Crumbling Skull Rule

Established by the Canadian Supreme Court in Athley v Leonati (1996), the crumbling skull rule is a legal doctrine which holds that where a plaintiff had a condition or injury that pre-dates the tort and would have naturally deteriorated or worsened over time, the defendant causing the tort is not responsible to the degree that the condition or injury would have naturally worsened over time.

In other words, if a plaintiff's pre-existing condition would have deteriorated or worsened over time, and the defendant's tort caused some degree of additional harm, the defendant is only responsible for the degree to which they caused additional harm or hastened the deterioration.

This rule is meant to limit a defendant's liability to only the damage they caused, and not to hold them responsible for harm that was inevitable due to the plaintiff's pre-existing condition.

It is important to noted that this rule was established by the Supreme Court of Canada and hence has binding authority on Canadian courts. Although not binding in other jurisdictions, it may have persuasive authority and used as a guide by courts in other common law countries.
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