Bitcoin has been labelled a speculative bubble by many including former Fed Chairman Alan Greenspan[ and economist John Quiggin. Nobel Memorial Prize laureate Robert Shiller said that bitcoin "exhibited many of the characteristics of a speculative bubble". Journalist Matthew Boesler in 2013 rejected the speculative bubble label and saw bitcoin's quick rise in price as nothing more than normal economic forces at work. Timothy B. Lee, in a 2013 piece for The Washington Post pointed out that the observed cycles of appreciation and depreciation don't correspond to the definition of speculative bubble. On 14 March 2014, the American business magnate Warren Buffett said, "Stay away from it. It's a mirage, basically." During their time as bitcoin developers, Gavin Andresen and Mike Hearn warned that bubbles may occur.