In 2006, David Holtzman decided to do an experiment. Holtzman, a security consultant and former intelligence analyst, was working on a book about privacy, and he wanted to see how much he could find out about himself from sources available to any tenacious stalker. So he did background checks. He pulled his credit file. He looked at Amazon.com transactions and his credit-card and telephone bills. He got his DNA analyzed and kept a log of all the people he called and e-mailed, along with the Web sites he visited.