evidence in court, it’s not in the defendant’s best interest to submit a polygraph test.

The accused will pass the lie detector test if he is truthful in denying the fact that he or she did not commit the crime. However, unless agreed upon, the accused can’t use these results as evidence in court.

If the accused does take the lie detector test and fails, the prosecutors and police will view that person as guilty and therefore much more likely to charge them with the crime.

Can you fail a polygraph test even if you’re telling the truth?

It’s important to note that the polygraph test doesn’t measure whether a person is lying or not. Instead, the device measures possible indications that would suggest that the accused may be lying. This means that the defendant can still fail the lie detector test, even if they tell the truth.

During the polygraph test, the examiner will ask the person various questions. A device will record the different physiological changes in the defendant, such as:

  • Pulse
  • Breathing rates
  • Perspiration
  • Blood pressure

The idea behind a polygraph test is that:

  • A defendant who is lying will demonstrate these physiological changes
  • A defendant who is telling the truth won’t demonstrate any physiological changes

It’s important to note that these lie detector tests can cause the individual an enormous amount of stress before and during the test. These are true if the accused individual is being asked detailed questions about:

  • An event at work
  • Or a particular crime

Emotional stress and distraught can result in fluctuations in a person’s physiological state. These fluctuations may lead to erroneous indications that the person is lying even though they tell the truth.

Can a person do a private lie detector test to prove their innocence?

An accused person has the right to do a private polygraph with a private investigator to prove their innocence.

This is a process where a private polygraph examiner will conduct the lie detector test. These tests are given to witnesses or defendants of the criminal case. These lie detector tests are categorized as private because they are not obligated to inform the authorities or the prosecutor that the polygraph test is being taken.

During the polygraph test, the examiner will ask the accused person or witness whether they committed or saw the crime take place. For the defendant, if the answer is no and the test shows that they are truthful, then these results can be exhibited to the authorities and prosecutor to help get the case dismissed. However, the advantage of these tests is that you don’t have to bring the results to the court. Meaning, if the results indicate that the accused is untruthful, they can keep the results a secret.

Typically, the polygraph test uses the results in cases that involve felony or misdemeanor offenses. It may be wise for the accused individual to take the private lie detector test in these situations:

  • Convince the prosecutor to agree to utilize a second test during the trial
  • Attempt to dismiss a charge against them during the pretrial process
  • Persuade the defendant to enter into a plead no contest or plead bargain.

It’s also important to note that professional polygraph examiners cost a fee ranging between $200 to $2000.

Why do some officers pressure you to take a polygraph test?

Some prosecutors and law enforcement officers may try to persuade you to take the lie detector test by making you believe that you will automatically admit guilt by refusing to take this test. Since they already believe that you’re guilty, they will want more evidence to prove their claim and make their case even stronger.

Many officers don’t even care about the results of the lie detector test. Instead, they want you to keep talking, so they can squeeze as much information from you as possible. However, remember that you have the constitutional right not to answer any of their questions without your legal counsel.

Can you fail a polygraph test if you are nervous?

Even after nearly a century of conducting the lie detector test and thorough scientific research, the evidence and validity of the polygraph test are inconclusive. It is possible to fail the test if you are nervous, especially when being questioned.

There are also potential errors that may occur or cause the results to tip to being untruthful. This includes an incompetent examiner, the lack of accuracy in the lie detector device, a nervous individual, inadequate control questions, inadequate phraseology, mental and physical discomfort, negligence, resentment towards the test, and personal problems.


Now that you know the basic principles of the polygraphs and whether they’re admissible in Wisconsin, you can be prepared if you or someone you know is being interrogated. But, as we mentioned, the polygraph test isn’t always accurate and may display misleading results—the psychological and physiological reaction to specific stimuli and other variables. For more legal help or information be sure to contact Nathan J. Dineen or go to one of our offices in Milwaukee, Germantown, Appelton, or even West Bend.