Scientific Evidence In Criminal Prosecutions Research Paper

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There is no doubt that forensic science today makes an enormous contribution to the detection and proof of crime in modern legal systems. Scientific evidence is often useful, and sometimes vital, in proving offenses and bringing perpetrators to justice who might otherwise evade detection or conviction. Strong scientific evidence implicating the accused often induces guilty pleas without the need for a contested trial. Scientific evidence simultaneously plays an important role in excluding innocent suspects from further official inquiries or surveillance. Yet scientific evidence can also be a potent source of injustice when errors are made.

This research paper explores the role of scientific evidence in criminal prosecutions, particularly with regard to common law jurisdictions such as those in the UK and the USA. It highlights challenges for criminal process actors and indicates issues of on-going controversy and concern. Prosecutors play a pivotal role in the use of scientific techniques of detection and in providing scientific evidence to the court. Common law prosecutors are instrumental in instructing expert witnesses and, in consultation with trial counsel, in adducing scientific reports and testimony in cases proceeding to trial. While the paper focuses primarily on prosecutors’ professional responsibilities within an institutional legal framework informed by particular ideals of justice, the final section briefly addresses a more socio-legal issue: the impact on prosecutions of the “CSI Effect.”

Scientific Evidence As Proof Of Crime

The use of scientific techniques and expertise in the investigation and prosecution of crime is not a new phenomenon. The appliance of cutting-edge technology in the administration of criminal justice goes hand-in-hand with the Enlightenment and modernity’s scientific revolution. Indeed, English law cases addressing issues surrounding expert evidence, such as Buckley v Rice Thomas (1554) 1 Plowden 118, CB, (which is still occasionally cited today), date back to the Renaissance. However, the twentieth century is properly regarded as the century of forensic science. This is when the early practical experimentation of the nineteenth century, immortalized by Sir Arthur Conan Doyle in the figure of Sherlock Holmes and his avowedly scientific methods of criminal detection, truly bore fruit. First pathology and fingerprinting, then blood-typing (serology) and a raft of comparison “sciences” (handwriting, ballistics, tool-marks, glass, hairs, fibers, footwear marks, dentition, facial-mapping, voice analysis, etc.), and most recently CCTV cameras, mobile phones, digital forensics, and – above all – DNA profiling have transformed criminal investigation, prosecutions, and trials around the globe.

Forensic science has become an integral part of modern criminal justice systems. Scientific evidence routinely features in high-profile serious crimes of murder, robbery, and sexual assault, but also plays an important role in relation to more mundane offending lower down the criminal calendar. Empirical research indicates that relying on DNA evidence in prosecutions of high volume crimes such as burglary yields many more identifications of perpetrators than more traditional methods of detection, including fingerprinting, at apparently reasonable cost (Roman et al. 2008: but cf. Williams and Johnson 2008: Chap. 6). It is not surprising that governments have invested heavily in the development of forensic science technologies, notably including the UK’s DNA expansion program and the creation of a large National DNA Database (NDNAD) (Williams and Johnson 2008: Chap. 5) – although the Coalition Government’s highly controversial recent decision to close the British Forensic Science Service (FSS) is regarded by many as a retrograde step (see House of Commons Science and Technology Committee 2011).

Acknowledging that the development and proliferation of forensic science over the course of the twentieth century has been a marvelous boon for the administration of criminal justice is not to say that it comes without any risks or drawbacks, much less to encourage complacency about it. Like any powerful tool, forensic science gets the job done efficiently and with impressive results but it is equally capable of doing appalling damage if it is not handled carefully and treated with respect. Precisely because scientific evidence has a – generally speaking, well-merited – reputation for objectivity and reliability, it is prone to be highly misleading in cases where it is either inherently unsound or put to improper uses. This is why forensic science is often implicated in miscarriages of justice. In a National Registry of Exonerations recently launched by Michigan and Northwestern law schools in the USA, “bad forensic science” features in 24 % of 891 confirmed false convictions since 1989 (Gross and Shaffer 2012: 63–65). It is often suggested that, whatever defects scientific evidence might suffer from, it has to be more reliable than other forms of evidence traditionally relied on in criminal adjudication, such as eyewitness identifications and confessions by suspects, which have often produced false convictions in the past and have been repeatedly torn to shreds by behavioral science researchers. While it might well be true that scientific evidence is comparatively more reliable than other forms of criminal proof, this is no reason not to strive to improve its performance and to make every effort to safeguard against errors, to the extent that this might be possible.

Prosecutors play a pivotal role in the use of scientific techniques of detection and in providing scientific evidence to the court. In many continental jurisdictions prosecutors direct criminal investigations, including recourse to scientific testing, and judges seek further technical advice, as required, from their own court-appointed experts. In common law jurisdictions, criminal investigations have traditionally been run by police detectives with substantial operational autonomy and comparatively little input from prosecutors. But this is changing. For example, many state prosecutors in the USA today adopt a proactive approach to participating in major criminal investigations, and even in England and Wales – where strict separation between the police and the Crown Prosecution Service (CPS) used to be an article of faith – there is far greater emphasis on interagency dialogue and cooperation between what is now, revealingly, described in official documents as “the prosecution team” (Moreno and Hughes 2008). Prosecutors are instrumental in instructing expert witnesses and, in consultation with trial counsel, in adducing scientific reports and testimony in those cases that proceed to trial.

