Expert Evidence: Tips and Traps for Counsel and Employee Experts

Courts and scholars have long grappled with the issue of expert evidence reliability and, in particular, the injustice which can arise when an expert exhibits bias. In Ontario (Natural Resources and Forestry) v. South Bruce Peninsula (Town) (“South Bruce Peninsula”), the Ontario Court of Appeal (“ONCA”) recently engaged these issues at length in the context of experts who are employed by their client. The expert at issue in this case involved a Ministry of Natural Resources and Forestry (“MNR”) employee. The expert testified at trial on behalf of her employer, the MNR, in a prosecution of the Town of the South Bruce Peninsula (“Town”) under the Endangered Species Act (“Act”).

The piping plover is an endangered bird species whose continued existence is threatened by human activity. After disappearing from Ontario in the early 1980s, the piping plover returned in 2007 to nest at Sauble Beach and the birds have returned there to nest each year. In the leadup to the prosecution, the Town and the MNR discussed how to avoid damaging the piping plover’s habitat. The Town indicated its willingness to comply with these guidelines at each meeting and passed a bylaw which prohibited raking Sauble Beach before the birds arrived in the spring or within 30 feet of the sand dunes at any time of the year. Despite these efforts, the Town carried out beach maintenance in a manner which not only exposed them to liability under the Act, but also conflicted with the agreed-to guidelines from the MNR and violated their own bylaw.

In order to establish contravention of the Act at trial, an expert opinion was required to demonstrate what “damage” to the habitat was in fact endured. Both the MNR experts testified that the Town’s maintenance damaged the piping plover habitat; it removed features used for nesting, foraging, shelter and camouflage, and exposed the ecosystem to potential further damage of erosion and flooding. Based in large part on the testimony of the MNR experts, the trial judge found that the Town contravened the Act. On appeal, the Town submitted that the trial judge erred in admitting the opinion evidence of one of those MNR experts, arguing that she lacked the necessary impartiality and independence to be a qualified witness. Despite the unanimous decision to uphold the lower court’s conviction, the ONCA discussed this issue at length with two sets of concurring reasons.

Overview of the Law:

The leading authority on the issue of witness bias is White Burgess Langille Inman v. Abbott and Haliburton Co. (“White Burgess”). The threshold test that emerged from White Burgess is as follows: if a witness is unable or unwilling to fulfil their duty to provide fair, non-partisan and objective assistance, their evidence will not be admitted. The expert must attest or testify on oath that they recognize and accept such duty. The Court in White Burgess notes that the fact that an expert is retained, instructed and paid by one of the adversaries does not, on its own, undermine the expert’s independence, impartiality and freedom from bias. Indeed, the court emphasized that it will be rare that a proposed expert’s evidence would be ruled inadmissible for failing to meet this threshold test.

Drawing on White Burgess, the ONCA in South Bruce Peninsula outlines the principles governing admission of expert witness evidence. First, it was reiterated by both sets of reasons that exclusion at the threshold stage will be a rare event and is only appropriate in clear cases. More commonly, deficiencies in independence or impartiality will be accounted for in the weighing of evidence or prescribing limits to testimony. Justice Peter Lauwers also highlighted that the trial judge’s gatekeeper role is ongoing; throughout the trial process, they retain the residual discretion to exclude evidence even after deeming it admissible if prejudice emerges. With respect to employment relationships in particular, being an employee to a party of the dispute is insufficient for a finding that the witness is biased. However, where an employee is involved in the prosecution’s case, there is an increased risk that the witness will be unable or unwilling to carry out their primary duty to provide fair, non-partisan, and objective assistance to the court. The deeper the witness’ involvement in the prosecution, the greater the risk. It was this latter issue that the Town raised as the basis for bias of one of the MNR witnesses.

The ONCA confirmed that the onus is on the party proffering the expert evidence to prove to the court that they are qualified and their testimony is admissible. This is consistent with White Burgess and the law of evidence generally. If the opposing party wishes to undermine that qualification, they do so through cross examination in order to cause the proffering party to fall short on balance of the threshold test.


The ONCA’s decision regarding the admission of the impugned expert testimony focused on two main submissions by the appellant: 1) the expert worked on the report during her maternity leave, indicating an overzealous commitment to the success of the prosecution; and 2) the expert vetted her report with the aim of making it more “persuasive” and to appear more “objective.” The appellant argued these facts indicated that the expert witness was an advocate for the prosecution. Ultimately, both submissions failed.

1. Maternity Leave

The court characterized the argument that the expert was inherently biased because she decided to work during her maternity leave as “absurd and offensive.” Women are criticized for working during a maternity leave, apparently now reflecting a bias, or historically said to reflect a lack of commitment as a mother; while at the same time, they have historically been criticized for taking a maternity leave at all, purportedly demonstrating a lack of commitment to their professional obligations. It was sexist to pursue this line of argument and the court clearly articulated this.

As the legal profession works towards improving equity in the profession, as lawyers, we should be cognizant of the arguments we put forward on behalf of our clients and avoid any arguments that are rooted in sexist or similarly offensive thinking to the court.

2. Vetting of the Report

The second submission discussed on appeal was the impugned expert’s request that her colleague review the report in order to make it more “persuasive” and Crown counsel’s request that the report appear more “objective.”

The duty of the expert witness is to deliver an impartial opinion to assist the court. Therefore, expert reports should be prepared and reviewed with the aim of presenting a comprehensive, clear and accurate opinion, rather than a “persuasive” document for trial. The fact that the report was vetted for persuasiveness and to ensure it appeared objective suggested a desire to advocate for the party calling the evidence. Although the ONCA did not, on appeal, disqualify the evidence on this basis, Lauwers left open the possibility that such evidence could bring into doubt the impartiality of the report, concluding:

…[P]roperly self-instructed, the trial justice would likely have permitted Ms. Robinson to testify, with any touches of partisanship in her evidence going to the weight that it would be accorded. I do not see this as one of the rare cases in which her complete disqualification would be a realistic or even a fair outcome. (emphasis added)


South Bruce Peninsula is a good application of the White Burgess decision in the context of an expert who is employed by a party. Experts must be vigilant in their work and ensure they can properly assist the court without partisanship. Further, counsel, as advocates, must avoid sexist and similar arguments in an attempt to discredit a professional witness on the basis of such arguments.