"49 There has been much written in recent years in this Court upon the relevance to the sentencing process of physical disease, mental illness, psychological disorders and intellectual deficit or handicap. ...
50 The following propositions at least would appear to emerge. The presence in the offender of such conditions as those referred to above will be relevant to the sentencing process in a number of different ways and for different reasons where there is a causal connection or link of a relevant kind established between the condition of the offender and the commission of the crimes for which he or she is to be sentenced. Generally speaking, where that is the case, the effect of the condition or disorder will be mitigatory, but that will not always be the case and indeed in some circumstances the effect may be one of aggravation, eg, where an intractable condition related to the offending behaviour leads to the conclusion that the offender will represent in the future a continuing danger to the community by reason of the commission of further offences. Such a condition may have an impact upon the type of disposition chosen and its severity.
51 Where it is advanced that an offender suffers from a condition or disability which should mitigate punishment, then as I have mentioned, it will be necessary to demonstrate a causal relationship between the offending and the condition, as I put it in CW, 'at least in the sense that as a result of the intellectual deficit the offender was not inhibited from committing the offence or offences in question'. In such a case the mitigation may be found in the conclusion that the offender's moral culpability, as opposed to his or her criminal responsibility, has been lessened so as to reduce the seriousness of the offending and the need for a denunciatory sentence.
52 Alternatively, or perhaps in addition to that factor, the offence and the offender may be seen to provide inappropriate vehicles for general and particular deterrence to be given their full weight. The extent to which such factors should be given weight will be a matter of degree depending upon the particular circumstances of the case in point, but it will often be the case, as Kennedy J put it in Dalgety, that such considerations of deterrence will continue to operate 'sensibly moderated'. Only in an extreme case will the relevance of such considerations be eliminated entirely.
53 For completeness I should add that quite apart from the situation where there is a causal link or connection between the offending and the condition in question and whether or not that is the case, if the offender's condition is such that a sentence which would otherwise be proportionate to the criminality involved may have a more severe impact upon the particular offender than upon others, then the court will be led in mercy, as well as by reason of the application of the general principles of sentencing, to moderate the punishment or choose an alternative disposition."