Scientific Evidence In Adversarial Criminal Process

Reliance on scientific evidence in particular criminal prosecutions is the outcome of a deliberate, self-conscious process, comprised of a series of reasonably distinguishable, though often overlapping, temporal phases. Forensic science evidence is just as much a product of police case-building, structured by a hypothesized “theory of the case,” as any other type of evidence. Research conducted for the Royal Commission on Criminal Justice identified nine key phases in the production of scientific evidence (Roberts and Willmore 1993; Roberts 1994), suggesting the following, somewhat stylized, model of “the typical case” to which real proceedings conform to a greater or lesser extent.

(1) Investigators must first of all decide to utilize scientific expertise. The initial decision is made by police and prosecutors and therefore answers to investigative, rather than strictly scientific, imperatives. This creates an observable tension between investigators’ needs and expectations, and the ability of science to satisfy them. In recent times attempts have been made to educate investigators about the possibilities and limitations of scientific evidence, harnessing technological innovation to financial imperatives driving “cost-effective” policing (Lawless and Williams 2010). Police officers have been encouraged to turn to science in the investigation of “routine” volume crimes such as burglary and theft, where the potential for scientific assistance has often been overlooked in the past, but progress is uneven across the regions. It remains the case that scientific evidence is produced only when investigators think they need it, which is not necessarily when science might in fact be of most – or indeed, of any – assistance to the prosecution.

(2) Having elected to employ scientific assistance, the first task for police or prosecutors is to locate an appropriate expert. In the UK, until very recently, the FSS has been on hand to provide what was generally acknowledged to be a world-class service in a range of forensic specialisms, including DNA profiling. With the closure of the FSS, reliance must now be placed on market provision by a range of commercial suppliers, some of which – like LGC Forensics, the old Lab of the Government Chemist – were formerly state-run but subsequently privatized. Effective regulation, validation of techniques and processes, and accreditation of laboratories is vital to the integrity of this market-based system. In England and Wales, much of this responsibility has been invested in the newly created post of Forensic Science Regulator, with the assistance of the Forensic Science Advisory Council. For other types of expertise, including clinical, medical, and the more esoteric forensic sciences, investigators have to look to the hospitals, universities, research institutes, and private consultancies to locate an appropriate expert. Although these arrangements seem to work out in the majority of cases, the process of hiring experts is surprisingly informal, and sometimes fails to produce the best evidence. It remains to be seen whether a post-FSS world of free-market provision will continue to supply an adequate range of high-quality forensic science expertise to the administration of criminal justice, with appropriate investment in quality control and research and development (Lawless 2011; Roberts 1996).

(3) Once an appropriate expert has been engaged, the next step is to supply the expert with relevant crime scene material (or other raw data) for analysis. Crime stain samples or material recovered from suspects or complainants must be identified, preserved, and transmitted to the laboratory free from contamination and protected from (further) degradation. Police and prosecutors must ensure that chain of custody is properly documented, since physical evidence is worthless unless the court can be confident about its provenance and integrity. For example, samples should be collected in “temper-evident” packaging (Lynch et al. 2008: Chap. 4). These simple administrative measures make an essential contribution to ensuring that justice is not only done, but also manifestly seen to be done.

(4) Another crucial aspect of the process of generating forensic science evidence concerns the nature of the instructions received by the expert. Scientists are inevitably influenced by the type and extent of background information provided to them by investigators, and even possibly by the form and wording of the police request for assistance. Researchers have drawn attention to the risks of unconscious biases creeping into scientific judgments through suggestive contextual information, leading forensic experts to “see,” and report to prosecutors and courts, what the information they were given has led them to expect (Risinger et al. 2002). One way to respond to the risk of “contamination” by such extraneous influences would be to insist that items must be sent to the laboratory without any accompanying background information and with a request for assistance in scrupulously neutral terms. At one time the FSS considered this best practice. Yet that approach, and the language employed in its justification, betrays a fundamental misconception about the nature of forensic science. Following from the essentially applied nature of their discipline, most forensic scientists prefer to be told as much background information as possible, in order to be able to tailor their approach to the needs of the investigation. There is, most obviously, little point in a scientist wasting time and energy on matters that are not disputed in the proceedings. Background information is not so much, on this view, an external source of “contamination” or “bias,” but an essential part of the scientist’s data for analysis. Nonetheless, the continued risk of inappropriate, and possibly unconscious, influence suggests that – to the extent that it can reliably be identified – irrelevant and potentially prejudicial material should be filtered out of the information provided to forensic scientists, at least until they have conducted relevant tests and produced their preliminary findings.

(5) The scientist next proceeds to conduct whatever testing or examinations are judged appropriate. (6) The results of the scientific investigation are then written up into a report. Both testing and reporting reflect investigators’ instructions and expectations, within the broader framework of criminal proceedings and the conventional practices of forensic science. The rules of substantive criminal law structure police investigations, which in turn influence the questions scientific experts are asked to consider, the tests they undertake, and the nature and content of the reports they write. For example, scientists asked to produce a DNA profile from bloodstained clothing in order to establish identity would not routinely comment on a self-defense theory of the case, or any other conceivable hypothesis such as provocation, mistake, or accident. But if specifically invited to do so, the expert could consider how blood spatter patterns, DNA mixtures or secondary transfer, etc., might bear on potential defense arguments and report any relevant findings or advice to prosecutors. According to one police training manual, “forensic awareness… involves the prosecution team providing the forensic scientist with all essential information in the case from the outset, keeping them continuously updated and asking them the right questions to progress the case and get the best results” (Moreno and Hughes 2008: 25). When producing reports detailing their results and conclusions, experienced forensic scientists are conscious of the instrumental role of science in criminal proceedings, and of a report’s intended audience. Investigators expect scientists to help them prove, or disprove, criminal charges against a suspect, and the style and language of expert reports is directed to that end. There are generalized pressures to work quickly and produce definite conclusions. Even if scientists were always paragons of the ideals of impartial and objective inquiry, which most of them espouse and the law demands, their work product would still remain a highly selective, constrained, stylized, and instrumentally orientated form of science. Unfortunately, forensic scientists have occasionally been seduced, or corrupted, by the institutional pressures inherent to an adversarial criminal process, allowing themselves to become partisan instruments of the prosecution rather than impartial purveyors of objective scientific facts and considered expert opinions to the courts (see Erzinc¸lioglu 1998; Giannelli 1997).

