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Dark Pools In Equity Trading: Significance and Recent Developments
Gary Shorter, Specialist in Financial Economics (gshorte crsc.gov, 7-7772)
Rena S. Miller, Specialist in Financial Economics (rsmiller@crs.loc.gov, 7-0826)
Dark pools are relatively recent and controversial electronic stock trading alternatives to traditional
exchanges, such as the New York Stock Exchange (NYSE), and now account for about 15% of overall
trading volume. A dark pool is a type of alternative trading system (ATS), a broker-dealer who matches
the stock trading orders of multiple buyers and sellers outside of exchanges. Orders sent to dark pools
to buy or sell certain stocks are not publicly displayed. When they emerged in the late 1990s, that
opacity attracted the pools' initial clients, institutional investors (such as pension and mutual funds),
who used it to conceal large trading interests, thus helping to reduce the risk of the market moving
against their trades. Quote concealment is a legacy of a regulation adopted by the Securities and
Exchange Commission (SEC) in 1998, Regulation ATS, which allowed ATSs with less than an average
5% share of the trading volume to not publicly display their quotes. This contrasts with the lit
venues, Nasdaq and the exchanges, which do.
Under Regulation ATS, dark pools are required to register either as exchanges with the SEC or as
broker-dealers with the Financial Industry Regulatory Authority (FINRA). FINRA, which the SEC
oversees, is the frontline regulator of SEC-registered broker-dealers. Dark pools are subject to the
same rules that govern trading either on an exchange or by a broker-dealer.
While large block trades of say, 1 million shares, were initially a focus of dark pool trading, in recent
years, average sizes have shrunken to a few hundred shares, comparable to levels on exchanges.
Major forms of dark pools include broker-dealer-owned pools, including Credit Suisse's CrossFinder and
Goldman Sachs' Sigma X; agency-broker pools, including Liquidnet and ITG Posit; and exchange-owned
pools, including those operated by BATS and the NYSE.
Pros and Cons of Dark Trading
Dark pools are at the center of a contentious policy debate on their impact on securities markets and
investors. Key elements are discussed below.
As described earlier, the pools' quote opacity can benefit large institutional trades. The pools are also
said to give traders comparatively more autonomy in the choice of the opposing buyers and sellers,
thus avoiding potentially problematic traders (i.e., those who may do forms of high frequency trading
described as predatory). The pools may also benefit traders because they tend to charge relatively less
for traders than do Nasdaq and the exchanges.
On the other hand, there are concerns that quote opacity prevents pools from contributing to the
critical price discovery process. Consequently, it is argued that the ability of the overall securities
market to equilibrate the most accurate securities prices is impaired. Summarizing the research on this,
SEC chair Mar  Jo White observed that the consensus appears to be that dark trading can sometimes
detract from market overall quality, including the informational efficiency of prices. She also said the
agency would continue examining whether dark trading volume was approaching a level that could
undermine the quality of price discovery on the lit venues.
The existence of the 40 or so dark pools has also contributed to a fragmented equities marketplace,
which encompasses about 11 exchanges and some 200 broker-dealers who execute retail trades
through their own stock inventory, called internalization. The fragmentation appears to have helped
boost competition, which has benefited traders in areas, such as lowered trading costs and trading
infrastructure innovations. But, particularly with respect to individual dark pools, it may impose multiple

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