Five California Jury Instructions Every Injured Person Should Know
When people think about a personal injury case, they often assume the biggest question is who caused the collision. In many cases, however, liability is not the real battle. Instead, the most contested issue is medical causation.
Insurance companies frequently admit their insured caused the collision but argue that the collision did not cause the injuries, did not make a prior condition worse, or did not necessitate the medical treatment the injured person received.
Fortunately, California law provides clear guidance to juries through the California Civil Jury Instructions (CACI). Five of these instructions are particularly important in helping jurors decide whether an incident legally caused a person's injuries.
Understanding these instructions also helps explain why a defense medical expert saying, "I'm not sure," or "It could have been something else," does not necessarily defeat an injured person's claim.
Medical Causation Is About Probability—Not Perfection
One of the biggest misconceptions people have is believing that a doctor must testify with 100% certainty that an incident caused an injury.
That is not the law.
California civil cases are decided using the "more likely than not" standard. Doctors often describe this as a reasonable medical probability.
That simply means:
- probable rather than merely possible;
- more likely true than not true; or
- if you prefer percentages, approximately 51% versus 49%.
Medicine rarely provides absolute certainty. Physicians routinely make treatment decisions based upon probabilities, not guarantees.
If a physician concludes that it is more probable than not that a collision caused an injury—or aggravated an existing one—that testimony can satisfy the legal standard for causation.
CACI No. 200: The Burden of Proof
Everything begins with California's burden of proof instruction.
CACI No. 200 tells jurors that the plaintiff must prove the facts are more likely true than not true. This is an important distinction.
Many people incorrectly assume that civil cases require proof beyond any doubt. That standard applies only in criminal prosecutions. Civil lawsuits require something much different.
Imagine a set of scales.
If the evidence tips ever so slightly in one direction—even by the smallest amount—the burden has been met. That means an injured person does not have to eliminate every conceivable alternative explanation for an injury. Instead, the plaintiff must simply convince the jury that the accident was more likely than not the cause.
CACI No. 430: A Defendant Does Not Have to Be the Only Cause
Insurance companies often argue:
"The MRI showed arthritis." or "The plaintiff was already getting older." or "The symptoms may have existed before the accident."
Even if those statements are true, they do not necessarily defeat the case. CACI No. 430 explains that the defendant's conduct need only be a substantial factor in causing the injury.
The instruction specifically states that the defendant does not have to be the only cause. This is critically important. Many injuries result from multiple contributing factors.
For example:
- an asymptomatic degenerative disc becomes painfully symptomatic after a collision;
- a repaired shoulder is reinjured in a crash;
- arthritis exists for years but becomes disabling after trauma;
- a minor back condition becomes permanent because of a violent impact.
Under California law, the defendant cannot escape responsibility simply because another factor also existed.
The question becomes:
Was the collision a substantial factor in producing the injury?
If the answer is yes, causation may be established.
CACI No. 431: Multiple Causes Can Exist at the Same Time
Medical conditions are often complex. Human bodies do not operate in a vacuum.
Someone may have:
- prior surgeries,
- degenerative changes,
- osteoporosis,
- prior sports injuries,
- diabetes,
- previous accidents,
- age-related wear and tear,
and still suffer a new injury caused by a negligent driver.
CACI No. 431 recognizes this reality.
The instruction tells jurors that negligence may combine with another condition, another event, or another person's conduct to produce harm. The defendant remains legally responsible if his or her negligence was a substantial factor, even if another condition also contributed.
Think about it this way.
If a person already has dry wood in a fireplace, and someone throws in a lit match, the existing wood may have contributed to the fire—but the match still caused it to ignite. Likewise, an accident can combine with an existing medical condition to produce real injury.
The law recognizes this.
CACI No. 3927: Aggravation of a Pre-Existing Condition:
The law and justice are not available only to a perfectly healthy 18-year-old football player who never had an injury. It is available to anyone injured through the negligence of another. Everyone has a past and for some people that means prior injuries or pains or conditions.
In this regard, the law is very logical. It works like this.
You cannot recover for any injury or pain you had before a car crash (Or fall or whatever), but you can recover for any new injuries or worsening (Exacerbation) of old injuries.
Example: Joe is 49 years old. He sometimes wakes up with a sore back if he sleeps wrong. He can usually stretch and the pain goes away. About twice per year, he sees a chiropractor for a week or two if the back pain does not go away with stretching; otherwise, his back is fine and he can work full time, play golf, whatever.
Joe is then crashed into by a drunk driver. Ever since the crash, his back pain is now severe, it will not go away on its own and the pain now shoots (Radiates) down his left leg to his foot, his leg is falling asleep constantly, and he can no longer work or play golf.