The production of a scientific report often effectively signals the end of particular criminal proceedings, either because the accused is induced to plead guilty in the face of compelling incriminating evidence, or because the prosecution is too weak to proceed in the absence of scientific corroboration. Scientific evidence plays an important – and possibly somewhat overlooked – role in exonerating innocent suspects from continued suspicion, in cases where DNA profiling indicates that the suspect could not, in fact, have been the perpetrator after all. In those minority of cases that do proceed to trial, the production of scientific evidence typically involves three further key stages.

(7) Defense lawyers sometimes appoint their own experts, occasionally to follow up exculpatory leads, but more often just to double-check the work already conducted by a prosecution expert. The appearance of a defense expert may present opportunities for communication, or even cooperation and the exchange of ideas, between experts on opposite sides of the adversarial divide. Forensic scientists seldom share the adversarial culture of the police and lawyers who hire them, and some scientists treat their notional adversaries as colleagues. On the occasions when scientists meet in the laboratory to review test results, defense experts have been known to persuade scientists working for the prosecution to undertake further tests or to reinterpret their results in the light of a different perspective or new information (Roberts and Willmore 1993: 56–57). Scientific investigations undertaken by the defense, in conjunction with prosecution scientists or independently, do occasionally produce significant exculpatory evidence. In the majority of cases, however, the defense examiner simply confirms the prosecution expert’s data and conclusions.

(8) Expert witnesses may attend a pretrial conference with counsel. This is potentially an important meeting, at which counsel can review the expert’s evidence and ensure that the expert is prepared to stand by the conclusions expressed in the expert’s report. Empirical research conducted in England and Wales in the early 1990s found that pretrial conferences between experts and counsel were frequently short or nonexistent (Roberts and Willmore 1993: 57–61). This was partly due to time pressures, but also reflected a deeply held aversion on the part of some barristers to meeting any witnesses prior to trial, for fear of attracting accusations of “witness coaching.” That fear seems misconceived with regard to expert witnesses, not because experts are immune to pressure and undue influence, but because counsel are unlikely to lead scientific evidence successfully if they do not themselves understand the expert’s evidence. Lack of pretrial communication between experts and counsel was said on occasion to have adversely affected the presentation of scientific evidence at trial. By contrast, witness preparation is a standard feature of criminal litigation in the USA. No US prosecuting attorney would proceed to trial, in any serious matter, without first having reviewed the evidence of key prosecution witnesses, including scientific experts, in person.

Over the ensuing years, the novel idea of conducting pretrial conferences with expert witnesses has become more familiar and culturally acceptable to barristers in England and Wales, too. More significantly still, English courts have adopted a general policy of proactive judicial trial management within the framework of the Criminal Procedure Rules (CrimPR), which were introduced in 2005. The CrimPR make explicit provision for clarifying, and if at all possible resolving, scientific issues prior to trial through conferences of experts and joint reports. The Court of Appeal has repeatedly emphasized the importance of these provisions, and constantly encourages trial judges to enforce them on the parties. It has come to be widely appreciated that adversarial trials involving “battles of experts” tend to obscure genuine scientific disagreements and sometimes give the impression of scientific dispute or uncertainty where none truly exists. The prevailing philosophy among the senior judiciary is consequently to try to deal with scientific aspects of the case outside the courtroom, while still preserving the determination of genuinely contested issues for the jury at trial.

(9) Although only a very small percentage of criminal cases results in a contested trial in any adversarial jurisdiction where guilty pleas and bargains are the norm, these are disproportionately serious and important cases. Scientific evidence features in many of these contested trials, to a greater or lesser extent. Such evidence is often uncontroversial, and may be agreed – possibly as a result of successful pretrial discussions between the parties and their experts. When not disputed, the expert’s report can simply be read out as documentary evidence, and is likely to be accepted at face-value by jurors (albeit that the jury is always at liberty to form its own view, and must be so directed by the trial judge in England and Wales: see e.g. R v Allen [2005] EWCA Crim 1344; R v Hookway and Noakes [2011] EWCA Crim 1989). Where the defense or, more infrequently, the prosecution wants to challenge scientific evidence, however, the expert will usually be called to court to testify in person by the party instructing the expert. There is formal provision for court-appointed experts in many common law jurisdictions. However, judges tend not to utilize court experts, in deference to the adversarial precept that the parties run their own cases at trial, with the judge adopting a relatively passive role as neutral “umpire” to ensure “fair play” and litigants’ adherence to the rules of criminal procedure. (The “umpireal” model of judging in adversarial trials is an overstated and increasingly anachronistic simplification, but still serves to encapsulate a deep-seated cultural difference between “adversarial” and “inquisitorial” conceptions of the trial judge’s role.)

