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Science and justicelocked

  • Jim Fraser


‘Science and justice’ investigates the relationship between the universal nature of science, and the local nature of the law. Forensic evidence is often presented in the courtroom by expert witnesses. The UK legal system states that these witnesses need ‘expertise’, but no criteria exist to define what level this expertise needs to be at. In the United States two tests are used to determine whether expert evidence is admissible. These determine whether the evidence is based on ‘scientific knowledge’ and whether it is generally accepted by the scientific community, but they also bring about their own issues. The development of an effective relationship between science and law is essential to ensure science continues to contribute to criminal justice.

The enormous conceptual change that [scientific] thinking require[s]d shows that science is not just about accounting for the ‘unfamiliar’ in terms of the familiar. Quite the contrary: science often explains the familiar in terms of the unfamiliar.

Lewis Wolpert, The Unnatural Nature of Science

More and more the problem of expert qualification and the risk of biased scientific evidence appear to stem from the institutional demands and limitations of criminal proceedings rather than reflecting the inadequacies of scientific method or failure of individual experts.

Paul Roberts and Christine Willmore, The Role of Forensic Science Evidence in Criminal Proceedings

The defining feature of forensic science is its relationship with the law. In this chapter, we come to what is typically the final stage of a criminal inquiry – the trial. This involves an encounter between science and law, and we will reflect on the issues this may raise. In doing so, we will explore the nature of science and law and the implications of their different worldviews for how forensic science is used, develops, and is constrained. Science is based on observing the external world. It has no particular view of how the world ought to be, but seeks to describe it on the basis of empirical p. 121observations and the development of predictive models. These models are continually retested and modified on the basis of experiments which use universal methodologies. Central to these methodologies is the use of statistical probability to describe the level of uncertainty in any set of observations. Science is the same wherever it is done and is unquestionably the best means we have of understanding the physical world. Scientific models do not always concur with our commonsense view of the world; they can be unfamiliar, even counterintuitive. In fact, Lewis Wolpert, the embryologist and well-known science writer quoted above, considers this to be one of the defining features of science: it not only explains the unfamiliar in terms of the familiar, but sometimes explains the familiar in new ways that we could not have foreseen.

Law proceeds by its own authority, by the power of statute or case law, with limited reference to any external authority. Different legal systems operate in different ways, and there is no universal law comparable to universality of science. In fact, the law is local, at the level of the nation state or provincial administration. The legal world can be divided into two main families of criminal law: inquisitorial and adversarial. In adversarial legal systems (the USA, the UK and its former colonies and dominions, Australia, Canada, New Zealand), legal outcomes are determined on the basis of a highly structured argument. Only certain types of facts and information can be used in such arguments as these need to be allowable (admissible) under the rules of evidence. The rules of evidence are an essentially ad hoc collection of mainly exclusionary directives derived from historical practice. Inquisitorial systems are less restrictive about the evidence they allow and more interested in the potential value of the evidence in determining the outcome to the case. In general, inquisitorial systems get nearer to the ‘truth’ than adversarial systems. Unlike science, which uses a consistent methodology and standards, the law uses different procedures depending on the circumstances. In criminal cases, the prosecution has to prove ‘beyond reasonable doubt’ that the defendant is guilty, but in civil cases the burden of proof is on the p. 122balance of probabilities. Such determinations are frequently made by juries, and the rationale for such judgements is ‘common sense’. It is difficult to imagine two systems of knowledge that go about their businesses so differently, and it should come as little surprise that there are inevitable conflicts between science and law about who is right and whose knowledge has most authority.

These issues most frequently arise when expert evidence is involved. The law has recognized for hundreds of years the need to use special types of witnesses called ‘expert witnesses’ to cover areas of evidence about which the courts lack knowledge and expertise in the matters before them. Expert witnesses differ from ordinary witnesses in a number of ways, but primarily in their right to give opinions in evidence. Ordinary witnesses must restrict their evidence to facts – what they observed or heard – but are not allowed to interpret these facts, as this is a matter for the jury. Expert witnesses can express an opinion about the meaning and significance of facts – whether an item is a firearm, how a fire started, how a weapon came to be bloodstained. However, they are only entitled to express opinions within their particular area of expertise.

This is a particularly difficult issue, which can be illustrated by a recent miscarriage of justice in the UK. In 1999, the solicitor Sally Clark was convicted of the murder of her two infant children. The expert witness, Sir Roy Meadow, then Professor of Paediatrics and Child Health at Leeds University, gave evidence at her trial which was highly influential in her conviction. He testified on a range of medical matters that were presumably within his area of expertise, but he was also allowed by the court to give expert opinion on the probability of two child deaths in the same family. He stated that this probability was 1 in 73 million. There were no statistical experts involved in the trial and no significant cross-examination on this point. Meadow derived this figure from the frequency of a single child death in a family (1 in 8,500), which he multiplied to account for both deaths to obtain 1 in 73 million. His mistake p. 123should now be clearly evident. In Chapter 5, we encountered how the significance of DNA profiles are estimated on the basis of the combination of genotype frequencies involved. Such frequencies can only be multiplied if they derive from independent events, but the deaths of two genetically related individuals (the children in this case) cannot be treated as independent events. The subsequent appeal which released Clark also resulted in the review of around 5,000 other cases.

