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.
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.),  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.