The first wave of the use of probability theory in jurisprudence was in the 17th and 18th centuries. The probability of a jury reaching the correct verdict given evidence XYZ was greatly discussed. What counted as good evidence in a court changed during this period and our present system is pretty much still based upon it. Evidence that a defendant owned the same car that was identified at the scene of a crime falls within this tradition (even if some numbers are added in the form of ‘there are only X number of cars in the country, thus the probability of it being the defendant’s is Y‘). Evidence that DNA evidence matches a defendant does not always fall within this tradition because it is presented by scientists and expert witnesses. In most cases this should not be a problem, indeed it should improve the system, but in some cases judges and juries can be misled by bad science and bias expert witnesses. For sure we have had the standards of DNA evidence kept reasonable high by decisions such as that made by Mr Justice Weir when he rejecting low-copy number (LCN) DNA tests in the Omagh judgement of 2007. It is nonetheless possible that a scientists might be able convince a judge and jury that it was possible to obtain the “ideal conditions” that could produce probabilities of one in 3 trillion. In practice these probabilities are impossible, the taking of DNA evidence being as much an art as an exact science.

I am inclined to agree with the Massachusetts Bar Association that:

DNA evidence is one of the most powerful evidentiary tools to become available in the history of criminal litigation. The evidence demands the highest degree of scrutiny that can be brought to bear and that includes a rigorous review of the highly complex procedures used to create it in each particular case. Anything less creates the risk that our criminal justice system will be converted to one driven by scientific test results, rather than legal principles and common sense.

Another problem is, as with the Bolam test, bad legal principles can be made when the courts listen to expert witnesses and forget about common sense. This is why we must be extremely circumspect when adopting a powerful new scientific method of collecting and storing evidence. The technical and ethical problems that are raised by the storage of DNA data and all the other data on us is yet another problem. I am please to hand that over to Ross Anderson and his colleagues.

I enjoy your blogs – well done.

And as you point out, there is nothing especially *dangerous* that can be done with someone’s DNA, it’s just another piece of information that is held about you, worrying only becausse it makes you realise just how much information certain organisations hold about you.