DPP v Wilson: DNA evidence on its own is sufficient, in principle, to prove guilt to the criminal standard

Here, The Supreme Court held that:

Gardaí did not breach Wilson’s constitutional right to privacy by collecting his discarded cigarette butts to extract a DNA sample.

“There is no reason in principle why a jury may not be satisfied to the criminal standard of the identity of the perpetrator of a crime where the only evidence of such identity derives from DNA profiling” [9].

The trial judge was not obliged to warn the jury of  a danger in convicting an accused based solely on DNA evidence.

Background

In 2010, Wilson, the appellant, shot and killed Daniel Gaynor on a public road in Dublin. As he ran away from the scene, Wilson discarded a revolver, a cotton glove, a peak cap and a hoodie. On forensic analysis, firearm residue was found on the glove, the sleeve of the hoodie and on the cap. And a DNA sample was also recovered. Gardai suspected Wilson, based on confidential information, and arrested him for questioning. Wilson refused to provide a DNA sample voluntarily. The detectives in charge of the investigation swept any area of the yard, photographed it and allowed Wilson outside to smoke a number of cigarettes. Wilson discarded the cigarette butts on the yard.  The detectives then collected those cigarette butts for DNA analysis. Wilson’s DNA from the cigarette butts matched the sample from the clothing recovered from the scene of the murder.

 

A jury convicted Wilson of murder based solely on the DNA evidence. The Court of Criminal Appeal upheld that conviction but certified a question for appeal to the Supreme Court:

Is evidence of DNA samples taken from cigarette butts used and discarded by the detained person whilst in custody admissible evidence at his trial?

During case management, the Court granted leave on two additional questions:

When the sole evidence against an accused person is DNA evidence, is such sufficient to convict an accused or upon the prosecution case being closed, should a judge withdraw a case from the jury upon an application of the defence that there was no case to answer?

Should it be mandatory for a trial judge to warn a jury of the dangers of convicting an accused in circumstances where the sole evidence against the accused is DNA evidence alone?

 

Supreme Court

A three judge panel heard the case and issued a joint judgment.

 

Admissibility

The Court acknowledged that any accused in custody is in a vulnerable position and dependent on the Gardaí for the provision of facilities. “It is partly for that reason that the courts will give extra scrutiny to events occurring during detention, to ensure that the rights of a vulnerable person are not breached” [4.30] . But that does not cause the substantive content of a person’s rights in custody to rise above the rights of a person at liberty [4.31].

Finding that the Gardaí could collect the cigarette butts, the Court stated:

4.35 Where the detained person indicated an intention to resist the taking of the sample, and the Gardaí had an alternative source lawfully in their hands, it would not accord with principle to elevate the privacy rights of the person in custody – the whole purpose of which is investigatory – beyond those of either a person who complied with the statutory regime or a person at liberty. Since it is accepted that the latter would have no cause for complaint if his cigarette butts were picked up in a public place, or in an authorised search of his premises, it is impossible to hold that the rights of the detained person are breached by the same procedure in respect of things that he discards. Equally, it would clearly be contrary to public policy to hold that the Gardaí were in the circumstances constrained to use force, thereby risking injury to both the suspect and themselves, and that a failure to use force rendered the picking up of the discarded items unlawful.

 

Probity of the evidence

Dr O’Sullivan, the expert witness, stated that, without testing every human alive, it is not possible to rule out that any two humans could share the same DNA profile. But the likelihood of any two people having a DNA match is one in a thousand billion. That reduces to one in 27,400 in the case of siblings.

 

Prosecutor’s fallacy

The prosecutor’s fallacy is the confusion of two different concepts: the likelihood that two randomly chosen individuals could share a set of genetic characteristics and the likelihood that any other person shares the genetic characteristics as an accused person.

To demonstrate the issues this raises, the judgment gives an example involving a crime committed by a male with a genetic characteristic shared by one in every one thousand people. That would mean that there would only be approximately 400 males in the Dublin area who share that genetic characteristic. But a suspect with that characteristic would not be any more likely to be guilty than the other 399. And guilt beyond a reasonable doubt could not be sustained on evidence that an accused held that genetic characteristic.

In this case, though, the trial court was dealing with much larger figures than one in one thousand. And the Court compared DNA evidence to fingerprint evidence, on which a conviction can be solely based, but on which no statistical evidence as to reliability compared to DNA evidence is considered [5.48]. The Court also considered comparisons to witness testimony which can be fallible, or even false, and for which no consideration as to statistical reliability is given [5.52].

Advising that a jury should consider the manner of the collection and analysis of DNA evidence, the Court stated:

5.59 We would wish to emphasise, therefore, that there are a whole range of factors which need to be assessed before determining whether a so-called “cold hit” DNA case, where there is no other evidence of identity beyond the DNA profiling evidence, can properly provide sufficient evidence to prove identity to the criminal standard. However, where the other elements of the equation are robust, it does not seem to us that the fact that there may be a highly theoretical and tiny mathematical possibility of a false positive can, in and of itself, require that the case be withdrawn from the jury. To take that view would be to suggest that we should be happy to take remote risks with evidence which is not capable of detailed statistical analysis but not happy to take potentially even more remote risks with evidence which may, in fact, be a lot more probative but is capable of statistical analysis to demonstrate an extremely small and very remote possibility of a false positive.

 

Direction to Jury

While providing general guidelines on directions, the Court rejected Wilson’s claim that the trial judge should have warned the jury about convicting solely on the DNA evidence. The Court held:

For the reasons set out earlier, we do not consider that this ground is well made out for we do not consider, having regard to the general principles by reference to which it has been determined that warnings are required in other cases, that a warning is required in a case where the only evidence of identity is confined to DNA profiling [9.4].

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2 Comments

  1. I’m curious about one thing: why did the Supreme Court mention that Wilson was initially found on the basis of ‘confidential’ information? Assuming this information was not admitted at the trial (and was, presumably, inadmissible), what was its relevance to the issues on appeal? To be explicit, I worry that this information would prejudice the court, by leading the judges to think that the defendant’s guilt was supported by non-DNA evidence, which would (perhaps) make the court more reluctant to rule that he ought to be freed based on the method of gathering his DNA or the inadequacy of DNA alone to support his guilt.

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  2. Hi Jeremy,

    Thanks for your comment/question.

    I suspect that the Court mentioned that Wilson was identified by a confidential source as a box ticking exercise: to demonstrate that the Gardai had a reasonable suspicion and acted legally when they arrested him. If Wilson could have demonstrated that his arrest and detention was not legal and in breach of his constitutional rights, he may have had another ground to argue that the DNA evidence gathered during his detention was inadmissible. And one which may have had a higher chance of succeeding.

    On the issue of prejudice, this appeal to the Supreme Court was not primarily concerned with Wilson’s guilt or innocence (in theory). He had already exercised his right of appeal in the Court of Appeal. The Supreme Court was answering general questions on the gathering, admissibility and probity of DNA evidence to guide the lower courts.

    Killian

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