Aytugrul v The Queen [2012] HCA 15: DNA testing “exclusion percentage”

In its unanimous decision today (18 April 2012) in Aytugrul v The Queen [2012] HCA 15 the High Court of Australia has held that evidence given at the trial of Aytugrul (the appellant) which expressed the results of DNA testing as an “exclusion percentage” was admissible when it was accompanied by an “equivalent frequency ratio” and an explanation of the relationship between the two pieces of data.

Background

The appellant was tried for and convicted of murder in the Supreme Court of New South Wales in a case where among other things the prosecution had relied on the evidence of an expert witness who had conducted a DNA analysis of a hair found on the deceased's thumbnail.

The results of the analysis showed

• that the appellant could have been the owner of the hair, and

• how common the DNA profile found in the hair was in the community.

In relation to point 2 of the results, the expert gave evidence to the effect that one in 1,600 people in the general population would be expected to share the DNA profile that was found in the hair – this known as the "the frequency ratio" and that 99.9% of people would not be expected to have a DNA profile matching that of the hair - this known as "the exclusion percentage".

The appellant appealed to the NSW Court of Criminal Appeal primarily claiming a miscarriage because of “the prejudicial way in which DNA evidence was expressed to the jury". The appeal was dismissed.

The High Court Appeal

The Evidence Act 1995 (NSW) s 137 ("the Act") requires a court in a criminal proceeding to refuse to admit evidence adduced by the prosecutor if its probative value is outweighed by the danger of unfair prejudice to the defendant. While s 135 of the Act relevantly gives the court discretion to refuse to admit evidence if its probative value is substantially outweighed by the danger that the evidence might be unfairly prejudicial to a party or be misleading or confusing.  On appeal to the High Court the appellant alleged that the NSW Court of Criminal Appeal should have held that the trial judge had erred "in admitting statistical evidence expressed in exclusion percentage terms".  The appellant submitted that the effect of s 137 of the Act was to require exclusion of evidence which expressed the results of the DNA testing as an “exclusion percentage”. The appellant further submitted that, if that were not so, the only proper exercise of the general discretion given by s 135 of the Act would have seen the evidence excluded.

The High Court decision

In dismissing the appeal the High Court found the evidence of the “exclusion percentage” was accompanied by both reference to the “relevant frequency ratio” and an explanation of how the exclusion percentage was derived from the frequency ratio. The evidence given was clear.

The court found that although the evidence was adverse to the appellant it was “in no sense unfairly prejudicial, or misleading or confusing”.

The majority of the Court of Appeal was right to conclude that neither ss 137 nor 135 of the Act was engaged. Neither a wrong decision on a question of law or a miscarriage of justice on any other ground existed. 

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