20 Part I: Recognition and usage Fourth, the tribunals determine whether the expert’s testimony has the effect of deciding the issue before the court. Termed the “ultimate issue” rule, this exclusionary rule limits the expert from offering his or her opinion on subjects such as the criminal responsibility of an accused. National and international tribunals are very strict in this matter. If an expert pronounces an opinion on the criminal liability of the accused in question, the expert risks that the entire opinion may be rejected. Fifth, the tribunal will generally exclude those parts of an expert’s opinion that are based on matters not directly within the expert’s own observations. Reliance on material that cannot be directly evaluated by the tribunal can violate the accused’s fair trial rights. Once the expert’s opinion is admitted into evidence, the tribunal must determine what weight it should be accorded. Bearing in mind that the opinion is intended to assist the trier of fact, expert reports should be short and simple, whilst providing sufficient reasoning for the opinions therein without excessive technical jargon. It is of paramount importance that the expert maintains objectivity by avoiding argumentative statements or statements that demonstrate a lack of impartiality. However, the fact that a witness has been involved in the investigation and preparation of the prosecution or defence case or is employed or paid by one party does not disqualify him or her as an expert witness or make the expert statement or report unreliable. Challenges to the impartiality of a witness statement do not go to its admissibility but to the weight it should be given. Conclusion Investigations and documentation can give rise to critically important evidence of torture, thereby significantly enhancing prosecutions and accountability. However, before it can be accepted as evidence, the documentation must fulfil the general rules of admissibility. At the international tribunals, the rules of admissibility are flexible, and judges typically refrain from excluding evidence as long as it is relevant and probative. Therefore, the utility of the evidence to the judges is determined by the acceptance of the reporting professional as