What is the difference between the thin skull and crumbling skull rules in Ontario negligence law?

Both rules deal with a plaintiff who had a pre-existing vulnerability, but they lead to very different results. The thin skull rule requires a defendant to compensate for the full extent of an injury even though a pre-existing weakness made it worse than it would have been for an average person. The crumbling skull rule applies where the pre-existing condition was already going to cause problems on its own, regardless of the accident - here, the defendant is only responsible for making the plaintiff's situation worse than it already would have become, not for the underlying condition itself.

In practice, courts try to work out what would likely have happened to the plaintiff's health over time even without the accident, sometimes called the "measurable risk" of the pre-existing condition manifesting anyway, and reduce the damages award to reflect only the genuine additional harm the defendant caused. This distinction often becomes the central dispute in claims involving prior injuries, degenerative conditions, or chronic pain histories, since a defendant will typically argue crumbling skull to reduce damages while a plaintiff argues thin skull to preserve full compensation. Detailed pre-accident medical records and expert opinion on the person's likely health trajectory are usually decisive.