Scientific experts testify in the courtroom like any other witness, through a series of answers to questions put by counsel. The expert witness is first taken through examination-in-chief (in the USA, direct examination) by the advocate calling the witness, and then undergoes cross-examination by counsel for the other party or parties in a multihanded trial. If thought necessary to clear up any matter raised in cross-examination, a third phase of questioning (“reexamination” in the UK, “redirect” in the USA) may be conducted by the side calling the expert. Needless to say, this is a highly artificial way of presenting scientific evidence to the court. Its success depends in large part on the skill and scientific understanding of counsel, which cannot always be relied upon, especially if pretrial preparation has been inadequate. It might be preferable, from the point of view of communicating scientific evidence effectively, if experts could present their evidence in a more direct and less constrained fashion. As things stand, counsel may through incompetence or as a deliberate strategy distort the intended meaning of an expert’s evidence, and the effect may be compounded where experts called by the prosecution and defense disagree, or appear to disagree, with each other. The evident limitations in this context of traditional oral trial procedure, whatever its efficacy or cultural significance in relation to ordinary witnesses of fact, underscore the wisdom of making more extensive use of pretrial mechanisms to fully and fairly exploit the potential of forensic science in criminal proceedings.

The Common Law Prosecutor’s Role And Responsibilities

In many continental legal systems prosecutors are formally part of the magistracy. As a matter of jurisprudential theory, they are unequivocally regarded as acting in the pursuit of justice and they are supposed to be objective and nonpartisan. In common law countries, too, prosecutors are required to be “ministers of justice” who select and pursue appropriate charges against those suspected, on the basis of pertinent and reliable evidence, of having committed particularized criminal offenses. “Prosecution counsel has to exercise independent judgment throughout, with the objective not of obtaining a conviction at any cost, but of ensuring that justice is done” (Buxton 2009: 427). English prosecutors, it is said, prosecute but do not persecute. The special responsibility that prosecutors owe to justice may be rather less self-evident and unequivocal in an adversarial procedural system, in which criminal trials can sometimes take on the appearance of a gladiatorial contest. But it is no less fundamental to the legal system’s claims to justice and legitimacy. There is a world of difference between a tough-minded lawyer who prosecutes firmly and fairly, and an ethically cavalier show-boater who tries to win cases and secure convictions at almost any cost.

Another important difference between civilian and common law jurisprudential theory is that, whereas many continental systems adopt some version of the “principle of legality” requiring compulsory prosecution in every serious case in which there is solid evidence of criminality, common law prosecutions are fundamentally discretionary. Of course, a suspect cannot be prosecuted in the absence of evidence capable of demonstrating his guilt. But incriminating evidence alone is not automatically enough to justify, let alone mandate, a prosecution in common law jurisdictions. Prosecution in the instant case must also be judged to be in the public interest more broadly conceived. This principle is encapsulated in the two-step test for prosecution specified by the Code for Crown Prosecutors in England and Wales (CPS 2010). The first part of the test poses the question of evidential sufficiency: is there a “reasonable prospect of conviction” of specified charges on the available evidence? This is generally understood to mean that the reviewing prosecutor must judge that conviction is more likely than not, on the balance of probabilities (ibid: para. 4.6). Otherwise, more evidence must be sought by the police; and the prosecution may ultimately have to be abandoned if better evidence is not forthcoming. Only if the evidential sufficiency test is satisfied should the Crown Prosecutor proceed to consider the second limb of the two-part Code test: is prosecution also justified in the public interest? The Code for Crown Prosecutors contains lists of public interest factors supporting or detracting from the case for prosecution, albeit that offense seriousness is always a weighty consideration and will often be dispositive in practice. Crown Prosecutors in England and Wales are under a duty of continuous review to ensure that the Code Test is satisfied in any case proceeding to trial. Since 2004, Crown Prosecutors have also been responsible for framing the initial charges in serious cases – a task formerly allocated to the police, on the traditional theory that common law prosecutions are initiated by private parties. Selection of appropriate charges likewise requires an (earlier) assessment of evidential sufficiency. Charging and evidential review are among the prosecutor’s prime responsibilities in all common law jurisdictions, though detailed doctrinal arrangements and the institutional micro-dynamics of police-prosecutor relation-ships will naturally differ from one legal system to another.

As forensic science and other kinds of technical expertise have become increasingly prominent features of modern criminal investigations, they have also inevitably presented prosecutors with new challenges and opportunities. Some cases are effectively impossible to prosecute without scientific or medical evidence; in others, expert testimony plays a vital supporting rule. Prosecutors must grasp scientific fundamentals in order to make informed assessments of evidential sufficiency. For example, if an expert DNA report states that the accused’s DNA profile matches a crime stain partial profile with a random match probability of 1 in 30,000, the prosecutor needs to be able to make sense of this jargon in order to assess the probative value of the DNA evidence, within the evidential context of the case as a whole. Or again: if a medical expert report states that the pattern of a child’s injuries is “consistent with nonaccidental trauma,” does this constitute strong evidence of child abuse? Or does it simply fail to rule out abuse as one among myriad other possibilities that could be “consistent with” the injuries observed? In short, widespread use of scientific evidence in criminal prosecutions demands at least a basic level of scientific literacy from prosecutors (as well as from other relevant criminal justice professionals, including defense lawyers and judges). Ideally, prosecutors should also be equipped to spot the potential for developing lines of scientific inquiry that the police might have initially overlooked or regarded as unnecessary. This implies that prosecutors cultivate reasonable familiarity with the range of scientific specialisms and technologies potentially available to support criminal investigations, and should know how to identify and instruct suitably qualified experts, in consultation with police investigators and trial counsel where appropriate.

