What it is
The eggshell plaintiff rule (sometimes called the "eggshell skull doctrine") provides that a tortfeasor takes the victim as they find them — frailty, pre-existing conditions, and all. The classic illustration: a defendant who negligently bumps a person with an unusually thin skull is liable for the full extent of the catastrophic head injury that results, even though the same blow to an average person would have caused only a bruise. The doctrine applies broadly to pre-existing physical conditions (degenerative disc disease, prior whiplash, arthritis, prior surgeries) and to mental/emotional vulnerabilities (prior PTSD, anxiety disorders). The doctrine has two related but distinct components. Direct application: the defendant pays for the actual injury caused, even when an average person would have suffered less. Aggravation: when an accident aggravates a previously asymptomatic or stable condition (e.g., a pre-existing herniated disc becomes symptomatic after a rear-end collision), the defendant is liable for the aggravation even though they are not liable for the underlying condition itself. The rule is universal across U.S. jurisdictions and is reflected in standard jury instructions.
How it works in practice
In practice, insurance adjusters and defense attorneys aggressively attack pre-existing conditions, arguing that the claimant's injuries are "really" from prior degenerative changes rather than from the accident. They will demand prior medical records spanning years, look for evidence of similar complaints before the accident, and characterize objective imaging findings (herniated discs, arthritis) as pre-existing degenerative changes rather than acute trauma. The eggshell plaintiff rule is the claimant's defense to these arguments. The proof structure: the claimant's treating physician (or, in contested cases, a retained expert) testifies that (1) the accident caused acute injury, OR (2) the accident aggravated a pre-existing condition that was previously asymptomatic or stable. The defendant remains liable for the full extent of the actual injury caused, NOT just the marginal increment. Concrete example: a claimant with a pre-existing 30% impairment from a prior accident is in a new accident that leaves her at 70% impairment. The defendant is liable for the 40-point increase (the marginal aggravation), not for the original 30% impairment that was already present. The dividing line between "caused" and "aggravated" can be medically and legally complex, especially for degenerative conditions like back pain.
How Eggshell Plaintiff (Eggshell Skull Rule) affects your settlement
The eggshell plaintiff rule is the single most important legal doctrine for claimants with prior similar injuries or degenerative conditions, and it often makes the difference between a successful claim and a defense verdict on causation. Without the rule, an insurer could argue that any pre-existing condition reduces the value of the present injury — turning a $100,000 case into a $30,000 case by attributing 70% of the symptoms to "prior degeneration." With the rule, the insurer is limited to arguing only about the INCREMENT caused by the accident, not the underlying condition. Three practical implications for claimants. First: have a frank conversation with your attorney about EVERY prior medical condition, accident, or treatment. Concealing prior injuries from your own attorney is fatal — defense attorneys WILL find them in subpoenaed medical records, and surprise destroys credibility. Second: get the treating physician to explicitly document whether the accident caused the symptoms outright or aggravated a previously stable condition. This documentation is the cornerstone of the eggshell argument. Third: in pre-existing-condition cases, the medical-causation expert often matters more than the medical-damages expert. A well-credentialed treating physician or retained expert who can clearly explain the difference between pre-existing degeneration and acute traumatic aggravation can transform settlement value. The Insurance Research Council and IRC data show that pre-existing-condition cases settle for substantially less when poorly defended on causation — claimants who hire attorneys experienced with the eggshell doctrine recover meaningfully more on average than those who attempt to handle these cases alone.
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Informational only and not legal advice. Settlement-dollar implications described here reflect typical patterns and may differ in any specific case. Confirm the analysis for your situation with a licensed attorney.