Too Smart For Our Own Good

Mapping DNA was one of the smartest things scientists have ever done. It has been a gift to the criminal justice system, freeing the wrongly accused and convicting perpetrators even when it's a cold case many years old.

Eye witness evidence is notoriously unreliable; no two witnesses will ever describe the same event in precisely the same way - and indeed, if they did it would be evidence not of what actually happened but of collusion.

Circumstantial evidence, despite the 'it's only circumstantial evidence' comment sometimes being made, is much more solid. Even so, it has to be part of a chain of evidence to be convincing.

DNA evidence, though, like fingerprints, is hard evidence. It can stand on its own. Even the most optimistic and persuasive defence agent is unlikely to get anywhere with a jury if he challenges it. If a man's DNA is found at a crime scene, then he was there too.

Or was he? When DNA evidence was first used, obtaining it was the big problem. You needed a substantial sample before you could get any sort of result. Then the technique got cleverer still; DNA from even the smallest fragment could be analysed, even if it was just a few cells.

Recently there was a notorious case where an individual's DNA was found on the hand of a murder victim and he was arrested. It was only after he'd been in prison for some time awaiting trial that it was established that he'd a cast iron alibi; on the night in question he was in a hospital bed in an alcoholic stupor. Eventually they discovered that the paramedics who had treated him had then rushed to the aid of the murder victim, transferring cells of the alcoholic's DNA as they did so.

And there's another problem too. Belief in the infallibility of DNA evidence led to the arrest of a man whose DNA was found on the till in a coffee shop that had been robbed – despite the fact that the CCTV footage showed someone of a completely different height and build.

I was talking a while ago to Senior Investigating Officer who said that we had become so clever at picking up smaller and smaller samples of DNA, that the evidence from a crime scene had started to be a bewildering mass of tiny mixed-up traces of evidence, obscuring rather than illuminating what had happened. We are, indeed, becoming too smart for our own good.

In writing a modern crime novel, you have to retain at least the impression of realism, but somehow using DNA to solve the murder has always seemed to me a cop out. However, thinking about it now I know this, I can see it might prove to be a very useful red herring.