There are various lists of forensic practitioners produced by professional associations such as the UK Forensic Science Society, the British Psychological Society, and the Society of Expert Witnesses. National prosecution services like the CPS and state District Attorneys offices in the USA might further assist individual prosecutors by building up local intelligence on the range and quality of forensic scientific assistance available for consultation. These largely invisible, office-based processes have rarely been studied by empirical researchers, but what little we do know tends to suggest that identifying and instructing scientific experts is a rather ad hoc affair, heavily influenced by the motivation and personal experience of individual prosecutors. As well as failing to capitalize on opportunities to develop better evidence in the instant case, failure to share experiences more systematically may allow poorly performing experts to remain in circulation long after their consulting forensic practices should have been closed down. It is difficult to believe that this curiously unscientific approach to drawing on scientific expertise is beyond practical improvement – though it has to be said that developments over the last several decades (including the short life and ignominious demise of the UK Council for the Registration of Forensic Practitioners) provide little grounds for optimism.

The CPS in England and Wales has adopted various topic-specific policies pertaining to scientific evidence. One significant interagency document provides guidance on charging in cases involving DNA evidence (The Prosecution Team 2004), including the important principle that “[a] suspect should not ordinarily be charged solely on the basis of a match between his own profile and a DNA profile found at the scene of the crime” (ibid 11.1). This effectively introduced an informal corroboration requirement for DNA profiles in relation to suspects first identified through a speculative search of the NDNAD. It attempts to forestall the embarrassment experienced in several early DNA cases in which suspects who, viewed objectively, could not possibly have committed the crime in question were nonetheless charged purely on the basis of an adventitiously matching DNA profile. This was an object lesson in the dangerous fallacy of treating DNA evidence as though it supplied infallible proof of guilt and an invaluable reminder that, just occasionally, DNA matches can occur purely by chance (if not through contamination, analytical discrepancy, or interpretational error). CPS charging policy in England and Wales now sensibly proceeds on the assumption that, if the suspect is truly guilty, there should be more evidence linking him to the offense than an unexpected “hit” on the DNA database. Yet unjustifiably late decisions to discontinue prosecutions involving DNA matches evidently still do occur (see, most recently, Dodd and Malik 2012).

CPS training and policy specifically draw Crown Prosecutors’ attention to the potential for utilizing scientific or medical evidence in a number of priority areas. In relation to domestic violence prosecutions, physical evidence of assaults backed-up by medical testimony may be vital in continuing with a prosecution if the complainant later – for whatever reason – withdraws her complaint (Dempsey 2004), by all accounts a frustratingly common occurrence. Scientific evidence can be important in building up the prosecution’s case in rape and sexual assault prosecutions, partly because it can help to deflect the perception that such cases often involve “one person’s word against another’s,” inducing the jury to acquit because it cannot be sure that the accused is guilty, despite jurors’ suspicions that he probably is. At the limit, expert testimony can sometimes effectively constitute compelling evidence of guilt or innocence by itself. However, prevailing scientific opinion can and does change over time, making it risky to base a prosecution primarily on expert evidence, at least where the relevant science is rapidly evolving. This dilemma was presented starkly in England and Wales by a recent series of criminal prosecutions of mothers for murdering their children, based in part on the alleged improbability of multiple innocent cot deaths in the same family.

R v Clark [2003] EWCA Crim 1020 was one notorious case in which the prosecution expert testified that the probability that both of the defendant’s children could have died from Sudden Infant Death Syndrome (SIDS) was 1 in 73 million. This opinion was invalid when given, since it improperly assumed that serial cot deaths are independent events when in fact they could be caused by the same underlying pathology or genetic predisposition (with the further implication that multiple SIDS might run in families). In R v Cannings [2004] 2 Cr App R 7, a case in which three of the accused’s infant children had died, the Court of Appeal observed:

Experts in many fields will acknowledge the possibility that later research may undermine the accepted wisdom of today… . That does not normally provide a basis for rejecting the expert evidence. With unexplained infant deaths, however .. . in many important respects we are still at the frontiers of knowledge… . In cases like the present, if the outcome of the trial depends exclusively or almost exclusively on a serious disagreement between distinguished and reputable experts, it will often be unwise, and therefore unsafe, to proceed. (ibid. 111)

Standing alone, this statement might have been interpreted as giving a strong hint to prosecutors that they should discontinue prosecutions in cases of suspicious infant death whenever there was disputed medical evidence. In R v Kai-Whitewind [2005] 2 Cr App R 3, however, the Court of Appeal said that it would be a “startling proposition” if “whenever there is a conflict between expert witnesses the case for the prosecution must fail unless the conviction is justified by evidence independent of the expert witnesses” (ibid. 480). The Court proceeded to elucidate an essential distinction:

In Cannings there was essentially no evidence beyond the inferences based on coincidence which the experts for the Crown were prepared to draw. Other reputable experts in the same specialist field took a different view about the inferences, if any, which could or should be drawn and hence the need for additional cogent evidence. With additional evidence, the jury would have been in a position to evaluate the respective arguments: without it, in cases like Cannings, they would not. (ibid.)

