 Forms of doxastic irrationality are still the mainstay of much policy and legal analysis about mental disorder. Assessments of criminal responsibility, of fitness to stand trial, and of the capacity to undertake binding contracts, for example, are cast in terms of the defendant’s ability to know and understand (Reznek 1987; Robinson 1996, 2013; Adshead 2008). Ostensibly, construing mental disorder as a want of rationality has been weakened in the face of evidence from behavioral economics showing that holding and acting on well-grounded and reasons-responsive beliefs occurs rarely in the general population, and common prejudices and superstitions seem to be indistinguishable from clinical delusions with respect not only to their prevalence but to the way they are adopted and maintained (Bortolotti 2009, 2013).