DNA evidence as a reasonable doubt?
An article in the Guardian on 2nd October has highlighted some of the problems surrounding DNA evidence. Since it was first used to secure a conviction in 1986, DNA evidence has developed something of an unshakeable aura about it. It is common to hear in court ‘there is DNA evidence’ as if that somehow proves guilt beyond any doubt, let alone beyond a reasonable doubt.
Whilst science has developed to the extent that in many cases there is no doubt about who the DNA is from, what science cannot yet do is tell us how someone’s DNA found it’s way to a particular place. Some years ago, I defended in a burglary case, where the sole evidence against my client was a cigarette butt with his DNA on it found inside the burgled premises. That was sufficient for him to be charged and the case to proceed to trial. There was no evidence of the circumstances in which his DNA came to be on the cigarette butt or when and how the cigarette had found it’s way into the building. Fortunately, the District Judge trying the case recognised this and he was found not guilty, but both the police and the CPS thought they had the right man.
DNA evidence problems
Scientific developments have brought their own, fresh set of problems. Early DNA profiling was restricted in the profiles it could produce, as it required larger amounts of material for testing. Modern technology can develop a profile from smaller and smaller quantities of cellular material, and can often identify more than one person’s profile within a small sample. This gives rise to problems of cross-contamination, where one person’s DNA finds its way onto a second person, who then deposits the DNA at a different location; or cases where a DNA deposit is left at a location that becomes a crime scene later on.
DNA technology is clearly a vital tool for the police and prosecuting authorities, but there remains a need for caution when a case is substantially dependent on DNA evidence.
John Howey, Senior Solicitor
