Expert Scientific Evidence in Court: The Legal Considerations

In this chapter, the approach to expert evidence in some of the main common law jurisdictions is considered. Evidence given by experts is a special form of evidence and it is treated as exceptional in that there are certain hurdles that must be overcome before it will be allowed to be led. As litigation in some jurisdictions is becoming more and more adversarial in nature, lawyers and judges today are far more sceptical than in the past about the stature of expert evidence. This is particularly so with scientific expert evidence given its status, impact and objectivity in any case where eyewitness evidence is either sparse or unreliable. Consideration is given to the legal criteria applied by courts in civil and criminal cases to the question of whether opinion evidence given by an individual qualifies as expert evidence. The critical concept of admissibility of evidence is examined, in order to discover the conditions under which expert evidence will be permitted in the courtroom. Key cases from the United States, the UK, Australia and Canada are dealt with. Novel scientific evidence is treated as a special case in some jurisdictions, and the conditions for the inclusion of such evidence are discussed. The way in which evidence is presented is also touched upon, since presentation can be as important as substance, particularly in jury cases. This chapter is designed to highlight, in particular for non-legally qualified readers, the legal perspective on the subject of scientific expert evidence. This is an important perspective given that lawyers and judges (as well as jurors) do not approach the justification and presentation of such evidence in the same way as the scientist does. The law has constructed some artificial barriers and checks and balances into this question, in order to prevent the “mystic infallibility” of expert evidence having a disproportionate effect on the decision maker. Knowing what these barriers are, and how to prepare to cross them, is a key part in the preparation of techniques new and old and in the presentation of evidence about them. All of this is useful in considering the presentation of soil forensics expert evidence, since courts adopt a universal approach to all expert evidence, irrespective of the subject matter. As the courts are introduced to expert evidence from more and more areas of expertise, so they have become increasingly wary of evidence of techniques with which they are unfamiliar. On the other hand, well researched, objectively justifiable and clearly presented expert evidence will pass muster every time. Knowing what the law regards as essential in advance is key to the successful gathering, analysing and presentation of such evidence, and, in turn, to success in the outcome sought.