Coincidence, in other words, cannot prove murder beyond reasonable doubt, if expert opinion is divided on whether the circumstances justify an inference of foul play. But the mere fact of expert disagreement will not block a prosecution supported by other evidence.

The Court of Appeal reiterated in R v Henderson [2010] 2 Cr App R 24 that conflicts in expert evidence are in principle to be resolved by juries just like any other conflict of factual testimony. Henderson concerned three consolidated appeals involving allegedly “shaken babies,” prosecutions which have provoked much controversy in both the UK (R v Harris [2006] 1 Cr App R 5) and the USA (Tuerkheimer 2011). Prosecution evidence in these cases often almost amounts to an uncorroborated medical diagnosis of murder. Doctors have testified repeatedly that babies exhibiting a particular combination of symptoms (known as “the triad”) were definitely injured intentionally, and even sometimes claiming that the temporal pattern of symptoms pinpointed the culprit. However, recent developments in medical research and scientific thinking have cast serious doubt on such diagnoses. In Henderson the Court of Appeal stated unequivocally that “the triad” is not to be treated as conclusively diagnostic of nonaccidental head injury. This is something else that Crown Prosecutors must now take into account in exercising their charging and prosecutorial discretions in cases of alleged child abuse. Current CPS policy states that prosecutions based only on the triad are unlikely to be brought or continued, in the absence of “appropriate supporting evidence (which in certain circumstances can be found in the absence of certain factors)” (CPS 2011).

Sexual assault cases based on physical examinations of the alleged victims have also posed difficulties for prosecutors. In the past, medical experts have sometimes been willing to infer intercourse from physical marks or injuries, e.g., a ruptured hymen or anal tearing. However, a recent report by the Royal College of Paediatrics and Child Health (2008) called into question the extent to which such examinations could warrant positive findings of abuse, prompting the Court of Appeal to quash a conviction of child sexual assault in R v PF [2009] EWCA Crim 1086. Medical evidence led by the prosecution at trial was also criticized for being too dogmatic by the appeal court in R v Martin T [2008] EWCA Crim 3229, but on this occasion the conviction was upheld on the strength of the complainant’s testimony. In these delicate situations, involving disputed allegations of sexual abuse or assault, prosecutors must balance their duty to present as strong a case as possible today, against the possibility that changes in medical opinion might tomorrow undermine the safety of any conviction secured at trial. Recent experiences of dramatic reversals in prevailing medical opinion will predictably make prosecutors more reluctant to pursue cases built upon the shifting sands of scientific knowledge (O’Brian 2011).

These observations and illustrations prompt a more general question: to what extent should prosecutors be responsible for assessing the quality and validity of scientific evidence (as opposed to assessing its probative value on the assumption that scientific evidence is valid)? Of course, if a prosecutor knew or strongly suspected that a particular expert witness was incompetent or a charlatan it would constitute a gross breach of professional responsibility to present that person to the court as a witness of truth. Such faulty evidence could not legitimately contribute towards satisfying the evidential sufficiency test for prosecutions in England and Wales. But this is presumably – to put it no higher – a very unusual scenario. Prosecutors are not themselves trained scientists or forensic experts. When any indications to the contrary are absent, why shouldn’t the prosecutor simply take at face value the validity of scientific evidence produced by ostensibly well-qualified experts? After all, scientific experts are called upon to assist in criminal proceedings precisely because they contribute knowledge and skills otherwise unavailable to the justice system. Expert evidence comprises scientific findings, interpretative judgments, and expressions of opinion which prosecutors are presumptively ill-equipped to second-guess.

Two mutually aggravating considerations belie this superficially plausible rationalization of the legal status quo. First, recent years have witnessed a growing realization that many of the traditional forensic sciences routinely employed in criminal prosecutions and trials actually lack formal, rigorous scientific validation. A report published in 2009 by the highly respected National Research Council of the National Academy of Sciences in the USA concluded that “[i]n a number of forensic science disciplines, forensic science professionals have yet to establish either the validity of their approach or the accuracy of their conclusions, and the courts have been utterly ineffective in addressing this problem” (National Research Council 2009: 1–14). Even the reliability of fingerprinting has been called into question by high-profile controversies and inquiries in the USA (see ibid: 5–7 to 5–14) and the UK (The Fingerprint Inquiry: Scotland 2011). The point is not to induce blanket skepticism about scientific evidence and the thousands of convictions based on it, which would be a counterproductive and unwarranted overreaction to these revelations. A more measured approach demands continued vigilance and scrutiny of scientific evidence’s methodological and epistemic credentials, a task to which prosecutors should arguably contribute through searching pretrial evaluations of evidential sufficiency.

Less initiative might be expected of prosecutors in this regard if the validity of scientific evidence were sure to be thoroughly investigated at trial. But herein lies the second cause for concern. Common law courts have not traditionally imposed exacting admissibility requirements on scientific evidence. To the contrary, the general attitude has been that relevant scientific evidence should be admitted, leaving any (rare) challenges to validity to be resolved as questions of fact for the jury in the course of the trial. In recent decades, many common law jurisdictions have introduced somewhat more rigorous approaches to scientific validity, requiring trial judges to undertake some measure of “gatekeeping” scrutiny, at least in relation to novel techniques or applications. The best known of these tests is the admissibility standard elucidated by the US Supreme Court in Daubert v Merrell Dow 113 S Ct 2786 (1993) and subsequently adopted in many US state procedural codes. The success, or otherwise, of such interventions is still hotly debated. Commentators have noted a tendency (not necessarily restricted to US judges: Beecher-Monas 2007: 94–121) for Daubert to be applied quite stringently to disqualify claimants’ scientific evidence in civil tort suits, while a far more relaxed and indulgent standard is applied to admit scientific evidence adduced by the prosecution in criminal proceedings.

