Can evidence of a polygraph test be used in court?

Posted to: 
Law
05/10/2016

Police in Canada use polygraph tests in their investigations, but the results generally cannot be used as evidence in court. Courts have found polygraph results to be unnecessary, unreliable, and risky as evidence in criminal trials, although the law is not quite so clear in family matters.

Polygraphs measure and record changes in your physiological symptoms. The test presumes that a person who is lying will have an elevated level of anxiety and exhibit physical symptoms like sweating, and increased blood pressure, breathing or heart rates. The polygraph operator must interpret these changes in your body to determine if you are untruthful. However, a person’s body can react differently for a variety of reasons, like stress or being asked uncomfortable questions. A psychopath might not be anxious about lying and might produce inconclusive results on a polygraph. The reliability of a polygraph will also be dependent on the expertise of the person administering the test.

Testimony in court is often not as simple as “the truth” or “a lie”. A person’s ability to observe, remember and describe what they recall may affect their testimony. There is sometimes a tendency to "fill in the gaps" in your memory, often without being conscious of doing this. In such a case, a witness could give a sincere, but inaccurate, description of what happened.
Focusing on polygraph evidence could take away from other factors that are important when assessing a witness’s accuracy and credibility. As a result, polygraph evidence could be more harmful than helpful in assisting a judge or jury to come to a just decision.

In a 1987 court case called R. v. Béland the Supreme Court of Canada said evidence of polygraph results cannot be introduced in criminal trials because it offends well-established rules of evidence, is unnecessary, and leads to complications and confusion that may derail the proceedings. The Court cited four main reasons why polygraph evidence cannot be used in criminal trials.


R. v. Béland’s four reasons to exclude polygraph evidence:
First, admitting polygraph evidence offends the evidence rule against oath-helping. This rule forbids a party from presenting evidence that would bolster the credibility of that party’s own witness. Polygraph evidence offends this rule because the only purpose it would serve would be to add support to the accused’s testimony (“Look, I’m innocent!”), and the polygraph operator would be telling the court that the accused was telling the truth.

Second, polygraph evidence offends the rule against admitting consistent out-of-court statements by a witness. This rule says that having another witness testify that the accused person told them the same thing they are telling the court does not add to the accused’s credibility. A polygraph operator’s testimony would be this type of corroboration of the accused’s testimony, so it offends the rule against past consistent statements.

Third, polygraph evidence offends the rule about character evidence. This rule holds that an accused may introduce evidence of his general reputation, but he cannot relate specific acts which might tend to establish his character. The results of a polygraph test would infringe this rule since it would amount to evidence that he did not lie in a specific event – the test.

Lastly, admitting polygraph evidence is contrary to the expert evidence rule. This rule says that an expert may only give their opinion about something if it will help the judge or jury understand a subject that is outside their understanding or experience. If the judge or jury can form their own opinion, then the testimony of experts is unnecessary. In applying this rule to polygraph evidence, the Supreme Court of Canada decided that such evidence would relate only to the issue of the accused’s credibility and this issue is well within the court’s ability and understanding.


While polygraph evidence cannot be used in criminal trials, the law is not so clear in family court matters. In a 1995 B.C. family court case, a parent who passed a polygraph was allowed to present that evidence to support testimony that he did not sexually abuse his child. The judge decided the polygraph evidence could be introduced, but weighed it with all other evidence in the trial. (see C. (R.M.) v. C. (J.R.), [1995] B.C.W.L.D. 1337) However, more recent family cases in British Columbia have applied the principles from R. v. Beland and ruled that polygraph results may not be used in evidence.

eNews 08/12/15 explains how your local courthouse library or public library can help you research if you want to learn more about this topic.