The human face is a remarkable piece of work. The astonishing variety of facial features helps people recognize each other and is important to the formation of complex societies. So is the face's ability to send emotional signals, whether through an unconscious reddening of face or a false smile. People spend much of their waking lives, in the office and the courtroom as well as the bar and the bedroom, reading faces, for signs of attraction, trust and cheat. They also spend plenty of time trying todissimulate.
Technology is rapidly catching up with the human ability to read faces. In America, facial recognition is used by churches to track prayers' attendance; in Britain, by shopkeepers to spot past thieves. This year Welsh police used it to arrest a suspect outside a football game. In China, it verifies the identities of ride-hailing (网约车) drivers, permits tourists to enter attractions and lets people pay for things with a smile. Apple's new iPhone is expected to use it to unlock the home screen.
Compared with human skills, such applications might expand steadily in scale. Some breakthroughs, such as flight or the Internet, obviously transform human abilities; facial recognition seems merely to encodethem. Although faces are peculiar to individuals, they are also public, so technology does not, at first sight, intrude on something that is private. And yet the ability to record, store and analyse images of faces cheaply, quickly and on a vast scale promises one day to bring about major changes to our understanding of privacy, fairness and trust.
Start with privacy. One big difference between faces and other biometric (计量生物学的) data, such as fingerprints, is that they work at a distance. Anyone with a phone can take a picture for facial-recognition programs to use. FindFace, an app in Russia, compares snaps of strangers with pictures on VKontakte, a social network, and can identify people with a 70% accuracy rate. Even if private firms are unable to join the dots between images and identity, the state often can. Photographs of half of America's adult population are stored in databases that can be used by the FBI. Law-enforcement agencies now have a powerful weapon in their ability to track criminals, but at enormous potential cost to citizens' privacy.