In the meantime, there is no equivalent admissibility test for scientific evidence in England and Wales, albeit that the Law Commission (2011) has proposed legislation to introduce a locally adapted (and rather elaborate) version of the Daubert standard. One might argue that prosecutors’ professional responsibility to inquire into the scientific validity of expert evidence they propose to adduce at trial is all the greater in the knowledge that scrutiny in the courtroom is likely to be minimal. Lay jurors can hardly be relied upon to detect unacknowledged weaknesses in prosecution scientific evidence that lawyers and judges have failed, or disdained, to notice. On the other hand, it is probably unrealistic, in terms of institutional culture and personal psychology, to expect prosecutors in adversarial proceedings to adopt an overly critical attitude towards scientific evidence which, on the face of it, supports their case and would easily satisfy the law’s undemanding admissibility requirements.

Prosecution And Defense

In R v Ward (1993) 96 Cr App R 1, 51, the English Court of Appeal observed:

[A] forensic scientist conjures up the image of a man in a white coat working in a laboratory, approaching his task with cold neutrality, and dedicated only to the pursuit of scientific truth. It is a somber thought that the reality is sometimes different. Forensic scientists may become partisan … Forensic scientists employed by the government may come to see their function as helping the police. They may lose their objectivity.

Consciously partisan scientific evidence is antithetical to the expert witness’s primary duty to the court. The judges in Ward were in no doubt that “the clear duty of government forensic scientists [is] to assist in a neutral and impartial way in criminal investigations. They must act in the cause of justice” (ibid. 52). This most basic of the expert witness’s duties has been reiterated many times (e.g. R v Harris, [2006] 1 Cr App R 5; R v B (T)[2006] 2 Cr App R 3), and is now embodied in rule 33.2 of the Criminal Procedure Rules, which states unequivocally that:

An expert must help the court .. . by giving objective, unbiased opinion on matters within his expertise. This duty overrides any obligation to the person from whom he receives instructions or by whom he is paid.

In the United States, Standard 3–3.3(a) of the ABA Standards for Criminal Justice: the Prosecution Function states to similar effect, but with the emphasis on the prosecutor’s duty:

A prosecutor who engages an expert for an opinion should respect the independence of the expert and should not seek to dictate the formation of the expert’s opinion on the subject. To the extent necessary, the prosecutor should explain to the expert his or her role in the trial as an impartial expert called to aid the fact finders .. ..

Impartiality and objectivity are indubitably laudable ideals for scientific evidence and expert witness testimony. Whether it is entirely realistic to expect the parties to adversarial litigation to interpret their respective roles with a primary emphasis on achieving justice, viewed objectively and impartiality, is another question. The expert witness’s abstract “duty to the court” and to justice provides useful, but very incomplete, guidance for ethical professional practice. Prosecutors and defense lawyers also owe duties to the court, but this hardly settles myriad difficult questions of legal ethics and strategic decision-making in adversarial trial proceedings.

It is accepted as axiomatic in all modern systems of criminal justice that the accused must have a fair opportunity to put his side of the case and adequate time and facilities to mount a defense. Justice is unlikely to be done, and certainly will not be seen to be done, if the accused is muzzled or unreasonably hampered in conducting his defense. However, active defense participation in the proceedings takes on additional functional and symbolic significance in adversarial systems. Adversary theory assumes that the trier of fact will best be able to discern the truth of contested events by hearing each side advance its best case at trial and adjudicating between them (Roberts and Zuckerman 2010: 46–65). There is no overriding duty on the court to discover the truth, as there would be in inquisitorial proceedings. It would consequently undermine not only the normative legitimacy, but also the epistemic efficacy of adversarial criminal trial procedure, if the trier of fact were allowed to hear only the prosecution’s side of the story. In fact, the common law jury is always presented with at least two versions of events in any contested criminal trial, even if the defense “story” is reduced to a blanket denial of the allegations advanced primarily through cross-examination of the prosecution’s witnesses.

Adversarialism has broad ramifications for defense participation in criminal trials. Specifically in relation to scientific evidence, the most obvious implication is that the defense should have appropriate access to high-quality forensic science assistance, to retest and verify the prosecution’s scientific evidence, and to pursue further scientific inquiries of its own (Giannelli 2004; Roberts and Willmore 1993: Chap. 3). This is partly a question of providing adequate resources, which in practice means legal aid funding for work that the uninitiated might regard as wasteful reduplication. Criminal legal aid is virtually always squeezed in times of austerity. In addition, there is the related question of adequate defense access to evidential material, data, test results, and government labs and technicians (to the extent that these public goods survive free-market provision; wherein commercial confidentiality becomes a further potential barrier to access). This brings us to the more general, and perennially controversial, topic of pretrial disclosure by the prosecution to the defense.

Contrary to what simplistic models of adversarial “contests” might lead one to expect, trial by ambush is largely a thing of the past and is deprecated by modern courts and judges. Most common law procedural systems today require extensive pretrial disclosure by the prosecution, and many jurisdictions also demand more circumscribed disclosure by the defense. (The presumption of innocence preempts fully reciprocal pretrial disclosure in criminal litigation.) Moreover, late or inadequate disclosure by the prosecution has been exposed as a potent cause of miscarriages of justice, not least in relation to scientific evidence.

