The “wisdom of crowds” has become a mantra of the Internet age. Need to choose a new vacuum cleaner? Check out the reviews on Amazon. Is that restaurant any good? See what Yelp has to say. But a new study suggests that such online scores don’t always reveal the best choice. A massive controlled experiment of Web users finds that such ratings are highly susceptible to irrational “herd behavior”—and that the herd can be manipulated.