“I think the machines just took over.”

That’s perhaps the best explanation I’ve seen on yesterday’s sudden stock market drop:

“I think the machines just took over. There’s not a lot of human interaction,” said Charlie Smith, chief investment officer at Fort Pitt Capital Group. “We’ve known that automated trading can run away from you, and I think that’s what we saw happen today.”

On the floor of the New York Stock Exchange, stone-faced traders huddled around electronic boards and televisions, silently watching and waiting. Traders’ screens were flashing numbers non-stop, with losses shown in solid blocks of red numbers.

As noted by the WSJ, about two-thirds of the market’s overall volume is comprised of trades from “high-frequency trading firms,” which are fully automated systems. The New York Times offers additional insight into how these trading systems can fail:

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.

Some circuit breakers do exist, a legacy of the reforms made following the 1987 stock market crash, but they only kick in after a huge drop — and only at certain hours. Before 2 p.m., a 10 percent drop in the Dow causes New York Stock Exchange to halt trading for one hour. Between 2 p.m. and 2:30 p.m., the pause shrinks to a half-hour and after 2:30, there is no halt in trading.

The damage was deep:

Multiple stocks, ranging from Accenture PLC to Boston Beer Co., momentarily lost nearly 100% of their value, changing hands for just one penny. Exchange-traded funds, which are index funds that trade like stocks on exchanges, were also temporarily vaporized. The $9.5 billion iShares Russell 1000 Value Index Fund went from $59 to around 8 cents in the blink of an eye.

“It happened so quickly, it was like a torpedo,” said Scott Redler, chief strategic officer at T3 Capital Management, a hedge fund. “It was mayhem.”

I am still looking for more details around what actually and specifically went wrong yesterday, but so far, there is not much information.

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