Given the influence of social conformity and prejudice, defendants pleading not guilty by reason of insanity face the significant challenges of securing fair and impartial juries. Attitudes and knowledge of the insanity defense are factors that may influence levels of impartiality. In the light of this, we set out to develop a scale to examine knowledge levels of the insanity defense and their influence on decision-making. Two studies were conducted to construct a scale designed to assess laypersons' knowledge of the insanity defense. Items measuring knowledge of the insanity defense were based on Perlin's (1995) insanity defense myths. The first study identified particular items in need of revision and subscales that required the development of additional items in order to improve reliability and construct validity in the second study. The second study used the revised scale, demonstrating improved validity and reliability. The scale also had acceptable predictive validity with reference to insanity defense verdicts. Copyright © 2010 John Wiley & Sons, Ltd.