Police can keep DNA samples of innocent people

As noted on Samizdata, White Rose and the Spy.org.uk blog, and reported in The Telegraph, it has recently been ruled that the police can keep the DNA samples of those who are arrested and charged and who either had the charges dropped or were cleared in a subsequent trial. This upholds a change in the law introduced in 2001 to allow the police to retain DNA samples for use in "crime prevention or investigation".

Thus anyone who is arrested and charged can have their DNA samples taken and stored indefinitely, regardless of whether they're susbequently cleared or not thus entirely innocent people will have their DNA samples stored indefinitely by the police.

According to the Telegraph, Lord Brown said the only logical reason for objecting to samples being kept by the police was that it would make it easier for authorities to arrest someone if they ever offended in future.

This is quite simply false:

• DNA samples contain potentially very sensitive medical information such as whether someone has a predisposition to certain diseases or may indeed suffer from a genetically caused disease. The storing of such samples by the police opens up opportunities for abuse of this information by corrupt/unscrupulous officers. By retaining the DNA of the innocent, you expose them this risk.
  
• Retaining DNA samples will also open up opportunities to frame people by planting their DNA at the scene of a crime, knowing that their DNA is on file and likely to be checked. This possibility is not even restricted to unscrupulous officers. Anyone who knows that someone was arrested and charged for a crime will know that their DNA is likely to be on file as a result. Again another risk that the innocent accused would be exposed to.
  
• DNA matching is not a fool-proof process. Problems such as contamination of samples, or erroneously labelling of samples can lead to false matches being reported. Those whose DNA has been stored will be subjected to these risks and may find themselves in the dock again as result.
• As spy.org.uk points out, the retention of the DNA samples may even affect one's ability to get a job. Should an employer ask for an Enhanced Disclosure from the Criminal Records Bureau, they will be told that the police have some sort of record on you (though not the details). To assume that some employers won't hold this against you would be naive.
• The fact that DNA samples can be retained indefinitely also gives the police the incentive to arrest and charged people as a means of expanding the DNA database, thus offering an incentive to arrest and charge people that has nothing to do with whether they're guilty of a crime or not.
  

The judge also argued that the DNA database should be expanded. According to the Telegraph:

In 2001, the law was changed to allow the police to keep a database of samples taken from suspects, though the retained samples may be used only in crime prevention or investigation.

Lord Brown said the benefits of this procedure were so manifest and the objections so threadbare that the cause of human rights would be better served by expanding the police database rather than by reducing it.

"The larger the database, the less call there will be to round up the usual suspects," he said. "Indeed, those amongst the usual suspects who are innocent will at once be exonerated."

But is this true? The larger the database the greater the probability of the following:

* human error leading to a false match leading to an innocent person in the dock

* a false match occuring due to limitations of the technology used in DNA profiling (there's a non-zero probability of a false match regardless of human error).

* that planting DNA at the scene of a crime will result in someone else being put in the frame.

* that someone gets put in the frame because they innocently left their DNA at the scene of a crime.

* that criminals will know that leaving DNA around is risky and will adapt their behaviour.

All of these will reduce the value of DNA evidence. Indeed suppose we were to create a DNA database of the entire population (ultimately taking samples at birth). Then a criminal could be certain of wasting police time by dropping randomly collected DNA at the scenes of his crimes, and taking steps to ensure he does not leave his own DNA there (will the police consider that none of the DNA samples they find at the scene of a crime come from the perpetrator?) .