Thin skull rule is a principle of common law which states that particularly fragile victims of torts should be fully compensated for their losses, even where the damages arising out of their predisposing condition were not foreseeable to the defendant’s particular susceptibility.
The term implies that if a person had a delicate skull, and a tortfeasor or assailant who did not know of that condition were to hit that person on the head, causing the skull unexpectedly to break, the responsible party would be held liable for all damages resulting from the wrongful contact, even though they were not foreseeable. The general maxim is that defendants must "take their victims as they find them".
It is an additional exposure in tort liability towards persons who are particularly vulnerable or more fragile than the norm, who may have inherent weaknesses or a pre-existing vulnerability or condition; the tort-feasor takes his/her victim as s/he finds them; s/he compensates for all damages s/he caused, even if damages are elevated compared to a norm because the plaintiff was thin skulled.
In American law, the term eggshell skull doctrine is also used.