Steven Levitt, one of the authors of the best-selling "Freakonomics" book, has set his sights on a number of cheaters, including Chicago public-school teachers and real-estate agents. In a new study published by Levitt, of the University of Chicago, and Ming-Jen Lin of the National Taiwan University, the economist was called in by a professor of a "top American university" after two students came forward to complain about cheating.