U.S. stocks slumped as investors speculated the Federal Reserve’s refusal to raise rates bespeaks larger concerns about the strength of the global economy.

The Standard & Poor’s 500 Index lost 1.2 percent to 1,966.72 at 12:52 p.m. in New York. The gauge pared its gain for the week to 0.3 percent. The Dow Jones Industrial Average dropped 238.61 points, or 1.4 percent, to 16,436.13. The Dow Jones Transportation Average slumped 1.5 percent.

Some futures and options on stocks and indexes expire today in a process known as quadruple witching, which may increase stock volatility. Trading in S&P 500 companies was 38 percent above the 30-day average.

The operator of the S&P 500 will also rebalance the index in a quarterly move to adjust member weightings. About $41 billion of shares will be traded as investors buy and sell stocks to mimic the changes, according to a Sept. 8 estimate by Howard Silverblatt, an index analyst at the New York-based S&P Dow Jones Indices.

Source: Bloomberg