<P> Gladwell also includes two chapters of case studies, situations in which tipping point concepts were used in specific situations . These situations include the athletic shoe company Airwalk, the diffusion model, how rumors are spread, decreasing the spread of syphilis in Baltimore, teen suicide in Micronesia, and teen smoking in the United States . </P> <P> Gladwell and his co-author, John Decker, both received an estimated US $1--1.5 million advance for The Tipping Point, which sold 1.7 million copies by 2006 . In the wake of the book's success, Gladwell was able to earn as much as $40,000 per lecture . Sales increased again in 2006 after the release of Gladwell's next book, Blink . </P> <P> Some of Gladwell's analysis as to why the phenomenon of the "tipping point" occurs (particularly in relation to his idea of the "law of the few") and its unpredictable elements is based on the 1967 small - world experiment by social psychologist Stanley Milgram . Milgram distributed letters to 160 students in Nebraska, with instructions that they be sent to a stockbroker in Boston (not personally known to them) by passing the letters to anyone else that they believed to be socially closer to the target . The study found that it took an average of six links to deliver each letter . Of particular interest to Gladwell was the finding that just three friends of the stockbroker provided the final link for half of the letters that arrived successfully . This gave rise to Gladwell's theory that certain types of people are key to the dissemination of information . </P> <P> In 2003, Duncan Watts, a network theory physicist at Columbia University, repeated the Milgram study by using a web site to recruit 61,000 people to send messages to 18 targets worldwide . He successfully reproduced Milgram's results (the average length of the chain was approximately six links). However, when he examined the pathways taken, he found that "hubs" (highly connected people) were not crucial . Only 5% of the e-mail messages had passed through one of the hubs . This casts doubt on Gladwell's assertion that specific types of people are responsible for bringing about large levels of change . </P>

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