Legal Causation vs. Medical Causation: What Attorneys Need to Know
The difference between clinical causality and legal proximate cause is one of the most difficult—and most important—concepts in personal injury litigation.
When you retain a psychologist for a personal injury case, you need them to answer a seemingly simple question: "Did the accident cause the plaintiff's psychological problems?" But this question has two very different meanings depending on whether you're asking a clinician or a forensic expert.
Understanding the Two Types of Causation
Medical/Clinical Causality
This is what clinicians are trained to do. It's a diagnosis: identifying the medical or psychological cause of symptoms.
Example: "The patient's memory problems are caused by the traumatic brain injury."
Legal Causality (Proximate Cause)
This is a legal doctrine. The court wants to know if the defendant's action is the legally responsible cause of the plaintiff's damages.
Example: "The car accident is the legally responsible cause of the plaintiff's PTSD, which led to lost wages and diminished quality of life."
As a forensic expert, my job is to bridge this gap—to translate clinical findings into legally meaningful opinions about causation.
Why This Distinction Matters
1. Clinical Causation Alone is Not Enough
A treating clinician might appropriately diagnose a patient with PTSD and note in their clinical records that the patient attributes their symptoms to a car accident. This is clinical causation—the doctor has identified what's wrong with the patient.
But this doesn't answer the legal question. The court needs to know:
- Is the accident the sole cause, or did pre-existing factors contribute?
- Would the plaintiff have developed PTSD anyway, or was it specifically caused by this event?
- Is the defendant's action the proximate (legally responsible) cause, or is the connection too remote?
- Can this opinion be stated to a "reasonable degree of psychological certainty"?
Critical Point
A treating clinician's opinion that "the accident caused the patient's PTSD" is based on what the patient told them. A forensic expert's opinion is based on independent analysis of all available data, including records review, testing, and structured interview.
2. The Proximate Cause Standard
In legal terms, "proximate cause" has two components:
Cause-in-Fact ("But For" Causation)
Would the harm have occurred "but for" the defendant's action? If the accident hadn't happened, would the plaintiff still have developed PTSD?
Legal Causation (Foreseeability)
Was the harm a foreseeable consequence of the defendant's action? Is the connection between the action and the harm direct enough to impose legal liability?
As a forensic expert, I must opine on both aspects to provide a complete answer to the legal question.
How Forensic Experts Establish Legal Causation
Establishing legal causation requires a systematic, multi-method approach that goes far beyond clinical diagnosis:
Establish Baseline Functioning
Through comprehensive record review (medical, psychiatric, employment, educational), I establish how the plaintiff was functioning before the incident. This is critical for distinguishing new-onset conditions from pre-existing problems.
Document Temporal Relationship
Did symptoms appear after the incident? How soon after? Was there a clear temporal link, or did symptoms develop months or years later? The timing is crucial for establishing causation.
Conduct Differential Diagnosis
I systematically rule out alternative explanations. Could the symptoms be due to a pre-existing condition, a medical illness, substance use, or another life stressor? This process eliminates other potential causes.
Assess Dose-Response Relationship
Is the severity of the psychological harm proportional to the severity of the trauma? A minor fender-bender causing severe, disabling PTSD requires more scrutiny than a serious accident with life-threatening injuries causing PTSD.
Evaluate Consistency of Evidence
Do the plaintiff's self-reports match their behavior? Are psychological test results consistent with claimed impairment? Do collateral sources (family, employers, medical providers) corroborate the plaintiff's account?
The Challenge: Pre-Existing Conditions
One of the most complex causation issues is distinguishing between:
- New-onset psychological injury (the incident caused a wholly new condition)
- Exacerbation of pre-existing condition (the incident made an existing condition worse)
- Unrelated worsening (the plaintiff's condition got worse, but not because of the incident)
This is where the "whole person" formulation approach is essential. I analyze:
- • Baseline psychological functioning: What was the plaintiff's mental health before the incident?
- • Prior traumas and coping: How did the plaintiff handle previous stressors?
- • Nature of the incident: What made this event psychologically traumatic compared to prior experiences?
- • Symptom trajectory: Did symptoms appear suddenly after the incident, or gradually over time?
My analysis then answers the critical question: "Is this a new injury, or the aggravation of an old one?"
Reasonable Degree of Psychological Certainty
Courts require expert opinions to be stated to a "reasonable degree of [professional] certainty." This doesn't mean 100% certainty—it means:
- The opinion is based on sufficient data and reliable methodology
- The opinion is more likely than not to be correct (greater than 50% probability)
- The expert can defend the opinion against challenges
Example of Properly Stated Opinion
"To a reasonable degree of psychological certainty, the motor vehicle accident of January 15, 2023, is the proximate cause of the plaintiff's Post-Traumatic Stress Disorder. This opinion is based on comprehensive record review, structured clinical interview, psychological testing, and analysis of the temporal relationship between the accident and symptom onset. Alternative explanations have been systematically ruled out."
What This Means for Your Case Strategy
Understanding the difference between clinical and legal causation helps you:
Choose the Right Expert
A treating clinician can testify about what they diagnosed and treated. But for causation opinions that will withstand scrutiny, you need a forensic expert who understands legal standards and can conduct the rigorous analysis required.
Frame the Right Questions
Don't just ask "Did the accident cause PTSD?" Ask:
- "What was the plaintiff's baseline psychological functioning before the incident?"
- "Can you establish a temporal relationship between the incident and symptom onset?"
- "Have you ruled out alternative causes?"
- "Is this a new condition or the exacerbation of a pre-existing one?"
- "Can you state your opinion to a reasonable degree of psychological certainty?"
Prepare for Defense Challenges
The defense will challenge causation by:
- Highlighting pre-existing mental health issues
- Identifying alternative stressors in the plaintiff's life
- Questioning the temporal relationship
- Challenging the severity of the incident
A rigorous forensic evaluation anticipates and addresses these challenges before they're raised.
The Bottom Line
Legal causation is not the same as clinical causation. To prove your case, you need more than a diagnosis—you need a comprehensive forensic analysis that:
- Establishes baseline psychological functioning
- Documents temporal relationships
- Conducts differential diagnosis to rule out alternatives
- Distinguishes new injury from exacerbation of pre-existing conditions
- States opinions to a reasonable degree of psychological certainty
This is the difference between clinical work and forensic expertise—and it's the foundation of a defensible causation opinion.
Need Expert Causation Analysis for Your Case?
I provide comprehensive forensic psychological evaluations with rigorous causation analysis designed to meet legal standards. My background in biological sciences provides the analytical foundation for complex causation opinions. Contact my office to discuss your case.
Schedule a ConsultationAbout the Author: Dr. Marco G. Morelli is a Licensed Clinical Psychologist (CA PSY 34288) with over 15 years of experience providing forensic psychological evaluations. His background in biological sciences (M.S. Biology, B.S. Biological Sciences) provides a rigorous, analytical foundation for complex causation analysis in personal injury and workers' compensation cases.