State v. Arrington

Amicus brief arguing that a categorical rule requiring a defendant to present expert testimony in order to raise an insanity defense is unconstitutional.

This case raises the question of whether a defendant can assert the insanity defense without support of an expert witness. The Appellate Division in this case ruled that expert testimony is a mandatory prerequisite to an affirmative defense of insanity. In its decision, the Appellate Division treated legal insanity like a medical diagnosis, when instead the defense has operated for nearly two centuries as a theory of social accountability grounded in the types of facts about state of mind that juries routinely encounter and appraise. To be sure, an insanity defense presented through lay testimony alone may often prove unpersuasive. But here, the Appellate Division held that lay testimony is categorically inadequate to establish insanity. Our brief argues that to prejudge the sufficiency of every defendant’s proof on insanity is unsound and unconstitutional.

The New Jersey Supreme Court and the U.S. Supreme Court have on several occasions confronted per se rules, designed to ensure trustworthy evidence, that bar whole categories of defense testimony. Overwhelmingly, these rules violate the right to present a complete defense. And so does the rule here: a categorical bar on lay-only insanity testimony cannot survive constitutional scrutiny.