[gwern]

'Hard cases make bad law', as the saying go. We know they won't lead to a massive crime wave of false convictions because DNA databases generally aren't abused like that, and the effects on crime are enormous: "The effects of DNA databases on the deterrence and detection of offenders" http://jenniferdoleac.com/wp-content/uploads/2015/03/DNA_Den... , Anker et al 2017.

[ubernostrum]

Part of the problem is that as the size of the database grows -- i.e., as every police encounter turns into a mandatory sample collection -- the false-positive rate will also go up.

Remember: DNA "matches" are probabilistic, and most of their utility has been based on the low probability of a sample taken from a crime scene "matching" a sample taken from a suspect. Prosecutors (committing the prosecutor's fallacy, of course) love to tout the "one in a billion" type statistics for a match being a false positive, but it doesn't take a very large database at all for "one in a billion" coincidences to start occurring.

[jacquesm]

These laws make hard cases easy cases. The only reason it didn't work here is because there were some very exceptional circumstances. Even the suspect wasn't sure he had not done it and the combination of DNA evidence + plea bargaining skews terribly against suspects, even innocent ones.

DNA evidence is like a magic bullet for crime, DNA found -> guilty is the norm, not the exception.

As far as the statistics are concerned: we do not actually know the false positive rate.

[bryanrasmussen]

DNA databases, that haven't been around that long and haven't been especially useful because of not having a lot of data in them in comparison to all the possible DNA data, generally aren't abused in the way they would be if they had all the DNA data and were a common tool.