Objective ‘reasonableness’ or ‘reasonable person’ tests are a common element of both offences and defences in the criminal law. When tasked to apply these tests to defendants with mental disorder, however, courts and commentators tend to encounter a conceptual block. The typical reasoning runs that where a defendant’s beliefs, perception or conduct were influenced by a mental disorder, it becomes impossible to hold that defendant to an objective standard; since the ‘reasonable person’ is not (for example) delusional, depressed, or hyper-vigilant, any defendant who is becomes unreasonable by definition, and is thereby incapable of passing any test applied on an objective basis. Complicating the problem is the fact that the usual concessions – permitting particular characteristics such as age, sex, or physical capability to modify the objective test – give rise to conceptual challenges where the relevant characteristic is a mental condition; the ‘reasonable man (with psychosis)’ is perceived as less a coherent standard than a contradiction in terms. The consequence of this is that defendants with mental disorder may find themselves excluded from the application of tests for culpability which apply an objective element, forcing them instead to rely on the limited protection offered by special mental condition defences such as insanity.

The purpose of this paper, therefore, is to propose a guiding heuristic for the reformulation of objective tests in terms of ‘reasonable expectations’. My contention is that, through this reformulation, objective standards can be made sensitive to the relevance of a defendant’s mental condition to their culpability without compromising the objective force of these standards. The effect of this would be to significantly expand the options available to us in assessing the culpability of defendants with mental disorder.