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The structure of the legal concept of unfitness to stand trial and how it corresponds to psychometric concepts is examined. We conclude that psychometric attempts at quantification and measurement are logically flawed, because they inappropriately treat fitness/unfitness as an individual trait rather than as situation-specific conjunctive/disjunctive concepts. It is argued that, whereas psychometric approaches may be suitable for measuring single components of unfitness, an overall "fitness" score is meaningless and that the assessment should focus on elements of unfitness. The determination of unfitness requires the simultaneous consideration of several different individual capabilities in reference to the demands of a specific trial. The quantification of these specific capabilities by psychologists can assist psychiatrists and the court, but the evaluation of their influence on unfitness in the instant case must be left to the court.

An analysis of the psychological aspects of the legal concept of competency/fitness to stand trial draws attention to the central role of understanding. The rationale of certain basic requirements for the construct validity of a psychometric test of understanding in people with mental retardation is presented, and a test that meets those requirements is described.

A historical account of the concept of lunacy in terms of its symptomatology and aetiological models, and a critical review of its validity and reliability are given. It is concluded that there is no unequivocal support for a belief in a lunar influence on human behaviour and that any future research should be directed towards the specification of the behaviour and people that might be affected by the phases of the moon.

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