Thin skull rule: when a cracked past opens a personal injury claim

Some clients walk into a personal injury file already carrying a history; it can be a bad back, an old concussion, or an anxiety that predates the accident by a decade. The instinct, from both sides, is to treat that history as a problem. However, the thin skull rule provides otherwise. This doctrine now sits at the heart of how courts handle pre-existing conditions in personal injury damages assessment, and recent court decisions have sharpened its edges considerably.

What is the thin skull rule?

The thin skull rule provides that a negligent party is fully liable for all the harm they caused, even if that harm is far worse than what an average person would have experienced, because the plaintiff happened to be more physically or psychologically vulnerable. It simply means that a defendant must take their victim as they find them.

The principle comes from tort law, and it has been part of Canadian law for a long time. The idea is straightforward: careless people do not get to choose who they hurt. If they happen to hurt someone who was already fragile, they are still on the hook for all of it.

The Supreme Court of Canada confirmed this in Athey v. Leonati, [1996] 3 SCR 458. The Court put it plainly: “The tortfeasor must take his or her victim as the tortfeasor finds the victim, and is therefore liable even though the plaintiff’s losses are more dramatic than they would be for the average person."

The rule also goes by other names in Canadian courts, including the eggshell skull rule and the eggshell plaintiff rule.

Pre-existing conditions matters most when it comes to the thin skull rule. More about it with this video:

Check out our article on “Personal injuries proceedings act: Limits, notices, and traps” to learn about the factors that lawyers and litigants should not miss in these cases.

How the thin skull rule applies in personal injury cases

The thin skull rule usually covers the following in practice:

• musculoskeletal conditions: degenerative disc disease, prior spinal injuries, or osteoarthritis that were stable before the accident but became severely symptomatic after it
• prior concussions or brain injuries: where a second head impact causes disproportionate cognitive harm because of existing neurological vulnerability
• psychological vulnerabilities: anxiety, depression, or past trauma that made a person more susceptible to a severe psychiatric response after the accident

The condition does not have to be visible or even diagnosed before the accident. What matters is that it was stable at the time. If the tort triggered or dramatically worsened it, the defendant is liable for the full result.

How do courts tell a thin skull apart from a crumbling skull?

This is where the analysis gets harder, and where most personal injury files involving pre-existing conditions actually turn.

The crumbling skull doctrine is the counterpart to the thin skull rule. Crumbling skull applies when a plaintiff’s pre-existing condition was already deteriorating before the accident, and would have caused harm at some point regardless of the tort. In that case, the defendant is not responsible for the full picture. They are only responsible for whatever their negligence accelerated or added.

The Court set out the distinction in Athey: the defendant “need not compensate the plaintiff for any debilitating effects of the pre-existing condition which the plaintiff would have experienced anyway.” But, the Court added, “if there is a measurable risk that the pre-existing condition would have detrimentally affected the plaintiff in the future, regardless of the defendant’s negligence, then this can be taken into account in reducing the overall award.”

The practical dividing line is stability:

• thin skull: where a stable, well-managed condition is pre-existing
• crumbling skull: an actively deteriorating condition may attract a reduction

What courts look for in thin skull vs. crumbling skull

At most, courts would ask the following questions in determining which rule should apply:

• Was the plaintiff functional before the accident, despite their condition? (Working, exercising, living normally)
• Was the condition producing significant symptoms, or was the condition largely dormant?
• Is there medical evidence of a trajectory toward independent deterioration, or is deterioration merely a theoretical possibility?

The burden of proving a crumbling skull reduction sits on the defendant. Since speculation is not enough, actual medical evidence of trajectory, prognosis, and likelihood of independent decline is required.

Common law on thin skull vs. crumbling skull

The New Brunswick Court of Appeal illustrated this clearly in Trainor v. DeArcos, 2025 NBCA 139. The trial judge had applied the crumbling skull doctrine to a plaintiff whose pre-existing psychological conditions were, on the court’s own finding, “well managed” before the accident. The appellate court reversed that finding, holding that a well-managed, stable condition makes someone a thin skull plaintiff, not a crumbling skull one. The 40 percent discount the trial judge had applied to future earning capacity damages was removed entirely.

Here’s another video explaining the thin skull and the crumbling skull rules:

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Does the thin skull rule apply to chronic pain and psychological injury claims?

Yes, the thin skull rule applies to chronic pain and psychological injury claims. Notably, this is one of the most actively litigated areas of personal injury law in Canada right now.

