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At 2:45p.m. on Thursday, May 6, George (Gus) Sauter received a frantic call from one of his traders to get in front of a Bloomberg terminal. The Dow Jones industrial average, already down 3.9 percent that day on fears about Greece, was in free fall. In just five minutes the index plunged 573 points. Less than two minutes later, the Dow had rocketed back up 543 points, going on to finish the day down 3.2 percent.

"It was just crazy," Sauter, chief investment officer of mutual fund giant Vanguard Group, told me a few days later. "I had to go to our fixed-income building, about a five-minute walk from my office. By the time I got there, the market had rallied."

Crazy, indeed. The aptly named "flash crash" temporarily wiped out more than a half trillion dollars in equity value, shaking what little faith nervous investors had in U.S. markets. Shares of Dow component Procter & Gamble Co., the ultimate defensive blue-chip stock, dropped more than one third in a matter of minutes before recovering almost as quickly, all for no apparent reason. A few other large U.S. companies, including accounting firm Accenture, saw their stocks trade as low as a penny a share, only to close not far from where they had begun the day (nearly $42 a share in the case of Accenture) - again, on no news. By the time the dust settled, a whopping 19.3 billion shares had changed hands, more than twice the average daily U.S. equity market volume this year and the second-biggest trading day ever.

But for me, the single most amazing fact about the flash crash was that no one had a clue as to what had triggered it. Not that I should have been surprised, based on the conversation I'd had two days earlier with Mary Schapiro, chairman of the Securities and Exchange Commission. Schapiro, whose organization is charged with maintaining "fair and orderly" markets, explained to me how the SEC did a detailed study after the October 1987 crash to reconstruct what had happened. "We've lost some of the capacity to do that given the dramatic volumes of trading that exist today," Schapiro said. "But we need to be able to do that to understand where are the vulnerabilities in our marketplace and what are the practices that have the potential to hurt investors and the marketplace in the long run."

In 1987 the SEC had a much easier task because the vast majority of listed U.S. equities were traded in one place - on the floor of the New York Stock Exchange, where specialists employed by the Big Board's member firms made a market based on an open-outcry auction system. Today, as a result of a series of regulatory changes designed to increase competition and make the market fairer for mom-and-pop investors, only about one quarter of all U.S. equity trading occurs through the now publicly held NYSE Euronext. And the majority of that trading is done electronically, either by the new NYSE floor specialists, called designated market makers, or on the fully automated NYSE Arca platform. The rest of the trading in U.S. equities is spread across a wide range of venues, including the three other major exchanges (Nasdaq Stock Market, BATS Exchange and Direct Edge) and dozens of broker-dealer-operated trading systems, electronic communications networks (ECNs) and dark pools, where buyers and sellers are matched up anonymously.

The past decade of fragmentation and automation has given rise to a whole new type of professional trading firm: one that uses sophisticated computer algorithms, often running on servers housed right next to exchanges' own machines, and high-speed market data feeds to buy and sell securities in rapid-fire fashion. Some of these high frequency traders place hundreds of millions, even billions, of buy and sell orders a day, continually canceling and replacing them, and are likely to be on the other side of your trade. Not that you'd know who they are - proprietary trading firms are not required to disclose their identity - or recognize their names. The bulge bracket of high frequency trading includes firms like Allston Trading, DRW Holdings, Global Electronic Trading Co. (Getco), Hudson River Trading, Quantlab Financial, RGM Advisors, Sun Trading, Tower Research Capital and Tradebot Systems.

High frequency trading has become a multibillion-dollar business, accounting for an estimated 50 to 70 percent of the total U.S. equity market volume on any given day. Since last summer it has also been a lightning rod for the populist anger directed at Wall Street, despite the fact that most of the largest high frequency firms operate far from the canyons of lower Manhattan, in places like Chicago, Kansas City and Austin, Texas. Critics accuse high frequency traders of being fair-weather market makers who, unlike the former NYSE specialists they've largely replaced, don't have a legal obligation to trade during periods of stress. They also say that the growth in high frequency trading has created a two-tiered market of technology haves and have-nots that is unfair to long-term investors and poses potential systemic risks.

I grew interested in high frequency trading last year when I was writing a feature on hedge fund firm Citadel Investment Group (more on that later). As an editor, however, it wasn't until January that I was able to dig into what I soon learned is an incredibly arcane world. My first stop was a company called Pragma Securities, an agency-only brokerage firm that aggregates more than 40 different dark pools, electronic trading venues and open market destinations into a single liquidity source for clients. Douglas Rivelli and David Mechner, Pragma's co-CEOs, spent two hours at the firm's spacious New York offices taking me through that world.

High frequency traders, Rivelli and Mechner explained, generally fall into one of two camps: proprietary trading shops that act as electronic market makers, using computers to generate and adjust buy and sell orders automatically throughout the day, and hedge funds that specialize in statistical arbitrage, seeking to exploit pricing inefficiencies among different securities and asset classes. The distinctions between the two sometimes blur, however, as proprietary trading firms often try to capitalize on some of the same buy and sell signals that statistical arbitrageurs use and hedge funds trade on ever-shorter time horizons. High frequency firms are best known for trading equities, but they also trade futures, options and foreign exchange - basically, anything that can be traded electronically. High frequency trading is also an increasingly global phenomenon, gaining ground in both Europe and Asia.

One thing is clear: Hedge funds don't like to be called high frequency traders, as I quickly discovered after visiting with some of the biggest quantitative managers, including AQR Capital Management in Greenwich, Connecticut, and D.E. Shaw & Co. and Renaissance Technologies Corp. in New York.

In the wake of the flash crash, as people scrambled to determine what had triggered the market plunge, it didn't take much longer than the 400 to 600 microseconds (millionths of a second) that high frequency traders typically need to identify and place a trade for fingers to start pointing at them. "The potential for giant high-speed computers to generate false trades and create market chaos reared its head again today," Delaware Senator Ted Kaufman said in a statement released that same afternoon. When I caught up with the Democratic lawmaker a week later, he was even more incensed, pointing out that regulators still didn't know what had caused the flash crash.

"We have a 300-pound gorilla in the room, and we're saying that we're going to keep it in a cage somewhere," he told me. "This thing will be 600 pounds."

"But isn't part of the problem that there are 300 gorillas?" I asked, referring to the fact that an estimated 200 to 400 firms do high frequency trading.

"Good point," he replied. "We have all these gorillas, and guess what? We put them in zoos where the people running the zoos don't have enough information and authority to take care of them."

Kaufman's interest in high frequency trading predates mine. When he was sworn into office in January 2009 to fill the Senate seat of his former boss Joe Biden, Kaufman was hell-bent on making sure that everybody responsible for the 2008 market meltdown paid for their actions. He soon focused on short-selling, urging the SEC to reinstate the uptick rule requiring short sales to be filled at a higher price; the rule had been eliminated in 2007. He told me that when he was in business school in the 1960s, it was "an article of faith" that the uptick rule was "one of the two or three things that helped deal with predatory bear raids." As a result of his interest in short-selling, Kaufman said, his office started getting calls from some fairly sophisticated people, including former Wall Streeters, telling him that if he thought that practice was bad, he should look at high frequency trading.

Kaufman likes to draw an analogy between high frequency trading and the swaps market. "With synthetic derivatives, you had a lot of money at stake, no transparency and then a major meltdown," he explained to me. "If you look at high frequency trading, I think the same Kaufman formula works."

A graduate of the Wharton School of the University of Pennsylvania, the 71-year-old Kaufman is a quick study and understands markets. If I were a high frequency trader, I'd take him seriously.

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