It’s easy to see why. Because of the vagaries of human traits and the way they vary from day to day, biometrics have to cope with the same person appearing a little different each time they front up. The systems also need to allow for different positioning of the hand (in the case of palm scanning), lighting (face recognition), background noise (voice) and so on, as well as the fact that the sensors get grimy and worn over time. So biometric recognition systems cut you some slack; the more slack, the easier they are to use. Inevitably this means that a biometric system will occasionally confuse one person with another; that is, it will commit a False Positive. So you can see the inherent trade-off in all biometrics, between their ability to discriminate between different people (necessary for security) and their ability to properly recognise all users (convenience).

You can’t have it both ways; a system that is very secure and discriminating will be more inclined to reject a legitimate user, and conversely, a system that never fails to recognise you will also be more likely to confuse you with someone else.

And when we imagine biometrics replacing cash one day, we need to be precise about what we have in mind. Is it thought that you could pay for your groceries by fingerprint, with no other form of ID, and in particular, no card? Well they’re trying this sort of cardless system in Japan with palm scanning ATMs, but it’s easier said than done. As we’ve seen, it is simply inevitable that a biometric ATM will make false positives. And when there is no card, the biometric matching is done against a database of all enrolled customers, hundreds of thousands of them, and the probability of a false positive rises exponentially. Think about it: a false match means the ATM will give you access to someone else’s money – surely the worst case scenario. So in Japan, the ATMs require your Date of Birth and PIN! This despite the hype that biometrics are “unique”! And it will be the same with biometric cash. In fact it will be worse because engineering tradeoffs in cost-effective point of sale equipment (or god help us, phones and tablets being pressed into service as virtual cash registers) means the security/convenience tradeoff is probably unsolvable.

Sadly, the people answering the user acceptance surveys would have got most of their understanding of biometrics from science fiction movies. This tech just doesn’t work like people think it does. With biometrics it is more true than in any other technology: The Devil Is In The Details.

Stephen Wilson, Lockstep Consulting.