This comes up frequently. The common example given is the “partial match” on a licence plate allowing the police to go investigate all the drivers in the partial match set.

DNA and car licence plates are very different. DNA is hugely personal and descriptive of you. As I’ve noted, in time, technology will be able to map from DNA (and even partial DNA) to an excellent physical description, a drawing better than a typical police sketch, and probably a good medical evaluation too. It’s our software, but we can’t help shedding it. It’s a special case that we must seriously consider extraordinary rules about how public it will be. Normally we say “If you go out in public, give up some privacy” but DNA is so detailed that it’s more akin to a diary you can’t avoid dropping copies of everywhere you walk. That we shed DNA is a fact of reality; our rules will take the unusual step of trying to get us back to where we were when we couldn’t read it.

Because of this there are rules about how and when they can take your DNA (usually a conviction, though in some places just an arrest) and how they can store and use it. For example, U.S. federal law does not allow the FBI to do this sort of partial matching of relatives, but some state laws do, which is why these cases arose.

It’s important because it means you’re in the database even if you fought not to be in it. Your own cells can be used to betray you in spite of all your efforts. You can become under surveillance in spite of trying to keep private, because your brother or cousin didn’t. This is the principle being protected.

And no, the privacy rules don’t just stop the police from “misusing” their powers on the innocent. In many cases, any use of those powers on the innocent (or rather those for whom there is no probable cause) is considered misuse. In fact, use of surveillance powers on the guilty without meeting the appropriate standards of probable cause or suspicion is considered abuse — innocence does not play a role. What is true is that we know if the standards are not being met on the guilty, they are also not being met on the innocent.

The key difference in technology/surveillance issues is scalability. Canvassing neighbours requires human legwork. It doesn’t scale. There is an inherent limit on state power. Technological surveillance, including DNA database scanning, scales. One human can check lots of samples against millions of database entries just as easily as checking against one database entry. When new forms of surveillance scale like this, it often results in a shifting of the balance between state power and the freedom of the individual, and we must examine this shift and decide if it should be corrected for.

It is not hard to shift the balance in a variety of ways, which would give the police the ability to catch more criminals at some cost in civil rights. That’s an ancient debate. We could have everybody wear a GPS transponder (half of us carrying them now voluntarily) and have the police be able to know everybody who was near a crime scene. This would solve a lot of crimes, but is it the sort of power you want to give the police? The type of society you want to live in?