Criminal law as it pertains to 'mentally incompetent defendants': a McNaughton rule in the light of cognitive neuroscience.

The McNaughton rules for determining whether a person can be successfully defended on the grounds of mental incompetence were determined by a committee of the House of Lords in 1843. They arose as a consequence of the trial of Daniel McNaughton for the killing of Prime Minister Sir Robert Peel's secretary. In retrospect it is clear that McNaughton suffered from schizophrenia. The successful defence of McNaughton on the grounds of mental incompetence by his advocate Sir Alexander Cockburn involved a profound shift in the criteria for such a defence, and was largely based on the then recently published 'scientific' thesis of the great US psychiatrist Isaac Ray, entitled 'A treatise on the medical jurisprudence of insanity'. Subsequent discussion of this defence in the House of Lords led to the McNaughton rules, still the basis of the defence of mental incompetence in the courts of much of the English-speaking world. This essay argues that the rules need to be reconsidered in the light of the discoveries of cognitive neuroscience made during the 160 years since Ray's treatise. It is shown, for instance, how the conflation of 'the power of self-control' with 'irresistible impulse' by Cockburn is not supported by cognitive neuroscience because these are separate capacities requiring normal activity in distinct brain structures for their expression. In this way cognitive neuroscience assists in distinguishing between different capacities. It is further shown that failure of appropriate restraint in the expression of a capacity can be related to failure of synapses in particular parts of the brain. This raises the question as to what level of synaptic loss will the legislature and the courts rule as sufficient for a subject to be no longer held responsible for their lack of restraint.