Rapists and murderers may be able to challenge their convictions after the Home Office admitted juries may have been misled by subjective DNA evidence.
Draft guidance from the forensics watchdog warned courts have been increasingly relying on ‘qualitative’ assessments of DNA evidence, which could overstate the prosecution case.
Scientists warn reliance on such subjective summaries, rather than raw data, could open the door for a number of appeals.
Until a landmark case last year, DNA evidence was typically deemed inconclusive unless an expert could conclusively and statistically show a sample was from the suspect.
But since then, where such statistic-based approaches have been impossible, criminal courts have allowed ‘subjective expressions of evidential weight’ to be offered to juries. This may mean a science saying he or she could describe the chance of the DNA pointing to the suspect as ‘moderate’ or ‘strong’, rather than using figures.
Peter Gill, who pioneered the use of DNA profiling in Britain, told The Times he had first raised the issue with the Home Office five months ago.
Recognition that subjective interpretations of DNA evidence were potentially unscientific could lead to appeals from those convicted, he warned.
‘Evidence given by a scientist is not the same as scientific evidence,’ said Professor Gill, now of the University of Oslo.
The rise in the use of subjective interpretations of DNA evidence came after a landmark appeal court ruling in the case of a Polish burglar who was convicted in 2011 of a pensioner’s killing.
Kuba Dlugosz, 36, was convicted of manslaughter in a case that hinged on a match with a mixed DNA sample. In such case where the likelihood of a false positive is much higher, making it more likely the wrong person will be convicted.
The court of appeal rejected Dlugosz’s attempt to challenge his conviction, a ruling which, according to the forensics regulator’s draft guidance, has been taken as a ‘broad license’ to allow subjective assessments to carry the same weight as genuine DNA matches.
‘Current application of qualitative methods appears to be largely ad hoc without specifically designed controls,’ the guidance warns.
University College London statistician David Balding told The Times that it seemed ‘from a rational perspective’ there were now grounds for appeal against convictions.
But, he added, ‘The trouble is that the Court of Appeal licensed all this ad hocery in the first place. Nobody in the system seems prepared to directly criticise the court of appeal.’
Crime prevention minister Norman Baker said the new guidance, which is out for consultation, would give no basis for appeals in cases where a conviction was already supported by existing academic literature.
Damien Gayle for MailOnline