I recently prepared for a mediation involving the death of a family patriarch. It was a high-stakes case—personal, tragic, and contentious. In the lead-up, I was struck by a troubling feature that has become increasingly common: the late service of a seemingly partisan expert report from the camp representing the defendant. The timing of the report alone raised questions about procedural fairness. But what was more concerning was the content—an unwavering defence of the defendant’s conduct that read more like a closing argument than an expert opinion.

The case reminded me why this topic continues to command attention in the personal injury and insurance bar. Whether it is a defence expert being excluded for opining beyond his specialty^[1], or a plaintiff’s expert being barred for failing the impartiality threshold^[2], the courts have drawn a clear line. An expert’s role is not to advocate but to assist. If the report reads like it was reverse-engineered to defend rather than to inform, its utility is diminished, and worse, it is potentially excluded.

This tension between the need for expert guidance and the risk of expert overreach sits at the heart of mediation in many of the cases I mediate. It also raises urgent questions about the utility of medical opinions that lack independence, transparency, or methodological rigour. In Ontario’s personal injury landscape, the credibility of medical experts is paramount. These professionals play a crucial role in shaping the outcomes of cases, influencing both settlements and trial verdicts. I see troubling pattern emerging where certain medical assessors, frequently retained by both defence or plaintiff counsel, have been repeatedly discredited in court decisions, raising serious concerns about their impartiality and the integrity of the justice system.

The issue has not gone unnoticed. In the 2025 Civil Rules Review: Phase Two Consultation Paper^[3], the Ontario judiciary has explicitly acknowledged that partisan expert evidence continues to erode confidence in the litigation process. The report recommends several targeted reforms. These include tighter timelines for the delivery of expert reports, enhanced judicial gatekeeping powers to exclude unreliable or overly partisan opinions, and a renewed emphasis on pre-trial discussions aimed at narrowing expert issues. These proposals reflect a growing recognition that the existing framework, while well-intentioned, is insufficient to curb advocacy disguised as expert analysis. The objective is not just procedural tidiness, but the restoration of expert evidence to its correct purpose to be illuminating, not further polarizing, the adjudicative process.

The Problem of Partisan Assessments

Medical experts are expected to provide unbiased, objective opinions to assist the court in understanding complex medical issues. Yet, some assessors have developed reputations for consistently producing reports that favor the retaining party. These reports sometimes exhibit striking similarities, with only the claimant’s name and specific details altered, a practice that undermines their credibility.

As a mediator, I encounter this issue regularly. The same medical assessors appear repeatedly on both sides of the fence, and their reports are often met with immediate skepticism. Their opinions are routinely canceled out by equally partisan reports from the opposing side, leading to a form of evidentiary gridlock. One party dismisses the other’s medical evidence as biased, and vice versa.

Yet, the system is structured so that parties are penalized if they show up to mediation without medical backing. When plaintiff’s counsel arrives without a report confirming loss of function or chronic pain, the insurer complains that there is no evidentiary basis for the claim. Conversely, when the insurer brings a medical report supporting the defense, the plaintiff side often brushes it off as inherently biased. The result is a stalemate, where experts do not resolve disputes. Instead, they deepen them.

Case Law Highlighting Discredited Experts

Ontario courts have not turned a blind eye to this issue. In Moustakis v. Agbuya 2023 ONSC 6012^[4], Justice Merritt critically examined the testimony of Dr. F., an orthopedic surgeon frequently retained by insurers. The court found that doctor’s opinions ventured beyond his area of expertise, particularly into psychiatric evaluations, and displayed a lack of impartiality. Justice Merritt concluded that Dr. F’s testimony was more prejudicial than probative, leading to its exclusion from evidence.

Similarly, in Parliament v. Conley 2021 ONCA 261^[5], the court overturned a jury verdict after determining that the defense’s medical expert provided opinions that exceeded his qualifications and lacked impartiality. The expert’s testimony was found to have usurped the jury’s role in assessing credibility and factual matters, necessitating a new trial.

In Mamado v. Fridson 2016 ONSC 4080^[6], Justice Baltman scrutinized the defense experts, a physiatrist and a psychiatrist. The court identified serious flaws in their opinions, noting that the physiatrist misread portions of the plaintiff’s medical history and failed to document several tests she claimed to have performed. Justice Baltman also highlighted that the doctor’s income predominantly came from assessments for defense lawyers and insurance companies, and she had rarely testified on behalf of plaintiffs.

