The media are now revealing that certain material was not admitted into evidence at the trial - The Guardian 29th October - "Vincent Tabak and the porn searches the jury did not hear about." On what basis could such information be withheld from the jurors who are, after all, constitutionally, the decision-makers in the case? It is certainly arguable that all known material ought to be placed before the jury and that they ought to be allowed to consider it along with the guidance given to them by the trial judge. Indeed, the prosecution had argued that the material ought to have been admitted into evidence but the trial judge ruled against them.
At a basic level, the
first requirement for evidence to be admitted is that it is factually relevant to the issues being tried. Another fundamental requirement is that a trial must be fair and, sometimes, this may require the exclusion of material which is likely to prejudice the jury against the defendant but which does not necessarily point to guilt. The legal phrase used is "more prejudicial than probative."
In R v Sang  AC 402, the House of Lords confirmed that that the judge in a criminal trial has a discretion to exclude legally admissible evidence tendered by the prosecution. The existence of this discretion had actually been accepted for quite some time prior to Sang - e.g. R v Christie  AC 545. The discretion is part and parcel of the trial judge's paramount duty which is to ensure a fair trial. All of this is English common law principle. The words of Roskill J in R v List  1 WLR 9 explain this further:
It is reported that Police analysis of Tabak's computer revealed (a) pornography involving sexual violence to women and (b) that he looked at websites on which the sexual services of women were advertised. He had already admitted to killing Miss Yeates so the issue before the jury on the murder charge would have been what was his mental state at the relevant time. Intent to kill or intent to cause her serious bodily harm had to be proved and proved to the criminal standard of "beyond a reasonable doubt." The question therefore became whether this material, if revealed to the jury, would have been likely to have a very prejudicial effect on the minds of the jury members. If so, it would then have been necessary to consider whether that prejudicial effect outweighed the question of whether the evidence actually showed that, at the relevant time, Vincent Tabak had the requisite intent to justify a murder conviction.
It is perhaps an obvious point, but nevertheless worth making, that the admission of evidence can be used to establish grounds for an appeal. Once admitted, we cannot know what the jury made of it since their deliberations are confidential (Contempt of Court Act 1981 s.8). Once the evidence is excluded we can say that the jury will not have been influenced by it. Even in cases where there is an appeal based on a judge's decision not to exclude, the Court of Appeal will only interfere with the judge's exercise of discretion if there is no material on which the judge could have based the decision or where he has erred in principle. It will not be enough that the appellate court considers that it would have exercised the discretion differently.
There are also various other rules relating to admissibility of evidence in criminal trials. (In civil cases, different arrangements apply for the admissibility of evidence). Thus, in criminal cases, there are special rules relating to matters such as hearsay evidence, confessions, evidence relating to character (good or bad) and so on. However, at the foundation level, evidence must be relevant and the trial judge's duty to ensure a fair trial is paramount.
Jack of Kent - "Criminal evidence - a very quick introduction."
Law Mentor - "A just verdict - Vincent Tabak"