In the case of Jenkins v. United States in 1964, the U.S Supreme Court ruled that a jury should be allowed to consider whether an individual was insane at the time of committing a crime if there is substantial evidence supporting this claim. The defendant, James Jenkins, had been convicted for housebreaking and attempted rape in Washington D.C., but his defense argued he was not guilty by reason of insanity. However, despite expert testimony suggesting mental illness on part of Jenkins during trial proceedings, the judge instructed jurors they could only consider this defense if it were proven that Jenkins did not know his actions were wrong when he committed them - effectively raising the bar for proving insanity beyond what was required by law. The Supreme Court unanimously overturned his conviction because it found that these instructions improperly limited juror discretion and violated due process rights.

In the dissenting opinion for Jenkins v. United States, Justice Harlan argued that the majority's decision to reverse and remand was incorrect because it misinterpreted a key aspect of District of Columbia law regarding insanity defenses. He contended that under D.C. law, an individual is only considered insane if they are unable to appreciate the wrongfulness of their actions or control their behavior at the time of committing a crime due to mental illness. In this case, he believed there was sufficient evidence presented during trial showing that Jenkins understood his actions were wrong when he attacked another man with a hatchet but did so anyway due to uncontrollable impulses caused by his mental condition. Therefore, Justice Harlan felt it should have been left up to jury discretion whether or not these circumstances constituted legal insanity rather than being decided by appellate courts based on technicalities in how instructions were given.