One of the most common insurance-company tactics in a New Jersey injury case is to point at your medical history and say the magic words: “pre-existing condition.” The implication is that because you had a prior back problem, an old injury, or some degeneration on a scan, the crash didn’t really hurt you. New Jersey law has a long-standing answer to that argument, and it’s strongly in the injured person’s favor.
You take your victim as you find them
The principle is often called the “eggshell plaintiff” (or “thin-skull”) rule. It means a negligent party is responsible for the full extent of the harm they cause — even if the victim was unusually fragile or had a pre-existing condition that made the injury worse than it would have been for a perfectly healthy person. A wrongdoer doesn’t get a discount because the person they injured was already vulnerable.
Why insurers push the pre-existing argument anyway
Despite the rule, insurers lean on pre-existing conditions constantly, because they know it can confuse a jury and discourage a claimant. It shows up most in cases involving back and spinal injuries (where “degenerative changes” appear on nearly everyone’s imaging after a certain age) and head injuries. The argument isn’t a winner under the law — but only if the case is built to answer it.
How you defeat the pre-existing defense
The eggshell rule protects you, but you still have to prove the aggravation. That’s done with a careful record:
- Comparing before and after — your condition and function before the incident versus after.
- Prompt, consistent treatment — documenting the new or worsened symptoms from the start.
- Treating-physician testimony — explaining how the incident aggravated the prior condition and what’s attributable to it.
- Prior medical records — which, handled correctly, often help by establishing your baseline.
It fits with New Jersey’s broader rules
The eggshell rule works alongside the other principles that govern injury cases — comparative negligence (which apportions fault, not the extent of harm) and, in auto cases, the verbal threshold. Aggravation of a pre-existing condition can itself satisfy the threshold when it’s permanent.
Being told your injury is “pre-existing”? Don’t accept it
A prior condition does not let a negligent party off the hook for making it worse. If an insurer is using your medical history to minimize a real injury, we’ll build the case to prove the aggravation their client caused. The consultation is free.
Part of our complete guide: For every related New Jersey offense, claim, and defense in one place, see our NJ Personal Injury Guide.