What is the thin skull rule and how does it affect damages in an Ontario injury claim?

The thin skull rule means a defendant who negligently injures you must compensate you for the full extent of the harm actually caused, even if you had a pre-existing vulnerability that made the injury worse than it would have been for an average, healthy person. The idea behind the name: if a person has an unusually thin skull and a minor blow causes a serious fracture, the defendant is still liable for the serious fracture, not just the bump a healthier person would have suffered. In Ontario injury claims, this matters most where a plaintiff has a pre-existing condition, chronic pain history, or psychological vulnerability that the defendant did not know about and could not have foreseen.

The rule exists because Ontario courts take the plaintiff "as they find them" rather than requiring proof the defendant should have anticipated the specific vulnerability. It is separate from questions about whether the injury was reasonably foreseeable in a general sense, and separate from the related crumbling skull principle, which addresses pre-existing conditions that would have caused problems regardless of the accident. Because pre-existing conditions are often disputed, medical records from before the incident and expert evidence about how the injury actually unfolded are usually central to how this argument plays out.