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In general, evidence is admissible (see admissibility of evidence) if it contributes towards proving one or more of the facts at issue in the case. The term 'relevant' or 'logically relevant' is used to describe such evidence. Although there are many reasons why relevant evidence may not be admitted -- because it is Hearsay, or obtained by oppression, or privileged, etc. -- relevance is a necessary condition for admissibility. Irrelevant evidence is never admissible.
Example: a man is convicted of a brutal assault. There is some question whether the defendant, or some other man, carried out the attack. The police search the defendant's home and find a stack of videos of violent movies. Most likely this evidence will be deemed irrelvant, as it does not got to a fact at issue. However, if there is some specific theme to these movies, that is evident also in the assuault -- perhaps the movies all feature attacks with power tools, and that is also a feature of the assault in this case -- then there is a greater likelihood that the evidence will be admitted.
Although relevance is a fundamental concept, it is difficult to define a single, conclusive test for relevance. Moreover, judges have often stated that relevance must be assessed in the context of the case as a whole. An interesting case to that effect is R v blastland (1986). Here, the defendendant was convicted of murder. He sought to adduce evidence that another person (M) had confessed to the crime. However, M had not been called to give evidence and, therefore, the trial judge had declared that D's account of M's confession was hearsay. On appeal, D argued that what the excluded evidence went to show was not that the facts of M's confession were true, but that M had a particular state of mind (a knowledge of the murder), and that M's state of mind was something that the jury should have known about. The House of lords accepted the argument that M's state of mind should not be excluded as hearsay. However, the House went on to state that this evidence ought to have been excluded anyway. The deciding question was: is M's state of mind logically relevant to the case against D? There were a number of ways that M could have acquired knowledge of the murder. For example, he could have witnessed it committed by D. Consequently, his state of mind was not probative of anything against D.
This decision has been criticised because, although M's knowledge of the murder might not be conclusive of anything, it has some probative value. It is clear, therefore, that there is a threshold for relevance -- evidence which has only a meagre probative effect is not relevant evidence. What that threshold is, is a matter of judicial commonsense, but there is clearly an interplay between the relevance and the reliability of evidence -- the weight it should be given by a jury. Evidence that is more reliable is more likely to be judged relevant. See, for example, R v kearly (1992), where the 3:2 decision in the House of lords shows just how difficult it is to decide on relevance.
Good examples of the difficulties encountered in finding a general test for relevance can be found in the recent case-law on supply of illegal drugs. If a suspect who would normally be thought to be of modest means is found with a supply of illegal drugs, and a large stash of money, it might be suspected that he has obtained that money by supplying drugs. Similarly, if a suspect of modest means is found in possession of drugs, and has been living lavishly, it might be suspected that he has been funding his lifestyle by money obtained from drug sales. But if the suspect is tried for possession with intent to supply, how much relevance does the money/lifestyle evidence have? Before answering that question, consider that (at least until recently) the courts would be reluctant to admit in evidence that the defendant has a previous conviction for supplying drugs. If a previous conviction is not admissible to show intent to supply, why should money or lifestyle be admissible?
It is clear that the presence of a large some of money might be relevant. However, this evidence will need careful presentation to the jury. In RVGrant1996, the Court of Appeal held the judge must direct the jury that it must be sure, beyond reasonable doubt, that any alternative explanation offered by the defendant for the presence of the money is not the correct one. Moreover, the jury must be satisfied that the money indicates an ongoing intention to supply drugs, and not merely the proceeds of a previous crime (which is not at issue in the present preceedings, of course).
This direction seems quite generous to the defendant, but the standard of proof must always be 'beyond reasonable doubt', and a person in possession of drugs might have an innocent explanation for the money.
As to the lavish lifestyle, in R v halpin (1996), the Court of Appeal held that lifestyle would only rarely be relevant in drugs trials. However, lifestyle has been admitted in subsequent cases