How do Ontario courts decide which parent gets to make the medical decision when they disagree?
Medical decisions for the child fall under decision-making responsibility (i.e. “custody” before recent amendments).
Courts have broad powers with respect to allocating decision-making responsibility. They can order that one parent has decision-making responsibility in one context, such as medical decisions, or even a more specific context such as vaccinations, while the other parent gets to make all the decisions related to other contexts, such as school choice. Alternatively, the court can provide that parents are to make decisions together, or one parent is to make all decisions for the child. However, courts must make the order according to the “best interests of the child”, pursuant to section 16 of the Divorce Act or section 24 of the CLRA.
If parents cannot agree on a medical decision for their child, and the medical decision has yet to be decided or contemplated by the court, a parent can ask the court to make the decision on that particular issue. Courts are willing to do this even where parents have a prior agreement in place outlining decision-making responsibility, but subsequently disagree (see examples, CMG v DWS, 2015 ONSC 2201; and, BLO v LJB, 2019 ONCJ 534). This is clearly outlined in section 56(1) of the Family Law Act:
In the determination of a matter respecting the education, moral training or decision-making responsibility or parenting time with respect to a child, the court may disregard any provision of a domestic contract pertaining to the matter where, in the opinion of the court, to do so is in the best interests of the child
Judges are not doctors nor scientists, whereas medical decisions clearly need to have regard to a medical authority. Courts can typically reconcile this by appealing to the medical authority. For parents making their submissions to the court, a medical decision is, by nature, more about what the medical authority says and, consequently, evidentiary principles. In other words, what does the prevailing medical authority say about the issue, how strong is the authority, and how is the parent to get this authority heard before the court? BLO v LJB [2019 ONCJ 534], illustrates the use of medical authority, demonstrating that proper medical evidence can strongly sway the court:
Dr. Robinson's testimony was properly before the Court. Her evidence was relevant, necessary to assist the Court, there was no exclusionary rule preventing it and she was properly qualified to testify about the vaccine dispute before the Court. See R. v. Mohan,  2 S.C.R. 9 (S.C.C.).
The Court completely accepts Dr. Robinson's evidence.
Quite apart from the strength of Dr. Robinson's evidence on its own, there is no evidence from a competing expert to the contrary. The mother's evidence is based on her own research, plus "Dr. Paul's" internet pamphlet. The mother is not an expert and the information from "Dr. Paul" is hearsay.
I acknowledge that pursuant to rule 14(19) of the Family Law Rules, the Court may admit and rely on hearsay. That said, the hearsay upon which the mother wishes to rely is in the nature of opinion. It does not come anywhere close to meeting the requirements for admissibility in R. v. Mohan. Even if the information from the internet is admissible hearsay, by virtue of rule 14 (19), which I seriously doubt, the Court would place no weight on it.
To be clear, I wish to acknowledge that counsel for the mother readily agreed that Dr. Robinson should be qualified as an expert. Acting on the mother's instructions, counsel for the mother cross-examined Dr. Robinson on much of the subject matter that is contained in the "Dr. Paul" document. However, the cross-examination could not undermine the science that underlies Dr. Robinson's opinion.
As iterated, however, this must be examined through the lens of the “best interests” test, which can complicate matters. For instance, what if the science says that a certain medical decision should be ordered, but the child, who is mature, does not want that medical treatment? This requires an additional analysis into whether the court should appeal to the child’s preferences relating to the medical decision in question. In Manitoba v C (A), 2009 SCC 30, the leading Supreme Court decision on the matter, the Court set out relevant factors on this very issue. Parties should always look to the best interests of the child in medical cases. Courts will make their decision according to that test.
Obvious takeaways? If parents disagree on a medical decision for their child, the court can decide which parent will get the final say on the matter. The case law suggests that the court will tend to follow the prevailing medical authority on the matter where it is available.