The Economist magazine stated, "The worldwide rise in house prices is the biggest bubble in history",  so any explanation needs to consider its global causes as well as those specific to the United States. The then Federal Reserve Board Chairman Alan Greenspan said in mid-2005 that "at a minimum, there's a little 'froth' (in the . housing market) ... it's hard not to see that there are a lot of local bubbles"; Greenspan admitted in 2007 that froth "was a euphemism for a bubble".  In early 2006, President Bush said of the . housing boom: "If houses get too expensive, people will stop buying them... Economies should cycle".