The glitch that sent markets tumbling Thursday was years in the making, driven by the rise of computers that transformed stock trading more in the last 20 years than in the previous 200.
The old system of floor traders matching buyers and sellers has been replaced by machines that process trades automatically, speeding the flow of buy and sell orders but also sometimes facilitating the kind of unexplained volatility that roiled markets Thursday.
“We have a market that responds in milliseconds, but the humans monitoring respond in minutes, and unfortunately billions of dollars of damage can occur in the meantime,” said James Angel…
In recent years, what is known as high-frequency trading — rapid automated buying and selling — has taken off and now accounts for 50 to 75 percent of daily trading volume. At the same time, new electronic exchanges have taken over much of the volume that used to be handled by the New York Stock Exchange.
In fact, more than 60 percent of trading in stocks listed on the New York Stock Exchange takes place on separate computerized exchanges.
Many questions were left unanswered even hours after the end of the trading day. Who or what was the culprit? Why did markets spin out of control so rapidly? What needs to be done to prevent this from happening again..?
One official said they identified “a huge, anomalous, unexplained surge in selling, it looks like in Chicago,” about 2:45 p.m. The source remained unknown, but that jolt apparently set off trading based on computer algorithms, which in turn rippled across indexes and spiraled out of control.
Many firms have computers that are programmed to automatically place buy or sell orders based on a variety of things that happen in the markets. Some of the simplest triggers are set off when a stock drops or rises a certain percent in the trading day, or when an index moves a specific amount.
But these orders can have a cascading effect. For example, if enough programs place sell orders when the overall market is down, say, 4 percent in a single day, those orders could push the market down even more — and set off programs that do not kick in until the market is down 5 percent, which in turn can have the effect of pushing stocks down even more…
“There was no pricing mechanism,” Dermott Clancy said. “There was nothing. No one knew what anything was worth. You didn’t know where to buy a stock or sell a stock.”
And most of the trades made during that enormous network fart – have been invalidated – whether they were opportunist, programmed or made by individual choice.