Chronic pain and central sensitization

Central sensitization is a condition where the nervous system becomes hypersensitive to pain signals, producing symptoms that are disproportionate to the underlying physical injury.

Plaintiffs with pre-existing central sensitization are classic thin skull plaintiffs. This is because their nervous systems amplify the damage in ways a defendant cannot predict.

In Morgan v. McCarthy, 2026 NSSC 248, the Nova Scotia Supreme Court dealt with this issue. Here, Madison Morgan had a documented history of fibromyalgia and chronic fatigue dating to 2011. In November 2018, a low-speed rear-end collision set off a chain of escalating pain, headaches, and cognitive impairment that ultimately left her unable to sustain employment.

The Nova Scotia Supreme Court found that “but for the [motor vehicle accident], the worsening of her symptomology and the onset of some new ones would not have occurred when it did.” The accident met the but-for causation test.

However, the Court also found that a crumbling skull reduction was warranted. Morgan’s fibromyalgia had a waxing-and-waning course, and the evidence supported a measurable risk that her symptoms would have returned to a debilitating level at some point regardless. The court applied a 25 percent crumbling skull reduction to the full damages award.

The result was still a substantial award. Before the reduction, the court assessed:

• general damages: $65,000
• diminished earning capacity: $150,000
• future care: $35,000
• lost income (2024 and 2025): $65,500 combined

After the 25 percent crumbling skull reduction and an additional 15 percent for failure to mitigate (Morgan had not pursued recommended psychological therapy), the final award came to $211,324.80.

Psychological injury

Psychological vulnerabilities are equally protected under the thin skull rule. The Supreme Court confirmed in Mustapha v. Culligan of Canada Ltd., 2008 SCC 27, that once it is established a person of ordinary fortitude would have suffered some compensable mental injury from the defendant’s conduct, the defendant must take the plaintiff as found, including all psychiatric fragility.

This was applied in Jane Doe v. His Majesty the King in Right of Ontario, B.H., and Toronto Police Services Board, 2026 ONSC 2807, involving a plaintiff with extensive pre-existing psychological trauma. The court found the plaintiff’s pre-existing history “rendered her psychologically fragile, but fragile is not the same as inevitably ruined.” As a result, a crumbling skull characterisation was rejected, and a full thin skull liability was applied to the psychological harm that the assault caused.

What does a pre-existing condition mean for damages assessment after Athey?

The Athey framework that was established in 1996 still governs how Canadian courts approach damages in pre-existing condition cases. Three principles are settled:

• No apportionment between tortious and non-tortious causes
• The crumbling skull reduces, but does not bar, recovery
• Medical evidence determines which doctrine applies

We’ll discuss these principles below.

1. No apportionment between tortious and non-tortious causes

If the defendant’s negligence materially contributed to the injury, they are fully liable. As such, courts cannot divide the loss between the tort and the pre-existing background condition.

The Court in Athey was direct on this. It said that “[t]here is no basis for a reduction of liability because of the existence of other preconditions.” The crumbling skull is the only legitimate mechanism for reducing an award based on pre-existing conditions.

2. The crumbling skull reduces, but does not bar, recovery

Even where a crumbling skull reduction applies, the plaintiff still recovers for the harm the tort caused or accelerated. The Morgan decision is a good illustration. In that case, while the 25 percent reduction reflected the probability that her fibromyalgia would have returned anyway, it did not wipe out recovery.

3. Medical evidence determines which doctrine applies

The fight in these files is always fought with expert evidence. What was the plaintiff’s functional baseline before the accident? What does the medical prognosis show? Was deterioration inevitable, or merely possible?

How courts answer those questions, on the strength of expert testimony, determines whether the plaintiff walks out with full compensation or a reduced award.

Thin skull rule: the skull, cracked or not, still holds a claim

Just as the thin skull rule protects plaintiffs who come into a courtroom carrying an imperfect physical history, so too does it resist the impulse to treat vulnerability as liability. In sum, a past that is cracked is not a past that is closed.

For personal injury lawyers working with files where the defence leans hard on pre-existing conditions, understanding where the thin skull ends and the crumbling skull begins, and who bears the burden of proving the difference, is the work that shapes outcomes on both sides.

Bookmark Canadian Lawyer’s Personal Injury page to read more about the doctrines for Canadian legal professionals, aside from the thin skull rule.