website for the Lincoln Institute for Land Policy. Housing bubbles may occur in local or global real estate markets. In their late stages, they are typically characterized by rapid increases in the valuations of real property until unsustainable levels are reached relative to incomes, price-to-rent ratios, and other economic indicators of affordability. This may be followed by decreases in home prices that result in many owners finding themselves in a position of negative equity—a mortgage debt higher than the value of the property. The underlying causes of the housing bubble are complex. Factors include tax policy (exemption of housing from capital gains), historically low interest rates, lax lending standards, failure of regulators to intervene, and speculative fever.<ref>Tax Break May Have Helped Cause Housing Bubble, The New York Times, December 18, 2008</ref> This bubble may be related to the stock market or dot-com bubble of the 1990s. This bubble roughly coincides with the real-estate bubbles of the United Kingdom, Hong Kong, Spain, Poland, Hungary and South Korea. While bubbles may be identifiable in progress, bubbles can be definitively measured only in hindsight after a market correction, which began in 2005–2006 for the U.S. housing market. Former U.S. Federal Reserve Board Chairman Alan Greenspan said "We had a bubble in housing", and also said in the wake of the subprime mortgage and credit crisis in 2007, "I really didn't get it until very late in 2005 and 2006." In 2001, Alan Greenspan dropped interest rates to a low 1% in order to jump the economy after the ".com" bubble. It was then bankers and other Wall Street firms started borrowing money due to its inexpensiveness. The mortgage and credit crisis was caused by the inability of a large number of home owners to pay their mortgages as their low introductory-rate mortgages reverted to regular interest rates. Freddie Mac CEO Richard Syron concluded, "We had a bubble", and concurred with Yale economist Robert Shiller's warning that home prices appear overvalued and that the correction could last years, with trillions of dollars of home value being lost. Greenspan warned of "large double digit declines" in home values "larger than most people expect". Problems for home owners with good credit surfaced in mid-2007, causing the United States' largest mortgage lender, Countrywide Financial, to warn that a recovery in the housing sector was not expected to occur at least until 2009 because home prices were falling