The principle appears to have emanated from a 1901 English case, Dulieu v. White and Sons, where it was stated:
“If a man is negligently run over or otherwise negligently injured in his body, it is no answer to the sufferer’s claim for damage that he would have suffered less injury, or no injury at all, if he had not had an unusually thin skull or an unusually weak heart.”
For example, if a person who has physical or psychological infirmities which extend, beyond a the norm, his/her recovery from injuries resulting from another's tort, the defendant’s damages are not discounted accordingly but, instead, are adjusted upwards to fit the "thin skulled" victim.
In American law, the term eggshell skull doctrine is also used. In Fleckner v Fleckner, Justice Sadler of the Court of Appeals of Ohio wrote, at ¶22:
"The thin skull or eggshell plaintiff rule is a creature of tort law, which states that, a defendant who negligently inflicts injury on another takes the injured party as he finds her, which means it is not a defense that some other person of greater strength, constitution, or emotional makeup might have been less injured, or differently injured, or quicker to recover."
It matters not, to the application of this rule, that the fragility or thin skull characteristic of a tort victim was latent or not.
In 1996, Canada’s Supreme Court, in Athley v Leonati
, noted that the thin skull rule:
“... makes the TortFeasor liable for the plaintiff’s injuries even if the injuries are unexpectedly severe owing to a pre-existing condition. 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.”
In 2001, R v Nette, Canada’s Supreme Court again spoke of this doctrine:
“The thin-skull rule, which is a long-standing principle of tort law, provides that a wrongdoer must take his victim as he finds him. Thus, the fact that a victim’s head injuries are aggravated beyond what would normally be expected because of the victim’s unusually thin skull does not relieve a TortFeasor of liability for the full extent of the harm that resulted from his wrongdoing.
“The thin-skull rule is a good and useful principle. It requires aggressors, once embarked on their dangerous course of conduct which may foreseeably injure others, to take responsibility for all the consequences that ensue, even to death.”
It is distinguished from the crumbling skull rule as stated in Shaw v. Clark:
“The distinction between a thin skull case and a crumbling skull case is that in the former the skull, although thinner than the average skull, is in a stable condition before the accident and, but for the accident, would have remained so. The latter is where the skull, whether thick or thin, is not in a stable condition before the accident but in a state of continuing deterioration which the accident has merely accelerated.”