Splitting Hairs: Aytugrul v The Queen

At first glance, amidst contemporary popular suspicion of the law’s onerous safeguards in favour of the accused, it may appear surreal, quixotic or even wilfully provocative to split hairs over the presentation of DNA evidence in criminal trials — particularly in cases, as below, in which all parties accept the expression of that evidence as mathematically correct and, in its own right, highly persuasive. Yet the clash of established behavioural science with uniform Australian evidence legislation and traditional expectations of jurors gives rise to questions which strike at the heart of the values of criminal law. In Aytugrul v The Queen (2012) 247 CLR 170, the High Court declined the appellant’s invitation to delve into psychological research on the effects of the ‘exclusion percentage’ on a juror’s subconscious mind. Dubious from the outset about the distinction between two alternative expressions of the same mathematical value, the Court found that the ‘exclusion percentage’ was not unfairly prejudicial as it was mitigated by prosecutorial and judicial explanation. Above all, the ‘exclusion percentage’ was salvaged by its presentation alongside the less problematic ‘ frequency ratio’. However, even if the exclusion percentage adds no further probative value beyond the ‘ frequency ratio’, the High Court preferred to analyse its merit on its own terms, in isolation. This case note draws on Heydon J’s discussion of a contrasting American decision, wide-ranging empirical studies and the policy of the law of evidence to argue that DNA evidence communicated as an ‘exclusion percentage’ should be excluded from criminal trials.