Why Extending the DNA Database is Dangerous

Part of the problem with extending the DNA database is that doing so increases the likelihood of this happening:

After a seven-day trial, Jama had been convicted of raping a 40-year-old woman in the toilets at a suburban nightclub.

The only evidence linking him to the crime was a DNA sample taken from the woman's rape kit.

...

Jama had steadfastly denied the charge of rape and said he had never been to that nightclub, not on that cold Melbourne night, not ever. He repeatedly stated he was with his critically ill father on the other side of Melbourne, reading him passages from the Koran.

But the judge and the jury did not buy his alibi, despite supporting evidence from his father, brother and friend. Instead, they believed the forensic scientist who testified there was a one in 800 billion chance that the DNA belonged to someone other than the accused man.

This week Jama gave the lie to that absurdly remote statistic. After prosecutors admitted human error in the DNA testing on which the case against Jama was built, his conviction was overturned.

Prosecutors said they could not rule out contamination of the DNA sample after it emerged the same forensic medical officer who used the rape kit had taken an earlier sample from Jama in an unrelated matter. They admitted a "serious miscarriage of justice".

DNA is an important forensic tool - when used properly. But it is not foolproof, not least because contamination can lead to false positives.

The more DNA profiles that are stored on a database, the more likely there will be a match found due to such false positives. And such is the belief in the infallibility of DNA testing - thanks to the impressive-sound "one in 800 billion chance that the DNA belonged to someone other than the accused man" - that it is likely to lead to more *innocent* people being convicted. The best solution is to keep the DNA database small, tight and useful.

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