How One Tweet Almost Broke US Financial Markets

When a phony Associated Press tweet reported explosions in the White House, Wall Street's computers reacted as if it were real.

—By Nick Baumann  | Thu Apr. 25, 2013 2:23 PM PDT

In the January/February issue of Mother Jones, I wrote about Wall Street's embrace of high-speed computer programs that execute thousands of trades per second. These algorithms, some of which can teach themselves and operate almost entirely without human interference, present a new and challenging danger to the stability of global financial markets because they work in timeframes that people can't begin to perceive. By the time an actual person realizes something is wrong, it might already be too late to fix the problem. The concern isn't that one firm's high-speed trading program will make a mistake, but rather that a bunch of them will make the same mistake at once, launching a chain reaction that could undermine the financial system.

On Tuesday, the world saw exactly how fast these sorts of programs can respond to bad news. Many high-speed trading algorithms are designed to read headlines and trade based on that information before human traders can react. So when the Associated Press Twitter account tweeted at 1:07 p.m. Eastern time on Tuesday that two explosions were reported in the White House and President Barack Obama was injured, the market fell immediately.