“The wheels of online commerce run on positive reviews,” said Bing Liu, a data-mining expert at the University of Illinois, Chicago, whose 2008 research showed that 60 percent of the millions of product reviews on Amazon are five stars and an additional 20 percent are four stars. “But almost no one wants to write five-star reviews, so many of them have to be created.”

Consumer reviews are powerful because, unlike old-style advertising and marketing, they offer the illusion of truth. They purport to be testimonials of real people, even though some are bought and sold just like everything else on the commercial Internet.

Mr. Liu estimates that about one-third of all consumer reviews on the Internet are fake. Yet it is all but impossible to tell when reviews were written by the marketers or retailers (or by the authors themselves under pseudonyms), by customers (who might get a deal from a merchant for giving a good score) or by a hired third-party service.