Experts giving evidence in an Ontario court are obliged to sign an acknowledgement that they are independent, with their obligation being to the court and not to the party who retained them. Nonetheless, scepticism regarding objectiveness and discrepancies between expert opinions remains, as demonstrated in the reasons of Justice Mesbur in Plese v. Herjavec, 2018 ONSC 7749:
 I have always been tempted to ask valuators whether their opinions would have been the same had the other party retained them. I have never given in to that temptation, but merely make the observation. It seems to me that in order to provide the court with truly independent, unbiased and reliable opinions, it would be preferable to require the parties to jointly retain a single expert, or, perhaps, to require the parties to fund an expert who would be retained by the court, at the parties’ expense.
I would welcome the procedure proposed by Justice Mesbur, but who is to say that only my opinion would be correct? I often am being called upon to critique an expert opinion and, if warranted, to rebut the opinion of distinguished colleagues. How is it that we can disagree to the point of holding contrary opinions?
Certainly, it is a well-known and acceptable concept in medicine to seek a second opinion. From what I can see, it is no different in law; needing a second opinion is the reason why we have the Court of Appeal for Ontario. When it comes to psychiatric expert testimony, I note that the Supreme Court of Canada has commented on the limitations of psychiatric evaluation in Starson v. Swayze, 2003 SCC 32:
 … Psychiatry is not an exact science, and “capable but dissident interpretations of information” are to be expected: see Weisstub Report, supra, at p. 229.
Psychiatric experts may disagree on the phenomenology of symptoms and signs resulting in differences of opinion on capacity evaluation. Rationality of thought and cognitive ability to understand and appreciate varies along a sliding scale that is open to varying interpretations.
Additionally, capacity evaluation is situation specific and so from a clinical context, the situation specific aspects of each individual case determine the approach to capacity evaluation.
Capacity, in law, serves as the effective threshold of autonomy, dividing the autonomous, on the one side, from the non‐autonomous, on the other. The key factor is an individual’s ability to engage in the process of rational thought, explained as the ability to exercise one’s will to reflect upon and choose between desires, and then to adopt a desire as one’s “own.”[i] When evaluating a patient based on the course of action (or inaction) chosen, it is important to distinguish between whether the person’s judgment is sound, versus whether they have the capacity to choose.
From the clinical view, one can argue that determining whether a patient is able to make autonomous decisions is not the only factor relevant to evaluating a patient’s capacity. Other factors, such as assessing risk tolerance and even actual harm to individuals comes into play. The Royal College of Physicians and Surgeons of Canada has defined the role of a “medical specialist” to include health advocacy. This mandate may bias the clinician towards taking into consideration the best interests of a patient. In other words, the clinician may consider the outcome of a finding of capacity or incapacity, and not only whether the patient meets the legal definition of “capacity.” Therein lies the tension.
I suggest that the most likely reasons that psychiatric experts may reach different conclusions following a capacity assessment is not only because of varying interpretations of phenomenology of symptoms and signs, but also because of varying opinions regarding the weight to be given to best interests, tolerance of risk, and likelihood of harm. These are clinical issues inherent to the role of psychiatrists as health advocates that differs from the legal perspective.
[i] Margaret Isabel Hall, “Mental Capacity in the (Civil) Law: Capacity, Autonomy, and Vulnerability” (2012) 58:1 McGill L. J. 61, 65.