Sandra Parker, a freelance writer who was hired by a review factory this spring to pump out Amazon reviews for $10 each, said her instructions were simple. “We were not asked to provide a five-star review, but would be asked to turn down an assignment if we could not give one,” said Ms. Parker, whose brief notices for a dozen memoirs are stuffed with superlatives like “a must-read” and “a lifetime’s worth of wisdom.”

Determining the number of fake reviews on the Web is difficult. But it is enough of a problem to attract a team of Cornell researchers, who recently published a paper about creating a computer algorithm for detecting fake reviewers. They were instantly approached by a dozen companies, including Amazon, Hilton, TripAdvisor and several specialist travel sites, all of which have a strong interest in limiting the spread of bogus reviews.

“The whole system falls apart if made-up reviews are given the same weight as honest ones,” said one of the researchers, Myle Ott. Among those seeking out Mr. Ott, a 22-year-old Ph.D. candidate in computer science, after the study was published was Google, which asked for his résumé, he said.

Linchi Kwok, an assistant professor at Syracuse University who is researching social media and the hospitality industry, explained that as Internet shopping has become more “social,” with customer reviews an essential part of the sales pitch, marketers are realizing they must watch over those opinions as much as they manage any other marketing campaign.

“Everyone’s trying to do something to make themselves look better,” he said. “Some of them, if they cannot generate authentic reviews, may hire somebody to do it.”