What is the "eggshell skull" rule and how does it work?

The eggshell skull rule (or thin skull rule) describes the principle that a defendant must "take the victim as he finds them". This means that particular vulnerabilities or frailties of a victim cannot be considered when determining the liability of the defendant. For example, if the defendant's action of punching the victim leads to death because of a rare bone disease that the victim has (whereas it would usually only cause mild injury) the defendant cannot rely on this as a defence, even if he was unaware of the victim's condition. In summary, where the full extent of the damage was unforeseeable due to the victim's particular vulnerability or characteristic, the defendant will still be liable for the full extent of the damage. This principle is important in tort and criminal law. 

R v Blaue is a useful case to illustrate the operation of the rule. The victim of an attack refused a blood transfusion on religious grounds even though the transfusion would have saved her life. The defendant was liable for the death because the victim's religious beliefs were an unexpected characteristic that fell within the scope of the eggshell skull rule.