WITH the construction of the railways in the 19th century, a new sociological phenomenon was born: the travelling criminal. Until then, police had relied on local communities to recognise a bad apple in their midst, but now the felons were on the move, wreaking havoc in communities which had no knowledge of their past and hence no reason to be wary. For law enforcers trying to contain the problem by sharing descriptions of known recidivists, it became imperative to answer one question: what is it that identifies someone as a particular person?

This article first appeared in the Economist on 17 December 2009. To continue reading click here.