Investing.com - The recent selloffs on Wall Street have a few analysts predicting the onset of a bear market in stocks. Though the generally accepted definition of a bear market is a 20% price decline lasting at least two months, bear markets of the past can be divided into two categories: bad and worse.That may sound overly simplistic, but the duration and depth of the dozen-plus bear markets in history varies widely.Unfortunately for current investors, two of the three worst happened in the past twenty years. The most recent coincided with the financial crisis, or Great Recession. It ran from October 2007 to March 2009, during which time the S&P 500 fell more than 56%.The other occurred between March 2000 and October 2002, marked by the bursting of the dot com bubble. The S&P 500 lost 49% of its value.The epic one, of course, accompanied the Great Depression, running from September 1929 to June 1932. It lasted almost three years and knocked 86% off of the S&P 500.Some bear markets since WW II have been mercifully brief; one lasted three months, another six months. In two cases, the decline maxed out at 28%.One thing is generally true of all bear markets. They are usually associated with a momentous event, such as a market crash, a recession or spike in inflation.