Forensic Evidence in Court

Evaluation and Scientific Opinion

Craig Adam

• Publisher: Wiley-Blackwell
• Year: 2016
• ISBN:
  9781119054436
• Edition: 1
• Topic: Derecho
• Subtopic: Ciencias forenses

The interpretation and evaluation of scientific evidence and its presentation in a court of law is central both to the role of the forensic scientist as an expert witness and to the interests of justice. This book aims to provide a thorough and detailed discussion of the principles and practice of evidence interpretation and evaluation by using real cases by way of illustration. The presentation is appropriate for students of forensic science or related disciplines at advanced undergraduate and master's level or for practitioners engaged in continuing professional development activity.

The book is structured in three sections. The first sets the scene by describing and debating the issues around the admissibility and reliability of scientific evidence presented to the court. In the second section, the principles underpinning interpretation and evaluation are explained, including discussion of those formal statistical methods founded on Bayesian inference. The following chapters present perspectives on the evaluation and presentation of evidence in the context of a single type or class of scientific evidence, from DNA to the analysis of documents. For each, the science underpinning the analysis and interpretation of the forensic materials is explained, followed by the presentation of cases which illustrate the variety of approaches that have been taken in providing expert scientific opinion.

Part 1

1
An Introduction to the Admissibility of Expert Scientific Opinion

1.1 Admissibility, Reliability and Scientific Evidence

The investigative and legal processes, from the discovery of a crime to the verdict of the court, should ultimately ensure that the guilty person is correctly identified and that the innocent are exonerated. However, in many cases, the complexity, both of these processes and of the contributions to the debate, may lead to difficulties and challenges that act to impede and divert both scientific and legal arguments and which may lead to an unsatisfactory outcome. This book is concerned principally with the contribution of scientific evidence to legal debate. Central to this is an understanding of how the scientist’s findings can be properly interpreted, evaluated and communicated to the court and how the court draws appropriate inferences from the expert opinion in reaching its decision on the ultimate issue. In doing so, the court must necessarily be satisfied that the science is valid and the evidence relevant to its deliberations.

Although the concept of relevance has been enshrined in law across most jurisdictions for many years, in more recent times a debate has emerged across wider aspects of the presentation of scientific evidence to the court and the role of the scientist as an expert witness. There are many reasons for this, which include significant advances in scientific techniques, the need for investigators to deal with more complex and high‐profile crimes, increasing attention to these concerns and the ongoing responses of the legal profession and lawmakers to those events. The chapters in the first part of this book are intended to describe and discuss these aspects in detail across several jurisdictions.

As a precursor to this, it will be helpful to outline some of the key issues that mark out the path of this later discussion.

The law provides for an expert witness to contribute both factual and opinion evidence to the court. Within the legal system, the judge is empowered to decide on whether any evidence is relevant to the case being debated. However, there is some diversification of rules and practice, which, in many jurisdictions, is largely driven by case law where so‐called landmark judgments by courts of appeal clarify points of law, which then apply to subsequent cases. In this way, across Anglo‐American and related jurisdictions in particular, the law governing the handling of scientific evidence by the courts has evolved, and continues to evolve, punctuated by changes to the law at a national level and other relevant activities by governments and agencies.

Over the past thirty years or so, concerns around scientific evidence in the court have focused on three main issues:

1. Is the evidence admissible to the court?
   Admissibility includes whether the evidence is relevant to and therefore has value for the legal debate (probative value), as well as a variety of other factors such as the status of the expert witness, the quality of the methodology and the underpinning science. Admissibility is determined by the judge and is categorical, since the expert witness will either be permitted or not permitted to give their testimony. The judge is said, by some legal authorities, to act as a ‘gatekeeper’ for expert evidence. The judge’s decision on admissibility and the grounds for that conclusion is crucial to the legal process.
2. Is the evidence reliable?
   Reliability may contribute to admissibility but is itself a complex concept which is subject to considerable variation in how it is dealt with across courts and jurisdictions. At a basic level, the court has to assess the validity of the scientific methodology and whether the evaluation delivered by the expert is soundly based on the results of their work. However, the extent to which this contributes to admissibility or whether reliability may instead be a factor in deciding the legal weight of the evidence is open to debate. Therefore, it is the degree of reliability and not necessarily whether evidence is reliable or unreliable that matters. The significance of these two factors has been summarised by Susan Haack in the phrase:

   ‘Admissibility is categorical, reliability is continuous’

   [Haack, quoted in Cole, 2007]

3. Has the significance of the evidence been properly assessed, then communicated to and understood by, the court?
   It is the responsibility of the scientist to design and carry out experimental measurements, to analyse results and then to interpret what they mean in the context and circumstances of the case. However, as an expert witness, the interpretation of this work must be presented to the court so that the judge, the jury and the legal professionals may fully understand its significance to their debate. The extent to which this may be achieved and the manner in which it is done varies hugely across the subdisciplines of forensic science, between individual scientists and across courts and jurisdictions. The process of the evaluation and presentation of scientific evidence forms the discussion in the main section of this book.

Given that expert evidence, as we understand it, has been delivered to the courts for around two hundred years, why has the debate over admissibility, reliability and the quality of scientific opinion come to the fore with such an intensity in more recent times? In the mid‐Victorian period, scientific expert witnesses were often pitted against each other in the courtroom. Such public displays of warring experts tended to dent the reputations both of science for objectivity and of the scientists themselves as impartial seekers of truth. The situation was somewhat mitigated in the first part of the twentieth century with the professionalisation of forensic science and the consolidation of much expertise into regional and national laboratories. The situation began to change from the 1970s onwards, with increasing criticisms of ‘junk science’, particularly in the United States, where scientists were held to be influenced by the interests of the big corporations that had hired them and by the continuing stream of miscarriages of justice where forensic science and forensic scientists were seen as significant contributors to the problem.

It is convenient and informative to begin this discussion with cases where the legal process has clearly broken down and miscarriages of justice have taken place. Though such events cannot be laid at the door of any single cause, the impact of DNA profiling has probably had a greater significance than any other factor in bringing these to light and so that shall be our starting point.

1.2 The Impact of the DNA Revolution

The conviction of Colin Pitchfork in 1988 for the rape and murder of Lynda Mann and Dawn Ashworth can rightly claim to be the first case where DNA profile evidence was key to a successful prosecution. However, this should not overshadow a second milestone achieved through this investigation. The reason the police initially approached Alec Jeffreys at Leicester University, with a view to trying out his new DNA identification technique, was that they had obtained a confession to one of these murders from a different man, Richard Buckland, and were looking for scientific proof of his involvement in the second case, which Buckland denied. When the analysis revealed that Buckland was not the source of the semen sample from either crime, this revolutionary technique prevented a potential miscarriage of justice. The impact of DNA profile evidence when it contradicted and quashed, not only other forensic evidence, but also witness testimony and suspect confessions, was to provide an impetus for challenges to wrongful convictions that would endure for the next thirty years.

However, DNA was to initiate a deeper and more subtle revolution, both in the paradigm of forensic identification sciences and, more particularly, in the evaluation of forensic evidence and its contribution to legal debate (Saks and Koehler, 2005). For just over a hundred years, fingerprint evidence had been the gold standard against which other forensic techniques were measured and, as an identifier of individuals, the fingerprint was unchallenged by the courts. Not only was it accepted that fingerprints were unique, but both legal professionals and lay people working in the courts had a general appreciation of the process of identification and trusted the word of the fingerprint examiner as an expert witness who stated that a crime scene mark matched a fingerprint taken from a suspect. By providing such categorical testimony, the fingerprint ranked above all other forensic evidence in the court.

Once the DNA profile arrived on the scene, as rival to the fingerprint, it is easy to see why the term ‘DNA fingerprint’ became fashionable, as the new technique sought to promote its apparent infallibility. However, DNA testimony did more than just state a match between two profiles, it was supported by an additional statement on the rarity of the profile within the relevant population and, unlike the uniqueness claim from the fingerprint expert, this was underpinned by rigorous scientific research in the field of population genetics. Thus, DNA was perceived as objective and scientific while the interpretation of fingerprints, for the most part, depended on subjective criteria, implemented according to the judgement and experience of the individual examiner. Over the following ten years or so, the DNA profile, with its strong scientific basis, became established as the new standard while fingerprinting, and indeed many other techniques within forensic science came under increasing scrutiny as their scientific foundation, validity and reliability became questioned by the legal profession, by many scientists and indeed by governments and their agencies. Consequently, the concept of a ‘DNA fingerprint’ fell into disuse and the ...