In the UK, the role and responsibilities of the expert witness are set out in case law, although there is also some legislation and practice guidance. Case law requires an expert to have ‘expertise’, but does not set out criteria by which this is to be judged or established. There is no need for experts to be formally qualified, indeed there is case law that warns against being over-impressed by the qualifications of expert witnesses, something that perhaps applied in the Clark case. The law also carefully prescribes the ambit of expert witnesses and how they ought to interact with the courts. The main case on this issue is worth quoting:

[The role of the expert is] to furnish the judge with the necessary scientific criteria for testing the accuracy of their conclusions, so as to enable the judge or jury to form their own independent judgement by the application of these criteria to the facts proved in evidence.

This carefully crafted judgement was set out by Lord Justice Cooper in a Scottish case (Davie v. Edinburgh Magistrates) in 1953. In effect, this amounts to an expectation that the expert witness will educate the court in each case on the specialist subject in hand – analysis of drugs, DNA profiling, paint comparison. This is a tall order in a classroom with willing volunteers such as students; it is almost impossible to meet this standard in a courtroom where the questioning is under the control of two lawyers who have opposing views about the significance of the evidence, and seek to convince the jury of their interpretation. The following example illustrates what can be achieved by tactical cross-examination:p. 124

  • Defence counsel:
    ‘Can you age hair?’
  • Witness:
  • Defence counsel:
    ‘Are you sure?’
  • Witness:
    ‘I am not aware of any means by which you can age biological materials.’
  • Defence counsel:
    ‘Have you done any research which proves that you cannot age hair?’
  • Witness:
    ‘… No’
  • Defence counsel:
    ‘Have you read any published papers which state that you cannot age hair?’
  • Witness:
    ‘I am not aware…’
  • Defence counsel:
    ‘You are not aware of any evidence which states that you cannot age hair…’

The example dates from the 1980s, but the principle it illustrates is still valid today. By careful questioning and control of the witness, defence counsel elicits a ‘fact’ from the witness (the possibility that hair can be aged cannot be excluded) which is untrue and which no one in the expert community believes to be true. Furthermore, the implication is left that the expert should perhaps have done her homework rather better and ought to have tried to age the hairs in the case. Most experts dislike this type of sophistry, but it is quite common in the courtroom. An experienced expert can respond in turn with carefully worded and qualified answers in what can become a war of words, but it rarely gets to the truth. Since it is not possible to carry out research on these issues, we cannot know the impact of such exchanges on juries. Such issues are not confined to cross-examination. This is not an environment that supports the level and quality and communication required of such important issues, and the quote from Roberts above appears to bear this out. Even the physical layout of the courtroom can intrude. The witness generally faces the questioning advocates, as is appropriate, but typically has to p. 125turn through 90 degrees to make eye contact with the jury who will evaluate her evidence, and the judge is frequently behind the witness. Those experts I know who have given evidence in inquisitorial systems consider them to be far more conducive to effective communication of evidence.

In the USA, expert evidence is subject to two tests before it can be deemed admissible. The first of these (Frye v. The United States, 1923) seeks to establish the reliability of the potential evidence by asking if it has ‘gained general acceptance in the particular field in which it belongs’. This is sound in principle but difficult to apply in practical terms. How does one establish ‘general acceptance’, and furthermore, what about new technology which by definition cannot have attained general acceptance? A second test is used in the USA which particularly addresses the issue of science. In Daubert v. Merrell Dow Pharmaceuticals Inc. (1993), it was determined that certain expert testimony must be based on ‘scientific knowledge’. The determination of what is and what is not science is more difficult than it appears since science has no single agreed definition. These considerations notwithstanding, both Frye and Daubert attempted to deal with the issue of expert evidence by setting standards for admissibility and reliability. The approach brings some clarity and shared understanding of this complex situation to lawyers and scientists, of the standards expected of expert witnesses and the restrictions that may be applied to evidence which does not meet the standard. There are no such declared standards in the UK at present, although the matter is under review by the Law Commission in England and Wales at the time of writing. The courts recognize the need for expert witnesses who have knowledge not in possession of the law. Yet there is a fundamental contradiction, since it is individual courts on an incremental case-by-case basis that determine who is to be an expert witness and what constitutes expert evidence.

The relationship between science and law is a complex one. Most lawyers are ignorant of science and most scientists ignorant of law. p. 126The courtroom is a complex environment which does not readily support the level or quality of communication that such evidence merits. Given that the amount of scientific evidence entering legal systems is higher than ever before, the development of an effective relationship between science and law is essential to ensure science continues to contribute to criminal justice.