In Denman v. Radovanovic, 2023 ONSC 1160^[7], Justice Ferguson conducted a mid-trial voir dire and ultimately excluded the defense’s expert who was tendered to testify on causation and standard of care in a catastrophic medical-malpractice case—due to significant bias issues. It emerged that the doctor was a “professional friend” and co-author with one of the defendant neurosurgeons yet failed to disclose that relationship in his reports. During cross-examination, he repeatedly expressed opinions favorable to the defense without grounding them in charted records or objective analysis, and he acknowledged that in malpractice litigation he exclusively testifies for defense clients and has never opined against a fellow physician. Justice Ferguson found that defence expert breached his duty of impartiality, including by impermissibly assessing witness credibility, cherry-picking evidence, and offering unqualified opinions. Applying the Supreme Court’s gatekeeping test for expert evidence, she determined these failings rendered him incapable of providing fair, objective, non-partisan testimony, leading her to reject his evidence at the preliminary stage.

In FortuneOzoike v. WalMart Canada Corp., 2023 ONSC 421^[8], Justice Vermette excoriated the defense’s expert witnesses following the catastrophic amputation of the plaintiff’s leg. The orthopedic expert, Dr. P, delivered what the judge called a “deplorable” performance, offering testimony that was evasive, non‐responsive to simple questions, and blatantly contradicted by the literature he himself cited in his reports. He refused to concede even the most obvious points, provided opinions that made no sense in light of standard texts, and failed to uphold the impartiality expected of an expert witness. Similarly, the defense’s emergency medicine expert Dr. B fared no better. Justice Vermette was astonished by how he makes too fine and selective points, provided inconsistent answers, refused to make obvious concessions, and effectively acted more as an advocate for the defense than as a neutral expert. The judge concluded both experts had breached their duty to the court by cherrypicking data, favoring adversarial positions, and abandoning their role as objective helpers, ultimately rejecting their evidence on critical points of standard of care and causation. This case stands as a stern reminder that expert witnesses must transcend partisan interests and faithfully support the trier of fact.

In Graul v. Kansal 2022 ONSC 1958^[9], Justice Lemon admonished two defense experts, a neurologist and a neuropsychologist, for their lack of impartiality. He encouraged them to focus their medical expertise on patients in need rather than serving as well-paid advocates in litigation.

In the very recent Cairns v. Ellis, 2025 ONSC 1762^[10], the Divisional Court upheld the trial judge’s exclusion of the plaintiff’s proposed expert neurologist, Dr. B. The trial judge had conducted a lengthy voir dire and concluded that Dr. B failed to meet the threshold for impartiality, citing his careless methodology, lack of documentation, and unwillingness to reconsider opinions in light of clear factual errors. The judge found that Dr. B’s reports were constructed using “macros and leading questions” designed to support predetermined conclusions, raising serious concerns about his objectivity. The trial judge’s ruling reflected the gatekeeping function outlined in the Supreme Court of Canada case White Burgess Langille Inman v. Abbott and Haliburton Co.^[11], and was affirmed on appeal. The case is a striking reminder that concerns about expert partisanship are not confined to defence experts alone. Plaintiff-side experts must equally satisfy the requirement of fairness, independence, and methodological transparency.

While much of the recent judicial scrutiny has focused on defence-retained experts, Ontario courts have not hesitated to discredit plaintiff-side experts when their opinions lack reliability or methodological rigour. In civil negligence matters like Parliament v. Conley (2021 ONSC 2785)^[12], courts have shown their willingness to police overreach. In that case, the plaintiff’s expert testimony was found to improperly assess witness credibility, prompting a new trial. And in Moore v. Getahun (2015 ONCA 55)^[13], the Court of Appeal struck a careful balance, cautioning that while some collaboration between counsel and experts is permissible, the independence of the expert must remain intact. In R. v. Hason 2024 ONCA 369^[14], a criminal case, the Ontario Court of Appeal affirmed that an expert’s conduct in prior proceedings, marked by boilerplate reasoning and resistance to correction, could justify exclusion in subsequent cases. These cases reinforce the principle that credibility, impartiality, and methodological soundness are non-negotiable, regardless of which side calls the expert.

