Expert witnesses can make or break a case. In British Columbia courtrooms, a qualified expert can help explain complex science, interpret technical records, or clarify patterns that are outside everyday experience. On the other hand, an expert opinion that is unreliable or overstated can be limited, challenged, or excluded entirely.
If you are facing charges, expert evidence is not just a courtroom detail. It can influence whether the Crown can prove key elements of an offence, whether a defence theory is believable, and how a judge or jury evaluates the evidence.
This article explains what makes someone an expert, how Canadian courts decide whether expert evidence is admissible, and how expert testimony is used and challenged in BC criminal trials.
What Makes Someone an Expert Witness
An expert witness is not simply someone with an impressive title. In Canadian criminal trials, an expert is a person with specialized knowledge that can assist the judge or jury on a topic outside ordinary experience.
Courts typically assess an expert’s education, training, professional experience, and practical familiarity with the subject area. Depending on the case, an expert may be asked to interpret DNA results, explain toxicology and impairment, analyze digital evidence, or speak to medical findings and injury timelines.
A key point is scope. Even a highly qualified professional can only give opinions within the specific area they are being qualified for. For example, a toxicologist may be well positioned to explain drug metabolism, but that does not automatically qualify them to provide accident reconstruction opinions.
The Legal Test for Admitting Expert Evidence in Canada
Expert evidence is an exception to the general rule that witnesses should not give opinion evidence. Because expert opinions can be persuasive, judges act as gatekeepers before expert testimony is presented at trial.
The Mohan Framework
The Supreme Court of Canada decision in R v Mohan sets the core requirements for admitting expert evidence. In plain language, expert evidence must be:
- Relevant to an issue in the trial
- Necessary to help the judge or jury understand something outside ordinary experience
- Not barred by another exclusionary rule
- Provided by a properly qualified expert
Reliability and Gatekeeping
Meeting the Mohan criteria is not the end of the analysis. Courts have repeatedly emphasized that expert evidence must be reliable and properly limited. Trial judges are expected to prevent opinions that are speculative, methodologically weak, or framed in a way that distracts from the real issues the court must decide.
Impartiality, Independence, and Expert Credibility
Admissibility also depends on whether the expert understands their duty to the court. In White Burgess Langille Inman v Abbott and Haliburton Co., the Supreme Court confirmed that experts must be willing and able to provide fair, objective, and non-partisan assistance.
If there are serious concerns that an expert is acting as an advocate for the side that hired them, that can affect whether their evidence is admitted at all.
How Experts Influence BC Criminal Cases
Expert evidence appears frequently in British Columbia criminal matters, including:
- Impaired driving cases involving alcohol or drugs
- Drug offences involving testing, quantities, or interpretations of substances
- Violent offences where medical evidence and causation are disputed
- Sexual offences involving digital evidence, timelines, or technical records
- Cases involving forensic identification such as DNA, firearms, or trace evidence
Experts tend to affect outcomes in two main ways.
1) Experts can strengthen the Crown’s case
The Crown may rely on expert evidence to prove key points such as:
- Blood alcohol concentration, drug levels, and impairment interpretation
- DNA comparisons, firearms analysis, trace evidence, or lab processes
- Cell tower mapping, device extraction, and metadata interpretation
- Medical evidence about injury patterns, timelines, and causation
Strong expert evidence can help the Crown connect an accused person to an event and support the Crown’s theory of what happened.
2) Experts can support the defence and create reasonable doubt
Defence experts can be used to challenge the Crown’s conclusions or offer an alternative explanation, such as:
- Identifying limits in testing methods, lab procedures, or chain of continuity
- Explaining alternative causes of injuries or different timelines
- Challenging assumptions in accident reconstruction, digital inferences, or identification evidence
- Clarifying how medication, tolerance, and testing delays affect toxicology interpretations
Defence experts are often most effective when they clearly explain why the Crown’s opinion goes beyond what the science can reliably support, rather than simply stating disagreement.
How Expert Evidence Is Challenged in Court
An expert report is not automatically accepted as truth. Expert evidence can be challenged at two stages: admissibility and weight.
Challenging admissibility before the jury hears it
A defence lawyer can argue that the opinion does not meet the Mohan criteria, is unnecessary, or lacks reliability. Courts are cautious about expert evidence that is speculative, unsupported, or broader than the expert’s true area of expertise.
Challenging the opinion through cross-examination
If the expert is admitted, cross-examination often determines how persuasive the opinion will be. Common pressure points include:
- Whether the expert followed a recognized methodology
- Error rates, limitations, and alternative explanations
- Assumptions the expert cannot actually prove
- Overstating certainty or presenting conclusions too strongly
- Ignoring contradictory information
This is often where credibility is won or lost. Judges and juries pay close attention to whether an expert is careful, balanced, and transparent about limits.
The “hired gun” concern
Courts are alert to experts who appear to be advocating for one side rather than assisting the court. That is why impartiality and independence remain central themes in how expert evidence is assessed.
Common Expert Types in BC Criminal Trials
Forensic experts
Forensic experts may testify about DNA, fingerprints, firearms analysis, trace evidence, or laboratory procedures. These opinions can be persuasive, but they often depend on proper protocols and continuity. Even small procedural issues can become important if the Crown relies on the opinion to link someone to the offence.
Toxicology experts
Toxicology evidence appears in impaired driving and drug-related prosecutions, and sometimes in overdose and interaction cases. Timing, metabolism, tolerance, and the limits of interpreting impairment from a number can all be contested issues.
Expert Witnesses Require Strategy, Not Just Science
Expert evidence is not only about what a report says. It is about how that opinion fits into the legal issues the court must decide, and whether it is admissible, reliable, and properly limited.
A strong defence strategy often includes:
- Identifying early whether the Crown’s theory depends on expert conclusions
- Deciding whether to challenge admissibility, weight, or both
- Retaining the right expert when an independent review is necessary
- Keeping opinions focused and avoiding speculation
- Ensuring the expert does not stray into conclusions the court must decide
In many cases, the most important step is recognizing that expert evidence can be contested. Courts do not accept it automatically, and a well-prepared challenge can meaningfully change the direction of a case.
Speak With Michael Shapray About Your Criminal Defence
If you are facing criminal charges in British Columbia and expert evidence is part of the case, you need defence advice that takes expert issues seriously. Whether the Crown is relying on forensic analysis, toxicology, or digital evidence, admissibility and credibility questions can shape the outcome.
Contact Michael Shapray to review the evidence, identify pressure points, and build a defence strategy grounded in both law and facts.