Egg-Shell Skull Rule

The rule's underlying logic was set out by the Court of Appeal in NHS Business Services Authority v Young, which considered whether the 'eggshell skull' concept in the law of tort could apply outside its usual personal-injury context. The court explained: the basis of the rule is that if A tortiously injures B and due to some inherent weakness in B's constitution the consequences are very much more serious than they would be in the normal case, that is nevertheless damage which has been caused by A's tortious wrong, for which A is liable in law.

The rule therefore does not ask whether the extent of the harm was foreseeable — only that some harm of the relevant kind was foreseeable. If a defendant's negligence or other tortious act would cause modest injury to an ordinary claimant but far more serious injury because of an unusual pre-existing vulnerability in the particular claimant, the defendant remains liable for the full, unexpectedly severe consequences rather than only the harm an average person would have suffered.

In Young, the Court of Appeal went further and accepted that the same underlying reasoning could sensibly extend by analogy beyond ordinary tort claims for personal injury, to a different situation in which, as a result of an injury sustained in an employment, an employee who suffers from a pre-existing, unusual weakness, suffers consequences which are very much more serious than they would be for an ordinary employee of ordinary constitution — illustrating that the principle, while rooted in tort, has been treated by the courts as reflecting a more general approach to causation and the assessment of loss.