As a matter of implementation, I wonder if this is in fact true at the moment. Of course in the future there's no reason that national biometrics databases cou

As a matter of implementation, I wonder if this is in fact true at the moment. Of course in the future there's no reason that national biometrics databases couldn't store unique identifiers that could be used to identify individuals (given the biometrics for an individual, you would be able to look up that individual in your database). However my very patchy understanding of systems as currently implemented is that they provide a way of tying a document reliably to an individual - so that you can present a passport and an automated kiosk can scan your irises/take fingerprints to work out if the document actually belongs to you. This is what's called a "closed search" problem. If you can generate the document in the first place then this doesn't really change the situation from before. Of course systems may get more sophisticated but I suspect the problems with collisions of biometric checksums are likely to put some sort of a limit on the practicality of an approach like this. Especially when retina scans etc can be defeated by wearing lenses etc.