The extent of the prosecution’s common law duty to disclose scientific evidence was clarified in Ward, in which the Court of Appeal stated:

An incident of a defendant’s right to a fair trial is a right to timely disclosure by the prosecution of all material matters which affect the scientific case relied on by the prosecution, that is, whether such matters strengthen or weaken the prosecution case or assist the defence case. This duty exists whether or not a specific request for disclosure of details of scientific evidence is made by the defence. Moreover, this duty is continuous: it applies not only in the pretrial period but also throughout the trial. The materiality of evidence on the scientific side of a case may sometimes be overlooked before a trial. If the significance of the evidence becomes clear during the trial there must be an immediate disclosure to the defence.

In Ward itself, negative test results seemingly contradicting the prosecution’s case were withheld from the prosecutor, and consequently were neither disclosed to the defense nor adduced at trial, an outcome which particularly infuriated the Court of Appeal because the jury had been kept in the dark about potentially significant information:

The consequence is that in a criminal trial involving grave charges three senior government forensic scientists deliberately withheld material experimental data on the ground that it might damage the prosecution case. Moreover [in their testimony at trial, two of them] misled the court as to the state of their knowledge about the possibility of contamination occurring from the debris of an explosion. No doubt they judged that the records of the firing cell tests would forever remain confidential. They were wrong. But the records were only disclosed about 17 years after Miss Ward’s conviction and imprisonment. (ibid. 49)

Pretrial disclosure in England and Wales is now governed by the Criminal Procedure and Investigations Act 1996, as amended, and the Criminal Procedure Rules. Although the principle of full and timely pretrial disclosure by the prosecution is accepted beyond question in English criminal proceedings, disputes can still arise over its precise requirements in individual cases, especially regarding “unused material” not forming part of the prosecution’s affirmative case. Defense lawyers and academic commentators continue to express skepticism about levels of compliance in practice with the prosecution’s disclosure requirements (see Lord Justice Gross 2011), possibly indicating deep cultural resistance to full disclosure in an adversarial system (Quirk 2006).

If it emerges at trial that the prosecution has failed to discharge the full extent of its disclosure duties in relation to scientific evidence, the proper course will normally be for the judge to consider granting an adjournment to allow the defense time to deal with new information. The defense would not necessarily require extra time in every case: it all depends on the precise nature of the evidence and the way in which the trial is being run by both sides. It is conceivable, however, that the evidence would have to be excluded altogether, in cases where the defense position has been irreparably damaged, for example, because the opportunity to conduct further testing on perishable samples has been lost. In the event that material nondisclosure of scientific evidence comes to light after the trial has been concluded, the Court of Appeal may determine that a conviction is no longer “safe,” in the terminology of the Criminal Appeal Act 1968, and must be quashed. Retrial might be a practical option in some, but by no means all, such cases. Viewed in this light, conscientious compliance with the prosecutor’s pretrial disclosure duties is calculated to safeguard the reliability and legitimacy of well-founded convictions of the guilty, at the same time as ensuring the defense has a fair opportunity to present its side of the story to the jury in an adversarial criminal trial.

A “CSI Effect”?

The “CSI Effect” has generated some anxious practitioner and academic discussion, predominantly in the USA (see Cole and Dioso-Villa 2009). The basic idea is that jurors weaned on a concentrated TV diet of the CSI: Crime Scene Investigation franchise, Law & Order, Bones, Silent Witness, Waking the Dead and similar top-rated serials may have come to entertain hopelessly inflated expectations of scientific evidence. The worry is that jurors in thrall to these cultural fantasies might effectively “punish” the prosecution by voting for an acquittal in any case entirely lacking in scientific proof, or in which the more banal reality of expert evidence fails to live up to the glamorous fictional portrayals driving jurors’ expectations. A handful of US prosecutors claims to have experienced the CSI Effect in cases in which the outcome has been, in their eyes, disappointing (e.g. Thomas 2005).

It is a perfectly plausible conjecture that factfinding by lay jurors in criminal trials is influenced by a range of social and cultural factors, including expectations generated by crime fiction (Tyler 2006). But such cultural influences are doubtlessly diffuse and operate in concert with countless other complementary and confounding perceptions and motivations. The empirical basis for positing a genuine CSI Effect is exceedingly thin. Most academic commentators who have addressed the issue are highly skeptical, starting with the rather protean and ill-defined concept of “the CSI Effect” itself. In most of the shows in question police and forensic scientists are portrayed as civic heroes and forensic science itself invariably wins the day and puts the bad guys behind bars where they belong. One might equally posit that jurors immersed in these predictable narratives will be only too ready to accept the truth of the prosecution’s case and, in particular, to defer to scientific proof of guilt whenever it is adduced (Godsey and Alao 2011).

Whatever the empirical truth of the matter, the theoretical possibility of a CSI Effect serves as a useful reminder that scientific evidence, for all its undoubted potency as a weapon in the armory of law enforcement, is neither infallible nor necessarily free from any conceivable drawbacks, downsides, or unanticipated side-effects. Part of the price of our increasing social investment in scientific proof may be eternal vigilance (not least on the part of prosecutors) to ensure that forensic science serves to promote, and not to thwart, the values and ideals of criminal justice.


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