The New York Times reports that the U.S. Supreme Court is reviewing the case of Eric M. Clark, who spent three years in a mental hospital before being found mentally competent to stand trial for killing a police officer, but was unable to use an insanity defense because of a combination of Arizona statutes and judicial decisions that limit evidence of mental illness.
The high court has never ruled that the U.S. Constitution requires a state to allow an explicit insanity defense, but most states either allow such a plea or give a defendant the ability to present evidence of diminished mental capacity. In Arizona, by contrast, such evidence is barred from the trial.
Link: http://www.nytimes.com/2006/04/20/washington/20scotus.html