Concept

The Causal Link Requirement

Product of Mental Disease or Defect

The Causal Link Requirement

A finding of mental disease or defect satisfies the first element of the product test and leaves the jury's work unfinished. The Durham court stated the point directly: the jury's task is not completed once it finds a disease or defect, because it must also consider the causal relationship between the abnormality and the act.¹ Carter v. United States, decided in 1957, restated that structure by describing two key factors in the defense, a mental disease and a critical relationship between that disease and the criminal act, and by adding that the mere fact of a disease or defect does not relieve anyone of responsibility.² The second factor is the causal link, and it does the sorting that a diagnosis cannot do, since it separates the defendant whose illness produced the crime from the defendant who is ill but committed the crime for some other reason.

The reason for a separate causal question lies in what the excuse is for. Criminal punishment is aimed at a will that freely chooses wrongdoing, so an accused who was not acting as a free agent in respect to the act falls outside the justification for punishing.³ Most people with a psychiatric diagnosis still act as free agents most of the time, which means the diagnosis alone cannot show that a particular act came from illness rather than from choice. The causal link supplies that showing. It converts a medical condition into a legal excuse by tying the act to the condition instead of to the defendant's own will. Without it, a sympathetic diagnosis could carry a verdict in a case where the crime had another source, such as a debt or a grievance, and requiring the jury to answer the disease question and the product question separately keeps the one from standing in for the other.

Exam Analysis Steps

The Durham court did not prescribe one mandatory instruction, observing that no single instruction suits every case, but it did state the substance that any instruction should convey.⁴ Each question is reached only if the one before it left the case open, so the jury moves through them in order, as follows:

If the jury believes beyond a reasonable doubt that the accused was not suffering from a diseased or defective mental condition at the time of the act, it may find the accused guilty, and the inquiry ends there.⁵
If the jury believes the accused was suffering from such a condition but also believes beyond a reasonable doubt that the act was not the product of that abnormality, it may find the accused guilty.⁶
Unless the jury believes beyond a reasonable doubt either that no such condition existed or that the act was not the product of it, the jury must find the accused not guilty by reason of insanity.⁷

The wording of the steps also allocates the burden of proof. A conviction is permitted only when the jury affirmatively believes, beyond a reasonable doubt, that the condition was absent or that the act was not its product, and acquittal follows whenever the jury holds neither belief.⁸ Once the issue is properly raised, then, the prosecution must defeat one of the two elements, and a jury that is merely unsure about the diagnosis or about causation must acquit. The defendant is not asked to prove either element to the jury's satisfaction. The order of the steps follows the same logic: the first asks whether there is anything for causation to operate on, and the second asks whether the condition in fact operated on the act.

The second step is where a defendant with a genuine diagnosis can still lose, and the rule behind it is that the offense must result from the condition. An offense qualifies as the product of a mental disease or defect when the defendant would not have committed the crime but for the condition, a standard known as but-for causation.⁹ The Durham opinion did not explain the word product, and the but-for gloss came later in Carter, which required a relationship that justifies a reasonable inference that the act would not have been committed if the person had not been suffering from the disease.¹⁰ The inquiry is counterfactual: the jury mentally subtracts the condition and asks whether the act would still have occurred. If it would have, the act was not a product of the condition, even though the condition was real and was present at the time. How a jury answers that question when an illness colors everything the accused does, and how reliably anyone can answer it, are taken up in the later sections on defining product and on the criticisms of the test.

Hypothetical Illustration

A defendant diagnosed with paranoid schizophrenia attacks a neighbor because he is convinced, through a delusion, that the neighbor is plotting to kill him. The jury concludes he would not have attacked but for the delusion, so it cannot believe beyond a reasonable doubt that the act was not the product of the disease, and the disease was present at the time. Neither negation is established, so the third step requires a verdict of not guilty by reason of insanity.

A second defendant has the same diagnosis and robs a store to raise the rent money. The plan has no connection to any delusion, and the jury concludes the robbery would have happened anyway. The condition existed, so the first step does not end the case, but the second step does: the jury believes beyond a reasonable doubt that the act was not the product of the abnormality, and it may find the defendant guilty.

A third defendant offers a psychiatrist's testimony that she had a mental disorder, but the jury believes beyond a reasonable doubt that no diseased or defective condition existed at the time of the act. The first step ends the inquiry, and the jury may convict without reaching the question of causation.

The first two defendants share a diagnosis and differ in a single fact, whether the condition drove the act, and that fact alone separates acquittal from conviction. The third defendant never reaches the causal question, because the first step closes the case once the jury is satisfied beyond a reasonable doubt that no diseased or defective condition existed. Students lose this issue when they treat the second step as a formality once a diagnosis is in the record, since a genuine disease that did not produce the act leaves the defendant responsible for it.¹¹

The causal link also fixes what an acquittal under this test communicates. The dissent in Foucha v. Louisiana contrasted the two rules: in a jurisdiction that follows the product test, a defendant acquitted by reason of insanity can fairly be said to have “committed the act because of mental illness,” whereas a verdict under the right-wrong test shows only that the defendant could not distinguish right from wrong.¹² The product test therefore ties the verdict to a finding of causation. That is why the third step cannot be reached on a diagnosis alone: the acquittal means the illness produced the act, and only the causal link can establish that.

The Causal Link Requirement

Endnotes13