Causation, Apportionment, and Impairment in Psychological Injury Cases

By Dr. Tyler Gerdin, PsyD, ABPP | Gerdin Psychological Services

The adjuster asks whether the worker’s depression is “from the injury.” The attorney asks “how much” of it is. The case manager asks whether he can return to work in March. It sounds like one question asked three ways.

It’s actually three different questions. Mixing them up is one of the most common sources of confusion in psychological injury cases.

You need to make sure that you have a psychologist who knows the difference.

The Three Questions Every Psychological Injury Evaluation Must Answer

Causation asks whether the event made a real difference to this person’s mental health.

Apportionment asks how much of the current picture belongs to the event, and how much to what came before or after it.

Impairment asks what this person can do, and not do, right now.

These answers don’t always line up. Someone can have clear, well documented limitations while the link to a specific event stays uncertain. An event can clearly have made things worse while the person still functions fairly well. Each question needs its own answer.

How Psychological Causation Works in Injury Cases

The most common mistake is the most natural one. Symptoms started after the event, so the event must have caused them.

Timing matters, but it isn’t enough on its own. Tortes Saint-Jammes (2026) argues that a causal opinion can’t rest only on the order of events, a plausible sounding story, or the presence of a trauma related diagnosis. Young (2015) adds that psychological distress rarely has a single cause. Pre-existing vulnerabilities, the event itself, and what happens afterward all play a part.

So the better question is usually: what role did the event play? The answer might be that the event was the main cause. It might be that the event contributed meaningfully alongside other factors, or that it had an effect through what followed, such as a lost job or a difficult recovery. Sometimes the honest answer is that the evidence isn’t strong enough to say.

That last answer isn’t a failure. It’s often the most useful thing a report can say.

Why the Language in a Forensic Report Stays Clinical

Psychologists don’t decide legal causation. Judges, juries, and adjudicators do.

The psychologist’s job is to describe the clinical picture and how strongly the evidence supports a link to the event. The legal conclusion belongs to the legal decision maker. Keeping that line clear is accurate, and it protects everyone, including the person being evaluated, from a report that claims more than the evidence can support.

Small wording choices matter here. “Symptoms are consistent with trauma” is not the same as “the event caused this condition.” A careful report chooses its words deliberately.

Apportionment in Psychological Injury Cases

Most people don’t arrive at an injury with a blank slate. Kohutis and McCall (2020) point out that the psychologist’s work is hardest, and most useful, when old and new conditions interact.

In legal terms, the “eggshell skull” idea generally means the responsible party takes the injured person as they find them, vulnerabilities and all. The “crumbling skull” idea means a condition that was already getting worse on its own shouldn’t be blamed on the new event. How these rules apply depends on the jurisdiction. The psychologist’s job is to describe the clinical picture clearly enough for the legal decision maker to apply them.

Clinically, the key distinctions are:

New condition means the event created something that wasn’t there before.

Aggravation means the event worsened something already present.

Natural progression means the condition likely would have reached this point anyway.

Honesty about precision matters too. A reasoned explanation is often more defensible than a tidy percentage that suggests more certainty than anyone actually has.

How Functional Impairment Is Assessed in Forensic Evaluations

A diagnosis is not the same as impairment. The American Academy of Psychiatry and the Law (Anfang et al., 2018) notes that having a psychiatric condition doesn’t automatically mean someone is impaired.

Impairment is about function: concentrating, keeping pace, working with others, managing stress, getting through a workday. It should rest on evidence such as testing, history, and input from others, not just a diagnostic label. It should also address the outlook, meaning whether things are likely to improve or have leveled off.

Why Validity Testing Is Essential in Psychological Injury Evaluations

All three answers depend on good data. Much of what these evaluations rely on is self report, which can be shaped by memory, distress, and the stress of litigation. Validity testing checks whether the information is consistent and reliable enough to build conclusions on. It’s not an accusation. It’s simply how a report earns the right to be trusted.

Why Separating These Three Questions Strengthens Your Case

When a report answers each question separately, you can see which parts of the opinion are strong, which are tentative, and where the real disagreement in the case lies.

Before retaining a psychologist for an evaluation, be sure that they understand and can address these fundamental distinctions.

A weak report answers the question it was asked. A strong report notices there were three questions and answers each one.

Forensic Psychological Injury Evaluations in Spokane, Washington

If you have a case that turns on a psychological injury question, reach out to discuss whether Dr. Gerdin can help. Contact drgerdin@gerdinpsych.com or 509-676-4313.

Frequently Asked Questions

Is causation the same as apportionment?

No. Causation asks whether an event contributed to a condition. Apportionment asks how the current condition divides among everything that contributed.

Does a diagnosis mean someone is impaired?

Not necessarily. Two people with the same diagnosis can function very differently. Impairment describes actual limitations in daily life and work.

Why don’t psychologists give legal causation opinions?

Legal causation is decided by the court or adjudicator. The psychologist describes the clinical picture and how strong the evidence is, so the decision maker can apply the law.

Dr. Tyler Gerdin, PsyD, ABPP is a board certified clinical psychologist in Spokane, Washington, specializing in forensic psychological evaluation, psychological and neuropsychological assessment, and independent medical examinations.

References

Anfang, S. A., Gold, L. H., & Meyer, D. J. (2018). AAPL practice resource for the forensic evaluation of psychiatric disability. Journal of the American Academy of Psychiatry and the Law, 46(1 Suppl), S2–S47.

Kohutis, E. A., & McCall, S. (2020). The eggshell and crumbling skull plaintiff: Psychological and legal considerations for assessment. Psychological Injury and Law, 13, 354–369.

Tortes Saint-Jammes, J. (2026). Causal attribution in forensic psychological injury: Legal causation, validity assessment, and expert reasoning. Frontiers in Psychology, 17, 1877102.

Young, G. (2015). Causality in criminal forensic and in civil disability cases: Legal and psychological comparison. International Journal of Law and Psychiatry, 42–43, 114–120.