The eggshell skull rule or thin-skull rule is a doctrine applied in both tort law and criminal law under which a defendant will be held responsible for all the consequence of his action, even if the plaintiff in a civil case or the victim in a criminal case had a preexisting medical condition or other vulnerability, such as if the plaintiff's skull is as fragile as an eggshell. This doctrine is often used in battery litigation.
A famous case that invoked the eggshell skull doctrine is Vosburg v. Putney, decided by the Wisconsin Supreme Court in 1891, in which a student kicked another student in the shin, which was injured earlier in a sledding accident. The kick caused the previous injury to worsen, leading to the boy losing the use of the limb. The court decided that the student was fully liable even if he did not intend to cause serious harm. In 2001, the Oregon Court of Appeals ruled in the case Fuller v. Merton that if the eggshell skull doctrine is applicable in a case the presiding judge must include the doctrine in the jury instructions.
It is controversial whether the eggshell skull doctrine should be applied to severe mental distress ("eggshell psyche"), due to that psychological injury can manifest in physical injury, and purely mental injury can be faked. The past cases on this issue have been inconclusive.
Crumbling skull rule - a doctrine that the actions of the tortfeasor did not contribute significantly to the already compromised condition of the victim.