The S&P 500 Index is flat so far this month after plummeting over 6% in August. Jeffrey Hirsch, Editor of the Stock Trader’s Almanac, said investors should remain cautious because September has been the worst month for returns since 1950. 'The latter part of the month is where the really trouble generally brews,' said Hirsch, who is also on the investment committee for Probabilities Fund Management, which runs a strategy based on the Stock Trader’s Almanac in its Probabilities Mutual Fund (PROTX), down 6.2% year-to-date, according to fund-tracker Morningstar. Hirsch added that the week after triple witching in September has historically been brutal, down 21 of the last 25, averaging an S&P 500 loss of 1.1%. In 2011, for example, the Dow Jones Industrial Average and the S&P 500 both lost in excess of 6%. One reason for the traditional end of September slump, according to Hirsch, is that end of third quarter portfolio restructuring causes a weak close for September. For evidence of this trend, he points to the fact that the DJIA and S&P 500 have only been up four times in the last 18 years on the last trading day of September. Hirsch said a bad August performance generally portends a better September result for stocks, except when August is 'really bad' like the one the market just experienced.