FACTBOX:The previous 10 S&P 500 bear markets

NEW YORK (Reuters) - The S&P 500 index .SPX plunged into a bear market on Wednesday -- more than 20 percent below its record high close of 1,565.15 points on October 9, 2007 -- after ceding to the pressure of a housing slump, a credit crisis, record-high oil prices and a weakening economy.

The S&P 500 was officially introduced in 1957 but its value has been extrapolated. Since 1929, whenever the index has fallen into a bear market, it has on average shed 29.4 percent of its value for the duration of the slump, which has averaged just over a year.

The S&P 500's worst bear market occurred in the early years of the Great Depression and stretched from April 10, 1930, to June 1, 1932.

The following is a recap of the previous 10 bear markets for the S&P 500, using "The Stock Traders Almanac 2008" data:

*January 4, 2002 to October 9, 2002

S&P 500 percentage loss: 33.8 percent

Number of days it lasted: 278

What happened? WorldCom and Enron's accounting fraud sparked fears that more firms could be artificially boosting their bottom lines.

*March 24, 2000 to September 21, 2001

S&P 500 percentage loss: 36.83 percent

Number of days it lasted: 546

What happened? The bursting of the dot-com bubble, following a period of soaring stock prices and exuberant speculation on new Internet companies. The United States entered a brief recession in March 2001.

July 17, 1998 to August 31, 1998

S&P 500 percentage loss: 19.3 percent

Number of days it lasted: 45 -- the shortest bear market on record.

What happened? Hedge fund Long Term Capital Management collapsed, requiring a rescue organized by the U.S. Federal Reserve.

July 16, 1990 to October 11, 1990

S&P 500 percentage loss: 19.9 percent

Number of days it lasted: 87

What happened? The United States entered recession in July. Iraq invaded Kuwait in early August.

August 25, 1987 to December 4, 1987

S&P 500 percentage loss: 33.5 percent

Number of days it lasted: 101

What happened? A heated debate between the United States and Germany over currency valuations fed an investor malaise that contributed to the Black Monday stock market crash.

October 10, 1983 to July 24, 1984

S&P 500 percentage loss: 14.4 percent

Number of days it lasted: 288

What happened? Inflation, thought to have been brought under control two years earlier, made a comeback, driving benchmark U.S. Treasury bond yields back to a peak near 14 percent by May 1984.

November 28, 1980 to August 12, 1982

S&P 500 percentage loss: 27.1 percent

Number of days it lasted: 622

What happened? Higher interest rates pushed economy into recession.

September 12, 1978 to March 27, 1980

S&P 500 percentage loss: 8.2 percent

Number of days it lasted: 562

What happened? Stagflation reached full maturity, with the annual inflation rate topping 14.5 percent by April 1980.

September 21, 1976 to March 6, 1978

S&P 500 percentage loss: 19.4 percent

Number of days it lasted: 531

What happened? Stagflation began emerging as a threat.

January 11, 1973 to October 3, 1974

S&P 500 percentage loss: 48.2 percent

Number of days it lasted: 630

What happened? The Watergate scandal erupted, ultimately forcing President Richard Nixon to resign. The Arab-Israeli war and oil embargo sent oil prices soaring, sparking a recession. The annual consumer inflation rate topped 10 percent. * Some market technicians consider the period from January 2000 through October 2002 to be a single bear market that contained a short-lived bear market rebound rather than two distinct bear markets with a brief bull market between them.

(Reporting by Rodrigo Campos and Kristina Cooke; Editing by Jan Paschal)