The Prosecutor’s fallacy

When the application of DNA Profiling in criminal identification was discovered, Sir Alec Jeffreys termed it as “DNA fingerprint”. However, this term was misleading, for the reason that it equated the power of DNA evidence with the Fingerprint evidence. The public and more specifically the jurors may have taken a wrong impression in respect of DNA evidence before their arrival in the court room.Although the media and press ought to use the term fingerprint more frequently to describe the DNA profiling process, the former differs significantly from our well known fingerprint.

Fingerprint identification has been accepted by the courts world widely for more than a hundred years. On the other hand, DNA profiling is a new process and has generated much debate. Whilst fingerprint relies upon a straightforward lift of prints from surfaces and their subsequent examination – a process with a few steps and very little potential for error, DNA profiling is a complicated and long scientific process, which as it has been evaluated, has a great potential for error.In addition to that, another significant difference is their discrimination power.

Whilst dermal fingerprint is unique for every individual and an expert when presenting it in court will state that he is certain that a mark in the crime scene was made by its originator, the same DNA profile ca be found in more than one individuals and therefore the expert will only give a numerical statement known as match probability.52It has been argued that events that happen outside the court may influence jurors’ judgements53. For example, a juror may read newspaper article, make a relevant discussion or observe an incident which will make him see things differently. This responds to the philosophical question whether it is possible for a juror to completely isolate himself from the influences induced to him by his upbringing, his “social class” and his surroundings, which the leading opinion in the philosophy of law teaches us is impossible. The juror could be affected by the above influences and it is not possible to deny them completely, although he may try to deny them as much as to be accepted by the society as an independent juror.

As a fact, these events will vary unsystematically among jurors and may be responsible for unpredictable differences in judgement that cannot be discovered at voir dire.Generally, even in recent days, the media prefers to use the term of Fingerprint rather than profile when describing DNA evidence. The Independent54 published an article after the multiple trains bombing in Madrid that suggested that the victims could be identified by a process described as “DNA Fingerprint Evidence.” Furthermore, the Guardian55 used the term DNA fingerprinting to describe the identification technique that was used to clarify the identity of Sadam Husein after his capture. Moreover, academic writers have published various books, which include in their title the term “Fingerprint” when referring to DNA profile evidence.It seems that the term “Fingerprint” is quite dangerous and it might have prejudicial effects to the defendant, especially when his conviction is truly based on DNA evidence. Although, Forensic science avoids to use this terminology for DNA evidence, other sciences such us molecular biology use it quite commonly. Given that the term “fingerprint” has been and will be used in the future, for the shake of the defendant, appropriate measures must be taken to distinguish in the minds of the Jurors what it really means.

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The first step for the Jurors is to estimate the probability that the defendant is guilty from the evidence drawn to their attention, before the likelihood ratio of the DNA was given to them. This result would represent the Prior Odds in the Bayes theorem. In the next step they will be given by the expert the likelihood ratio or the match probability of the profiling process. The product of Prior odds times the likelihood ratio will be the Posterior odds showing the defendants guilt after the incorporation of the probability given by the DNA evidence.If there is more evidence that need to be considered, the jury will have to apply the Bayes theorem separately each time. This means that if there were five pieces of evidence which had to be compared with the DNA evidence, then the Bayes theorem would have to be applied twenty times. It is very clear that Bayes theorem is based on a complex mathematical formula.

Zabell argued that generally, human behaviour is not able to estimate the prescriptions of Bayesian model. He also commented that the Bayesian probability model provides certain consistency constraints on probabilities that may help us police our beliefs, but the theory says nothing about where probabilities come from in the first place. Furthermore, studies by Tversky and Kahneman have concluded that when people make assessment of probability they commit serious mathematical errors60.Moreover Mark Kaplan has remarked that the Bayesian conception of subjective probability presumes that our mental architecture contains the equivalent of a black box stocked with subjective probabilities just waiting to be elicited.

 A similar approach took the Court of Appeal in R v Adams (Denis John)61 , where the issue was identification. In this case the prosecution relied entirely on DNA evidence. The prosecution expert gave evidence that the chance of a randomly selected individual to match the crime stain was one in two hundred million. However the victim failed to identify the defendant on an identification parade, and furthermore at the committal of the proceedings she said that the appellant did not look like the man who raped her. On the other side, the appellant had an alibi which was also supported by his girlfriend.

The prosecution expert alleged that the most appropriate method to compare non-DNA evidence with DNA evidence could be made using Bayes Theorem. As mentioned above, by using this method, the Jurors would have to apply the theorem several times, incorporating evidence such us the likelihood ratio that the perpetrator was a local man and the likelihood that the defendants alibi was true.After considering the defence arguments the court allowed the appeal ordering a retrial for several grounds. Firstly, there was a fear that the objectivity which encompasses the numerical figures that are used when applying Bayes theorem, might hide the element of judgement on which the calculation was based.

Secondly, Bayes theorem was deemed to rigid for the jurors given that they had to assess the evidence separately.Thirdly, it was inherited that jurors must evaluate evidence by joint decision using their “common sense” and “experience of the world” and not by the means of an automated formula, since this would deny them the free human will. Fourthly, it was accepted that Jurors would have different figures to be attached to the evidence and therefore they wouldn’t be able to alter their views individually at a later stage if they decided so. The court concluded that the introduction of the Bayes theorem into a criminal trial would plunge the jury into inappropriate and unnecessary realms of theory and complexity which would have a negative effect in their task62.Moreover it has been argued63 that the concept of Prior Odds is incompatible with the presumption of innocence. The presumption of innocence means that a defendant at the beginning of the trial is innocent. This mathematically expresses that the prior odds in the beginning of the trial would be zero. In fact this would render the process useless given that when a number is multiplied with zero equals zero.

Nevertheless, the first prior probability usually for the sake of the process is a small figure close to zero commonly the likelihood ratio of the population that leave around the area of the crime. Even if this figure is not reflective a common response is that over time the evidence will wash out the differences in initial starting points.On the other hand, Jowett (2001) argues that the courts should accept Bayes theorem as the one mathematically sound method of fully understanding the implications of DNA matches. In order to attack the above criticism he suggests that the court should implement the Bayesian networks approach, which despite being based on Bayes theorem, it has the advantage that it bypasses the complex workings of Bayesian probability.Nevertheless, Jowett in continuance parallels Bayesian networks with computers.

Just as people use computers without knowing exactly how they work, he believes that we too can use Bayesian networks. However his suggestion was inconsistent with the view of the court in Adams in respect of Bayes theorem. Using a Bayesian systems approach wouldn’t give the opportunity to jurors to alter their decisions.

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