CSI may have spawned innumerable spinoffs, but it turns out you can’t trust TV — or sniffer dogs: new research is questioning the scientific basis for the dogs’ use by police, as well as aspects of bloodstain pattern analysis and ballistics evidence routinely used in courts.
One published paper in The Alberta Law Review and two forthcoming articles in The Canadian Bar Review and The Dalhousie Law Journal examine how courts treat these forms of evidence and argue that greater scrutiny is needed.
“Just because a technique has been used in court for decades doesn’t mean we should take its reliability for granted, especially when someone’s liberty is at stake in a criminal trial,” said author Sara Gordon, associate professor at Peter A. Allard School of Law at UBC. “My hope is that this research will help judges and lawyers better understand the limitations of these techniques and ensure that forensic evidence is presented in a way that accurately reflects what the science can — and can’t — tell us.”
Gordon noticed a lag between advances in the scientific understanding of forensic evidence and its treatment in court. She examined several forensic and investigative techniques commonly used by police and regularly admitted as evidence in North American courts — some of which have played an important role in securing convictions — and found significant gaps in the scientific support.
Ballistics evidence is perhaps the most familiar, where bullets and cartridge cases are matched to a particular firearm.
While ballistics evidence has been accepted in Canada for more than a century, Gordon found that scientific research does not support the level of confidence to which ballistics experts often testify. In practical testing, for example, examiners misidentified bullets 37 per cent of the time.
Gordon’s forensic firearms article focuses on the use of this evidence in Canada, where it continues to be routinely admitted. In 2023, however, a Chicago court became the first in the United States to bar the use of ballistics-matching evidence in a criminal case.
In Canada, police may use a drug-detection or “sniffer” dog to conduct a search where they have reasonable suspicion that the search will reveal evidence of a drug offence — a lower threshold than ordinarily required for a warrantless search. The Supreme Court of Canada has justified this on the basis that sniffer-dog searches are minimally intrusive, narrowly targeted and highly accurate.
But Gordon’s review of the scientific evidence that has emerged since the Court’s latest rulings in 2013 indicates otherwise. “They’re not nearly as reliable as many courts seem to assume. And in fact, they can be quite invasive.”
The accuracy of sniffer dogs has been called into question repeatedly around the world. One Australian study, for example, found that drugs were not discovered following nearly 75 per cent of police dog alerts. Dogs may alert when no drugs are present for a variety of reasons, including poor training, residual odours, and even odours associated with some common baking ingredients. A handler can also unintentionally influence a dog — for example, by slowing down or behaving differently near a person the handler suspects.
The searches that follow a dog alert can also cause fear, anxiety, anger and humiliation, and may have a particularly negative impact on racialized people.
Gordon proposes several reforms, including clearer standards for training, certification and performance monitoring for the dogs. She also argues that the same standard that generally applies to other warrantless searches should apply to a sniffer-dog search.
Bloodstain pattern analysis involves examining the shapes, sizes and locations of bloodstains at a crime scene in an effort to interpret the physical events that caused them. It has long been accepted by Canadian courts as a physical science.
However, Gordon found that the discipline grew out of the work of a chemist experimenting in his basement, who later helped establish the field’s scientific journal and early textbooks, giving the emerging discipline many of the institutional markers of a scientific field. Practitioners today often invoke concepts such as fluid dynamics despite having little or no formal training in physics, Gordon said.
“If you dig a little deeper, you start to see that the field rests on a much shakier scientific foundation than courts have often assumed. There haven’t been many rigorous scientific studies, and the studies that are now emerging suggest that many of the conclusions police officers have testified to in court are not supported by the science.”
These studies reveal significant error rates: One study found that analysts correctly classified bloodstain patterns only about 70 per cent of the time on rigid surfaces and just 62 per cent of the time on fabrics. Another found that different analysts frequently reached different conclusions when examining the same evidence, while other research showed that what examiners were told about the case or crime scene could influence their conclusions.
Together, the papers call on judges to more actively exercise their gatekeeping role by ensuring that forensic evidence is reliable and that experts do not stray beyond what the science can support, and for greater scientific literacy among lawyers and judges.
Alberta Law Review