Privacy is a public Rorschach test: say the word aloud, and you can start any number of passionate discussions. One person worries about governmental abuse of power; another blushes about his drug use and sexual history; a third vents outrage about how corporations collect private data to target their ads or how insurance companies dig through personal medical records to deny coverage to certain people. Some fear a world of pervasive commercialization, in which data are used to sort everyone into one or another “market segment”—the better to cater to people’s deepest desires or to exploit their most frivolous whims. Others fret over state intrusion and social strictures.