A MAIN benefit for those who own real estate investment trusts has been their low historical correlation to the rest of the financial markets. In other words, REITs have tended not to move in tandem with most stocks and bonds, thus making them good portfolio diversifiers. All of that seemed to change this year amid the deepening credit crisis and Wall Street’s meltdown. Just as stocks fell fast and furiously, so, too, did most REIT shares. (And at those times when stocks were rallying back, REITs also moved higher.)