First Principles of Business Law

6. Causation

6.3.4. The 'eggshell skull' rule

We have seen that a defendant is liable for harm that is reasonably foreseeable as a consequence of their negligent conduct. But is a defendant liable if the harm is greater in its extent because the particular plaintiff was exceptionally vulnerable? Consider the following example and then answer the question.

A takes his dog for a walk. The dog is large and very friendly. When A gets to the park, he takes off the dog's lead to let it run freely. It does not worry A that there are other people in the park. But the dog runs up to B, an elderly lady, and jumps up to greet her. B, who is unsteady on her feet, is knocked to the ground and, because she suffers from osteoporosis, a degenerative bone disease, she breaks several bones when falling. A argues that even if letting the dog run free was negligent, and even if it was foreseeable that a large dog might cause physical harm of some kind to another user of the park, he could not have foreseen the extent of the harm to B, who was especially frail. A says he is not liable for the harm that is due to B's particular frailty.  Is A's argument correct?