The Eggshell Plaintiff Doctrine: Pre-Existing Conditions in Personal Injury Cases

The Eggshell Plaintiff Doctrine Explained

The eggshell plaintiff doctrine, also called the thin skull rule, is one of the most consequential common-law principles in personal injury practice.

The rule: a defendant takes the victim as found.

If the defendant's negligence aggravates a pre-existing condition or causes more severe injury than would befall a healthy person, the defendant is liable for the full extent of the harm.

The plaintiff's susceptibility does not reduce the defendant's responsibility.

If you had a prior back injury and a car accident aggravated it into a herniated disc requiring surgery, the at-fault driver is liable for the worsening, not just the portion that would have hurt a healthy claimant.

Insurance adjusters know the rule. They argue around it anyway, telling claimants their pre-existing conditions are "the real cause" of current symptoms.

The eggshell plaintiff doctrine is the legal counter to that tactic.

This page walks through the rule, its origins (including the 1891 Wisconsin Supreme Court case Vosburg v. Putney that gave us the modern formulation), how it interacts with apportionment and causation, and how it is applied in modern personal injury practice across U.S. jurisdictions.

What the Eggshell Plaintiff Doctrine Actually Says

The Restatement (Second) of Torts § 461 captures the rule directly:

"The negligent actor is subject to liability for harm to another although a physical condition of the other which is neither known nor should be known to the actor makes the injury greater than that which the actor as a reasonable man would have foreseen as a probable result of his conduct."

Translated: foreseeability does not limit damages once liability is established. The defendant who negligently caused some harm is liable for whatever harm actually resulted, even if the actual harm was much greater than what a reasonable person would have anticipated.

The rule has three operational components in practice:

• Pre-existing physical conditions. Brittle bones, prior surgeries, degenerative disc disease, prior fractures, prior soft tissue injuries, hemophilia, autoimmune conditions, cardiovascular conditions, and similar physiological vulnerabilities
• Pre-existing psychological conditions. Anxiety, depression, PTSD from prior trauma, panic disorder, and other mental health conditions that the accident worsens
• Pre-existing asymptomatic conditions. Conditions present in the body but causing no symptoms before the accident (degenerative disc disease without prior pain, asymptomatic aneurysm, asymptomatic tumor in the impact zone). These are particularly favorable for the plaintiff because the defendant cannot point to prior symptoms to apportion

 

 

The Origin Case: Vosburg v. Putney (Wisconsin 1891)

The modern eggshell plaintiff doctrine traces to a Wisconsin schoolyard. In Vosburg v. Putney, 80 Wis. 523 (1891), an 11-year-old boy named Andrew Vosburg sat in his classroom with a previously injured leg that was healing. His classmate George Putney lightly kicked him in the shin under the desk. The kick was minor. To a healthy boy, it would have been nothing.

For Vosburg, the kick reactivated the prior injury, triggered a serious infection, and ultimately resulted in permanent loss of use of the leg. Vosburg sued. Putney's defense was that he could not have foreseen the catastrophic injury from such a minor contact.

The Wisconsin Supreme Court rejected the defense. The court held that the unlawful contact was sufficient to establish liability, and once liability was established, the defendant was responsible for the full extent of the actual harm regardless of whether it was foreseeable. The case became the foundational American precedent for the rule.

Modern courts and the Restatement (Second) of Torts have adopted the same principle. The doctrine is recognized in every U.S. jurisdiction and is built into pattern jury instructions in nearly every state.

Apportionment: When the Defendant Can Reduce Damages

The eggshell plaintiff rule is not absolute. The defendant can reduce damages by proving how much of the post-accident condition is attributable to the pre-existing baseline rather than to the negligence.

This is called apportionment. It applies when the pre-existing condition was already producing symptoms, limitations, or treatment costs before the accident. The defendant pays for the worsening, not for the baseline that was already there.

When Apportionment Applies

Apportionment is allowed when:

• The pre-existing condition was symptomatic before the accident (prior pain, prior treatment, prior limitations)
• Medical evidence permits a reasoned division between baseline and aggravation
• The defendant carries the burden of proving the apportionment with competent evidence

When Apportionment Does Not Apply

Apportionment is denied or limited when:

• The pre-existing condition was asymptomatic before the accident
• Medical evidence cannot reasonably divide between baseline and aggravation
• The defendant fails to carry the burden of proof on the division
• The pre-existing condition was already worsening but the accident materially accelerated it

The Inability-to-Apportion Rule

When apportionment between pre-existing baseline and accident-caused worsening cannot be reasonably accomplished, the modern majority rule places the full damages on the defendant. The defendant who creates the apportionment problem by negligently injuring an already-injured plaintiff cannot benefit from the impossibility of clean division. The Restatement and the Restatement (Third) of Torts § 26 reflect this approach.

How Insurance Adjusters Use Pre-Existing Conditions Against You

The eggshell plaintiff doctrine is the law. Adjuster behavior is a different matter. The standard adjuster pattern when a pre-existing condition appears in the records:

"It Was the Pre-Existing Condition, Not the Crash"

The adjuster argues your current symptoms come from the pre-existing condition rather than the accident. The argument ignores the eggshell rule entirely. The legal counter is the doctrine itself: even if the pre-existing condition contributed, the defendant takes you as found and is liable for the worsening.

Pulling Years of Pre-Accident Records

If you signed a blanket medical authorization, the adjuster will pull every record from every provider you ever visited. The records get scoured for any prior complaint that could be characterized as "the same" condition. This is why blanket medical authorizations should not be signed at the start of a claim.

Demanding Apportionment Without the Burden of Proof

The adjuster's first counter-offer often assumes a 50/50 split between pre-existing baseline and accident worsening. The legal allocation does not work that way. The defendant carries the burden of proving the apportionment with competent medical evidence. An assumed split is not a defense.

Arguing "Asymptomatic" Was Really "Symptomatic"

If you had any prior medical contact involving the body part, the adjuster will argue the condition was symptomatic before the accident. The medical records often tell a different story. Pulling complete records and showing the absence of pre-accident symptoms is the standard rebuttal.

How Your Attorney Counters

Plaintiff's counsel responds with the medical records that establish the asymptomatic baseline (when present), treating physician opinions distinguishing baseline from aggravation, and where needed expert testimony from orthopedists, neurologists, or pain medicine specialists who can articulate the medical division. The eggshell instruction is requested at trial. Pattern jury instructions in nearly every state include the rule.