The thin skull rule is well known to lawyers. It makes the at-fault party liable for the plaintiff’s injuries even if the injuries are more severe than expected, owing to a pre-existing condition. The person causing a loss must take his or her victim as he or she finds the victim, and is liable even though the losses are more extensive than they would be for the average person.
If an injury claimant suffers more dramatic injuries than might be expected, and falls under the thin skull rule, money damages are not reduced by reason of the injuries being more extensive than expected.
The crumbling skull rule recognizes that some injury victims have pre-existing conditions which detract from their health and wellness prior to an injury. An example might be a person who is in an automobile accident, and suffers a spinal disc herniation several months later while doing exercises. In this situation, if it can be shown that the disc herniation would have occurred without the accident, then money damages would not be awarded for the occurrence of the herniation.
But in many situations it is not possible to state with certainty whether an injury would inevitably have occurred, absent the accident. Often, all that can be said by medical people is that there was a risk of deterioration, and a likelihood can be assigned to it – say 25%. In the example of the disc herniation, expert evidence must be offered to a court that there was a measurable risk that the disc herniation would have occurred without the accident. Usually, the burden to prove this evidence rests on the defendant, and defendants often overlook to produce this evidence. Without expert evidence of measurable risk, there is no basis to reduce an award to take into account any such risk.
From the standpoint of recovering full compensation for injuries, plaintiffs are better off coming under the thin skull rule. The crumbling skull rule may result in a reduction of damages otherwise obtainable, based on the principle of fairness, that the defendant need not put the plaintiff in a position better than his or her original position, before the injury occurred. But for the court to apply the crumbling skull rule and make a reduction in damages, requires expert evidence of measurable risk that the plaintiff’s skull was crumbling, which often defendants do not provide.