by Bob Couttie
Cliff Richard famously sang “We’ve seen it in the movies, now let see if it’s true.” We see DNA evidence in the movies, and on TV CSI-type series and it always gets their man. Right? Wrong.
A dead woman is found with a man’s DNA on her, including her fingernails. As it happens, a search on the police database turns up a matching DNA sample from a burglary. The police have him bang to rights. Wrong.
The man’s name was David Butler. He was a taxi driver who suffered from an unusual skin condition which meant that he shed more flakes of dead skin than normal. The victim wore a glitter nail polish that attracted DNA. She had ridden in his taxi.
Despite firm evidence that Butler had been nowhere near the scene of the crime, and warnings from forensic scientists who carried out the test that the DNA was of poor quality, he spent eight months in prison on remand awaiting trial based on the false DNA evidence. He has now been released.
Earlier this year 19 year old Adam Scott, from Truro, Cornwall, was arrested for a rape in Manchester, in the British Midlands. Scott had never been to Manchester. He had a DNA sample taken for an unrelated case. That sample, and an evidence sample from the Manchester rape case, were both being examined in the same laboratory and had inadvertently become mixed.
In 2008 German police were looking for ‘a woman without a face’ who had carried out six murders and numerous thefts over a 15 year period. She had covered much of southern Germany and had crossed borders into France and Austria. The mysterious woman seemed to have had a number of accomplices during the crimes, and eye-witness accounts led to suggestions that the ‘she’ might have been a ‘he’. After a $14 million investigation it was discovered that she did not exist. The female DNA came from an employee at the packaging plant where the original swabs used for gathering the DNA evidence had been sourced.
Even without contamination and poor laboratory practice the chances of false matches are rising. Many DNA databases only record ten DNA markers but as the number of entries on the database rises the chances of a false match increase.
In 2004 Professor Sir Alec Jeffreys, of Leicester University, whose work pioneered DNA fingerprinting, warned that ten markers was not enough and recommended increasing it to 15 or 16 markers. The ten marker standard was not foolproof, he said.
The Guardian newspaper quotes the then-head of the now defunct Forensic science service:
“…before a decision to prosecute can be given, all of the evidence in the case must be carefully considered. DNA is never reported in isolation.”
David Butler and Adam Scott respectfully disagree.
Ultimately, DNA is, literally, a numbers game. It is a matter of statistics. Most jurors, indeed most of us don’t really understand statistics. The birthday problem is a good example: how many people need to be in a room to ensure that there is a 50 per cent chance of two sharing the same birthday? The number is 23.
So here’s a scenario, familiar to any baseball fan: It’s a major league game, the stands are full. You don’t need 365 players on the field to be sure that two share the same birthday, there’s a 50 per cent chance that on each team of 25 active players, two players share the same birthday, and a far higher chance of two players on both teams sharing the same birthday.
Now let’s look at the stands, at the people chomping on their hotdogs. With a chance of a DNA match being around one in a billion, how many people in the stadium are likely to share the same DNA? Instinct suggests there would be few matches, but statistics show that at least two people will share the same DNA profile.
CSI-type crime shows make no attempt to reveal the variables that are involved in obtaining a valid DNA match.
More from Bob Couttie:
The Day I Said No to the French Connection
My First Murder – The Blue Anchor Scandal
Dispatch From Cambodia: Murder In A Sleepy Town
U.S. Navy Cold Case : A Sister’s Persistence Restores the Honor of Murdered Ensign Andrew Lee Muns
The Moors Murderers: Myra Hindley and Ian Brady