Current Rules Addressing Expert Bias

In response to longstanding concerns about biased expert testimony, Ontario’s legal system has enacted a series of procedures aimed at promoting objectivity and fairness. Chief among these is Rule 4.1^[15] of the Rules of Civil Procedure, introduced following the recommendations of the 2007 Osborne Report. This rule codifies the principle that expert witnesses owe their primary duty to the court—not to the party who retains them. It requires that their opinion evidence be fair, objective, and non-partisan, and that they limit their testimony to matters within their demonstrated area of expertise.

Complementing this, Rule 53.03^[16] stipulates that expert witnesses must sign Form 53, thereby formally acknowledging their overriding duty to the court. This rule also prescribes the core contents of an expert report, including a clear statement of the expert’s qualifications, a description of the instructions received from counsel, and the factual and theoretical basis for the opinions provided.

Procedural safeguards also exist at the pre-trial stage. Under Rule 50.06^[17], parties are required to address the number of expert witnesses they intend to call and the anticipated timing of expert reports. These discussions are intended to reduce the likelihood of trial ambush and to encourage efficiency and transparency.

However, despite these regulatory efforts, concerns about expert bias remain deeply entrenched in the judiciary and in the legal profession as a whole. The inherently adversarial nature of personal injury and malpractice litigation incentivizes counsel to retain experts whose views support their theory of the case. As a result, the problem of partisan advocacy masquerading as objective opinion persists, fueling judicial skepticism and prompting calls for more rigorous gatekeeping.

The Future of Expert Evidence: Key Reform Proposals in the 2025 Civil Rules Review

Ontario’s ongoing Civil Rules Review has identified the problematic use of expert evidence as a significant barrier to fairness and efficiency in civil litigation. The 2025 Phase Two Consultation Paper^[18] released by the Attorney General’s Advisory Committee includes several key proposals aimed at reforming the expert evidence regime.

Most notably, the report proposes amending Rule 53.03 to require earlier delivery of expert reports, well in advance of trial, and ideally prior to mediation. This change is intended to reduce trial ambushes and promote more meaningful pre-trial resolution. The Committee noted that late delivery of expert evidence is often used tactically and undermines fairness.

The report also recommends stronger judicial gatekeeping powers to exclude experts who are not truly independent. Drawing inspiration from cases like White Burgess Langille Inman v. Abbott and Haliburton Co., the report supports codifying a formal pre-trial admissibility process to challenge experts who fail the neutrality threshold.

Another major reform under consideration is the mandatory disclosure of expert retainer letters, intended to increase transparency around the scope of expert engagement and prevent improper coaching. The Committee is considering whether these should be provided automatically at the same time as the report itself.

Finally, the paper raises the possibility of curbing repeat-use “hired gun” experts through judicial discretion to limit their participation based on past findings of bias or exclusion. This recommendation reflects growing concern that some assessors have become too embedded in the litigation process, particularly in personal injury and medical malpractice cases.

If enacted, these changes could significantly shift the culture surrounding expert evidence in Ontario, reinforcing the principle that experts serve the court, not the party paying them.

The Role of Mediators

As a mediator, I often find myself navigating the complexities introduced by partisan expert reports. When both sides present conflicting medical opinions, it becomes challenging to facilitate a resolution. The credibility of these reports is frequently questioned, and their value in advancing settlement discussions is diminished.

To mitigate the challenges posed by biased or unhelpful expert evidence, parties should consider adopting several best practices. Disclosing expert reports well in advance of mediation promotes transparency, allows for meaningful review, and reduces the risk of disruptive surprises that can derail productive negotiations. I also believe that shifting the focus of expert discussion away from rigid medical classifications and toward the claimant’s actual functional limitations often leads to a more grounded and constructive resolution process.

Conclusion

The integrity of the personal injury legal system in Ontario depends on the reliability of expert testimony. While safeguards have been implemented to address the issue of biased medical assessors, challenges still remain, as evidenced by the 2025 Civil Rules Review. As mediators, legal professionals, and stakeholders, we must continue to advocate for practices that prioritize impartiality and fairness, ensuring that expert evidence serves its intended purpose of aiding, rather than hindering, the pursuit of justice.