On 2 November 2011 about 70 students walked out of Economics 10, Harvard University's introductory economics class taught by Gregory Mankiw. The students, who were inspired by the Occupy Wall Street movement that had begun in New York's Zucotti Park two months earlier, published their reasons for the walk-out in an open letter to Mankiw. They wrote that: ‘since the biased nature of Economics 10 contributes to and symbolizes the increasing economic inequality in America, we are walking out of your class today both to protest your inadequate discussion of basic economic theory and to lend our support to a movement that is changing American discourse on economic injustice’. The letter went on to question Mankiw's class for its teaching of conservative economic theories and for an overreliance on his own macroeconomic textbook. Principles of Economics has sold over one million copies since its first publication in 1997; according to the Open Syllabus Project, Mankiw is the most frequently cited author on college economics course syllabi. He is therefore an authoritative source for many economics students, some of whom are beginning to question his teachings and the orthodox economics they underpin.