Religious fundamentalism typically has been associated with negative perceptions of the insanity defense and defendants who use it. This association also has translated into verdict and sentencing decisions in insanity cases, such that higher endorsement of fundamentalist beliefs was associated with more punitive decisions. However, the mechanisms underlying the relationship between fundamentalism and insanity defense attitudes is unclear. Two possible explanations for this relationship include that (a) fundamentalism is associated with more dispositional attributions, and (b) fundamentalism is associated with more authoritarian attitudes toward mental illness and the mentally ill. Using structural equation modeling, attributions and attitudes were tested as mediators of the relationship between fundamentalism and insanity defense attitudes (i.e., strict liability attitudes and injustice–danger attitudes). These relationships were examined for theists and nontheists separately. Results suggest that personal attributions mediate the relationship between fundamentalism and injustice–danger attitudes, but authoritarian attitudes mediated the relationship between fundamentalism and strict liability attitudes. These relationships were only significant for theists. Implications for research and practice are discussed at the end.