Fake Post Erasing $136 Billion Shows Markets Need Humans

A false report of explosions at the White House that wiped out $136 billion from the Standard & Poor’s 500 Index in about two minutes highlighted the risks of the computerized trading that dominates the $18 trillion market.

The S&P 500 was up about 1 percent at about 1,578 at 1:07 p.m. New York time today when a posting on the Associated Press Twitter account said there had been explosions at the White House and President Barack Obama had been injured. The benchmark gauge for American stocks erased almost the entire gain, falling as low as 1,563.03 by 1:10 p.m. The index recovered from the plunge within three minutes as the news service said its Twitter account had been hacked and there were no explosions. The S&P 500 ended the session up 1 percent at 1,578.78.