The 'insanity defense' and diminished capacity

United States v. Hinckley, a recent successful use of the insanity defense

On March 30, 1981, John W. Hinckley, Jr., shot President Ronald Reagan, attempting to assassinate him. His defense attorneys did not dispute that he had planned and committed the attack. His attorneys instead argued that he was acting according to the impulses of a diseased or impaired mind.

The legal argument

Hinckley's attorneys argued that Hinckley had not acted of his own volition, but that his life was controlled by his pathological obsession with the movie, Taxi Driver, starring Jodie Foster. In that movie, the title character stalks the president and fights in a shootout. Hinckley's attorneys said he saw the movie 15 times, and identified with the hero and was seeking to reenact the events of the movie in his own life.

Hinckley's attorneys argued that Hinckley was schizophrenic. They argued that the movie was the actual planning force behind the defendant's assassination attempt against the President.

The judge allowed the defense to introduce evidence, in the form of a CAT scan, that Hinckley's brain showed signs of shrunken brain tissue, one of the common symptoms of schizophrenia. The prosecution opposed this evidence, on the grounds that the technical nature of the evidence would cause the jury to place too heavy an emphasis on it. The judge rejected this argument, on the grounds that the evidence was relevant. (See the law of evidence on admissibility.)