Eggshell Plaintiff Rule: Using It in a PI Demand Letter

The adjuster's response arrives two weeks after the demand. "The MRI shows degenerative disc disease. These findings predate the accident." Your client had never complained about their neck before the crash. Now the carrier wants to pay for a sprain.

That letter is why the eggshell plaintiff rule matters in pre-lit, not just at trial. This guide covers what the rule says and how it differs from the aggravation rule. Then it covers the evidence that makes either one stick, and how to write it into the demand so the adjuster has nothing easy to argue.

What Is the Eggshell Plaintiff Rule?

The eggshell plaintiff rule holds a defendant liable for the full harm their wrongful act causes. That's true even if the injured person was unusually fragile and a healthier person would have been hurt less. The defendant takes the plaintiff as they find them. It's a common law doctrine, also called the eggshell skull or thin skull rule.

Cornell's Legal Information Institute defines it the same way. If the wrongful act was the proximate cause of the injury, the defendant answers for the harm, however severe. The classic example is a person with a thin skull. A blow that wouldn't hurt most people gives them a serious head injury.

California's pattern jury instruction puts it in one sentence. Under CACI 3928, jurors award full compensation for the harm caused. That holds "even if" the plaintiff was more susceptible to injury than a normally healthy person. Other states have their own pattern instructions. Check yours before you quote one.

Eggshell Plaintiff vs. Aggravation of a Preexisting Condition

Two rules usually travel together, and mixing them up is how demands overreach. The eggshell rule is about fragility: the defendant pays for the full harm caused. The aggravation rule is about an existing condition: the defendant pays for the worsening, not for the condition your client already had.

	Eggshell plaintiff (thin skull)	Aggravation of a preexisting condition
California instruction	CACI 3928, Unusually Susceptible Plaintiff	CACI 3927, Aggravation of Preexisting Condition or Disability
The idea	A fragile plaintiff recovers the full harm caused, even if a healthy person would have been hurt less	No damages for the condition itself; damages for the effect the conduct had on it
What you claim	All the harm the incident caused	The difference between before and after
What you must prove	The incident caused this harm, in this person	A baseline before, a change after, and a doctor linking the change to the incident
What the carrier argues	The harm was unforeseeable or came from something else	Nothing changed; the symptoms would have come anyway

In a demand, you'll usually lean on both. The eggshell rule answers "a healthy person wouldn't have been hurt this badly." The aggravation rule frames what you're claiming for the old condition. Claim the change, prove the change, and the rules do the rest.

How Do Adjusters Attack Preexisting Conditions?

Adjusters attack preexisting conditions by arguing the symptoms would have happened anyway. They cite degenerative findings on imaging, prior treatment for the same body part, the client's age, or a gap in care. Each argument shifts the cause away from the incident. Your demand should answer each one before the adjuster raises it.

• "Degenerative" on the MRI report. A finding on a scan isn't the same as a condition that hurt. Ask whether your client had symptoms or treatment before the crash, not whether the scan shows wear.
• Prior treatment. A chiropractic visit three years ago becomes "ongoing back problems." Show when that treatment ended and what the client could do after it.
• Age. The adjuster says older spines hurt. The eggshell rule says that's the defendant's problem, not your client's.
• A gap before the incident. This one helps you. A long stretch with no treatment is evidence of a stable baseline.
• "No objective change." Answer with new findings, new restrictions, or new treatment that didn't exist before.

What Evidence Proves an Aggravation?

Proving an aggravation takes three things. You need the client's baseline before the incident and the change after it. You also need a treating doctor who connects the change to the incident in their own words. The records carry the claim. The client's account and people who saw the change support it, but they don't replace the paper.

What you need	Where it comes from
The baseline	Prior records of the same body part: the last visit, how the client was doing, any restrictions. Also work and activity before the crash
The change	New symptoms, new findings, new diagnoses, new treatment or restrictions after the incident, with dates
The link	A treating doctor's note or letter connecting the change to the incident
Support	The client's own account, family or coworkers who saw the difference, employer records

Get the prior records before the carrier does. Our guide to medical record retrieval covers how to request prior treatment narrowly, for the same body parts and the period the case needs. Build the before-and-after into the medical chronology so the change is visible by date. Our guide to medical record review covers flagging prior conditions while you read.

The doctor's words matter most. Ask the treating physician to address the prior condition directly: what it looked like before, what changed, and whether the incident caused the change. In the demo case below, the spine specialist answers the age question before the carrier asks it. He calls the disc injuries "most likely traumatic in origin" given the client's age and the mechanism of injury.

How to Write It Into the Demand Letter

Put the prior condition in the letter yourself, in the causation section, before the injuries. Say what it was, how the client was doing before the crash, what changed, and who says the crash caused the change. Then claim the change. Don't argue the doctrine at length. Adjusters know the rule; they want the facts that trigger it.

Here's an illustration. It's a made-up client, not a real case.

Weak: Mr. Alvarez's preexisting condition was severely aggravated by the collision, and under the eggshell plaintiff rule your insured is liable for all of his damages.

Better: Mr. Alvarez treated for low back pain at Valley Chiropractic in 2022 and was discharged with no restrictions (p. 14). He worked full shifts as a warehouse lead for the next three years without further treatment. Two days after the collision, he reported new pain radiating to his left leg (p. 31). His MRI showed a new disc herniation at L4-5 (p. 40). His treating physician wrote that the collision caused the herniation and aggravated his prior condition (p. 52).

The weak version asserts a conclusion and names a doctrine. The better version gives the adjuster a baseline, a change, a doctor and a page for each fact. It never says "eggshell." It doesn't need to.

Our guide to writing each section of a demand letter shows where causation sits. The PI demand letter template includes an optional subsection for preexisting conditions under causation.

When the Eggshell Rule Won't Carry the Claim

The rule has limits, and a demand that ignores them loses credibility. It doesn't pay for the condition your client already had. California's CACI 3927 is direct about it. There are no damages for a condition the plaintiff already had, only for the effect the conduct had on it.

It also doesn't prove causation for you. If nothing in the records shows a change, the rule has nothing to work with. Watch for events in between, too. If your client had a second accident or a fall before the new symptoms, expect the carrier to blame that event.

When the prior condition was serious and active right up to the crash, be honest about what you're claiming. A demand that claims the whole condition invites a low offer on everything. A demand that claims the change, and proves it, keeps the rest of the letter credible.

Where Lexyno Fits

Lexyno's AI demand letter includes a causation section with a subsection for preexisting conditions, drafted from the case's records. Medical Summary lists each diagnosis with the date it was first diagnosed and its source page. A prior diagnosis and a new one sit side by side.

The causation theory, including how to frame a prior condition, stays with the attorney. Your team can review, correct or remove any extracted event before the letter goes out. Our post on what AI should and shouldn't do for PI lawyers explains that split. Our medical record summary guide shows how to quote the treating doctor instead of arguing causation yourself.

Prove the Change, Claim the Change

The eggshell rule means a fragile client isn't a discounted client. The aggravation rule means you claim what changed. Get the prior records, show the baseline, show the change, and let the treating doctor connect them.

The defendant takes your client as they found them. Your demand should show the adjuster exactly how that was.

For the rest of the letter, see the demand letter to the insurance company, the car accident sample and the records-to-settlement workflow. Or try Lexyno on one case from your own file, free.