The insurance company for the drunk driver argues that Joe already had back pain before the crash, so that is the cause of his problems. Joe’s lawyer argues (and Joe along with medical testimony) argue that the crash caused new or exacerbation of back injuries, such that clinically, Joe is now much worse, his pain is radicular (when before it was localized) and that before it was minor and did not interfere with Joe’s life, but now it is significant and has taken over his life.
Who wins?
Hopefully Joe, because he did not have the same type of pain, limits, life impact, etc. before the drunk driver hit him that he does after the drunk driver hit him. The right lawyer can have a medical expert examine Joe, review all the medical records, get a low back MRI and pursue proper justice, using CACI 3927.
CACI 3928- Unusually Susceptible Plaintiff: The Eggshell Plaintiff Rule in Practice
You must decide the full amount of money that will reasonably and fairly compensate [name of plaintiff] for all damages caused by the wrongful conduct of [name of defendant], even if [name of plaintiff] was more susceptible to injury than a normally healthy person would have been, and even if a normally healthy person would not have suffered similar injury.
This instruction is similar but different from 3927. Perhaps no instruction is more misunderstood than CACI No. 3928.
Remember Humpty Dumpty. He was an egg. He sat on a wall, had a great fall and was seriously injured. Most people who sit on a wall and have a small fall will not be hurt, but because Humpty is an eggshell he breaks.
The insurance company might argue “It is not our fault you are an eggshell. Most people would not be hurt”; however, under the law, the negligent “Tort-Feasor” takes their victim as they find them, fragile or not.
This principle reflects a long-standing rule in tort law:
You take your victim as you find them.
Suppose two people experience the exact same collision. One walks away unharmed.
The other suffers severe injury because of a previously weakened spine. The negligent driver is still responsible for the additional harm caused.
The law does not reward a defendant simply because the injured person happened to be medically vulnerable.
Instead, the jury is asked to determine:
(Both for CACI 3927 and CASCI 3928) How much worse did the accident make the plaintiff's condition?
Damages are awarded for that aggravation.
Why Defense Experts Often Focus on "Possibility"
Defense medical experts frequently use words such as:
- maybe,
- could,
- possibly,
- cannot rule out,
- might have,
- age-related,
- degenerative.
Those words can sound persuasive. But remember the legal standard.
The plaintiff is not required to eliminate every possible explanation. The issue is whether the incident was more likely than not a substantial factor in producing the injury. Medicine frequently involves competing possibilities.
The jury's job is to determine which explanation is most probably true, not whether every alternative theory has been disproven.
Good Medicine and Good Law Often Align
One of the strongest personal injury cases is one where the medical records tell a consistent story.
Jurors often look for:
- a healthy baseline before the incident;
- immediate or reasonably prompt complaints afterward;
- objective medical findings;
- consistent treatment;
- diagnostic imaging when appropriate;
- credible physicians;
- believable testimony from the injured person.
No single piece of evidence wins a case. Rather, juries evaluate the entire picture. That is precisely what California's jury instructions require.
Why These Jury Instructions Matter
Together, these four CACI instructions tell jurors several important things:
- A plaintiff only has to prove causation is more likely true than not true (CACI 200).
- The defendant's negligence does not have to be the only cause of the injury (CACI 430).
- Multiple causes can exist simultaneously, and the defendant remains responsible if their negligence was a substantial factor (CACI 431).
- A defendant is responsible for worsening a preexisting condition, even if they did not create it (CACI 3927).
These principles reflect both common sense and California law. They recognize that real people often have prior injuries, age-related changes, or underlying medical conditions. The law does not require perfect health before an accident, nor does it allow a negligent person to avoid responsibility simply because someone was more susceptible to injury.
Final Thoughts
Medical causation is often the centerpiece of a California personal injury case. It is where medicine and law intersect, and it is frequently where insurance companies devote the greatest effort to disputing a claim.
Understanding these jury instructions helps explain why a case is not won or lost simply because an MRI shows degeneration or a defense expert suggests another possible cause. The focus is whether the evidence, taken as a whole, shows that the defendant's negligence was more likely than not a substantial factor in causing or aggravating the plaintiff's injuries.
At the Law Office of Adam Sorrells, we work closely with treating physicians, medical specialists, and qualified experts to present clear, persuasive evidence of medical causation. Whether an injury is entirely new or represents the aggravation of a preexisting condition, proving causation requires careful preparation, a thorough understanding of California law, and the ability to explain complex medical issues in a way that jurors can understand.
If you have been injured in an accident and an insurance company is claiming that your injuries were "preexisting" or "not caused by the crash," understanding these legal principles may make all the difference. California law protects individuals whose injuries were caused—or made worse—by another person's negligence, and the right evidence can be critical to achieving a just result.
Adam Sorrells
Law Office of Adam Sorrells
Serving injury victims throughout California.