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The Supreme Court of Canada building is seen in Ottawa.DAVE CHAN/The Globe and Mail

The Supreme Court of Canada has told the country's prosecutors that expert witnesses, including police officers, must not give opinions based on their experience.

The ruling, in a major drug case, has wide implications for the prosecution of serious crimes, from high-value theft to murder, in which experts are called on to draw inferences or analyze evidence before the court. Such testimony has been linked to wrongful convictions, and the court was unanimous in trying to call a halt to opinion it feels has been disguised as fact.

The court went so far as to mock the expert testimony of an RCMP officer in a case in which a man accused of being a drug courier, Ajitpal Singh Sekhon, said he hadn't been aware that 50 kilograms of cocaine were hidden in the truck he was trying to drive into British Columbia from Washington State. The Mountie had testified that in the 1,000 cases he had been involved in, he had never personally found a single one that involved a "blind" courier – someone kept in the dark about what he was transporting.

That testimony, wrote Justice Michael Moldaver, is no different from a "stolen goods investigator testifying that he or she has never seen a case of innocent possession of stolen property, or an experienced fraud investigator testifying that he or she has never seen a case where a senior manager was not aware of fraudulent conduct occurring within the company. The inherent danger of admitting such evidence is obvious." The trial judge had allowed the evidence, and so had the B.C. Court of Appeal, by a 2-1 count.

Still, the Supreme Court upheld the drug importing and trafficking conviction against Mr. Sekhon by a 5-2 margin. Justice Moldaver, writing for the majority, said the police expert's testimony formed only a small part of the trial judge's reasons for finding Mr. Sekhon guilty. The minority dissented sharply, saying that Justice Moldaver had engaged in "pure speculation" by concluding that the judge would have found Mr. Sekhon guilty without the expert testimony. Mr. Sekhon had been sentenced to 10 years in prison.

Eric Gottardi, a lawyer for Mr. Sekhon, said he was disappointed for his client but called the ruling a big victory for civil rights that will apply widely, to everything from serious theft cases to murder.

He said that Canadian courts had in effect been "contracting out" questions of guilt or innocence to experts. From here on, though, "any time 'experiential' evidence points the finger directly at the accused, that's going to be offside. It's not just drug cases – it's any serious case."

He said that in cases in which police officers testified as experts, it was very difficult for defence lawyers to find people with the expertise to challenge them.

"For a while there was a proliferation of 'profiling' experts. There's been a murder and the killer has staged the house to look like there was a break-in. The Crown would call experts who would testify that the crime scene was staged. A lot of experts would then take an extra step and say, 'the profile tells us the killer is often someone who is known to the victim'" – which would point directly at the guilt of the accused individual.

Ben Berger, a criminal-law specialist at Osgoode Hall Law School in Toronto, said the ruling is an attempt by the court to rein in expert witnesses, part of a trend since Ontario's 2008 Goudge inquiry into more than a dozen wrongful convictions in baby deaths, stemming from the testimony of disgraced Toronto pathologist Charles Smith.

The Supreme Court found that "police experience on its own is something that has to be looked at with suspicion and a critical eye," Prof. Berger said. "That's important not just in a drug setting but in pretty much any criminal investigatory setting."

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