R v Hayward (1908) 21 Cox CC 692 is a leading case on the issue of causation in the context of unlawful act manslaughter.
In this case, the defendant's unlawful act of threatening his wife with violence caused her to be frightened, which combined with her pre-existing medical condition to lead to her death. The court held that the defendant was responsible for the death, even though he did not physically touch his wife, because his unlawful act was the cause of the chain of events that led to her death.
The court also applied the eggshell skull rule, which means that a defendant is responsible for the full extent of the harm caused to a victim, even if the victim has a pre-existing medical condition that makes them more vulnerable to harm. In other words, the defendant must take the victim as they find them, regardless of their pre-existing medical condition.
The thin skull rule, also known as the eggshell skull rule, is a legal doctrine that holds a defendant responsible for the full extent of the harm caused to a victim, even if the victim has a pre-existing medical condition or vulnerability that makes them more susceptible to injury, as in Smith v Leech Brain & Co (1962). In other words, a defendant must take a victim as they find them, and cannot use the victim's pre-existing condition as a defence to limit their liability, as in R v Blaue (1975).
The rule is called the thin skull rule because it was used in cases where a victim's skull or brain was particularly vulnerable to injury due to a pre-existing condition such as a thin skull, which might increase the risk of harm from a head injury. However, the rule is not limited to cases involving head injuries or medical conditions affecting the skull. It can apply to any pre-existing condition that makes a victim more susceptible to injury or harm.
The thin skull rule is often applied in cases of negligence, personal injury, and criminal law, particularly in cases of assault or battery where a victim's pre-existing condition may make them more vulnerable to harm. The rule serves to protect vulnerable individuals and ensure that defendants are held accountable for the full extent of the harm they cause, regardless of the victim's pre-existing condition or vulnerability.