Market Makers and How Liquidity Works
Liquidity is the ability to buy or sell a security quickly without significantly moving the price. It is one of the most underappreciated features of developed stock markets. When an investor places an order for 100 shares of Microsoft and receives a fill within milliseconds at a price nearly identical to the last trade, that is liquidity at work. It does not happen by accident. Behind every liquid market stands a network of market makers, firms that commit capital to stand on both sides of the order book, ready to buy when others want to sell and sell when others want to buy.
What Market Makers Do
A market maker continuously posts bid prices (the price at which it will buy) and ask prices (the price at which it will sell) for a given security. The difference between the bid and ask is the spread. For a stock like Apple, the spread might be one cent. For a micro-cap stock with minimal institutional interest, it could be twenty cents or more.
The market maker earns revenue from this spread. If it buys 1,000 shares at $150.00 (the bid) and sells 1,000 shares at $150.01 (the ask), it earns $10. Multiply that by thousands of transactions per day across thousands of securities, and the economics become substantial. Citadel Securities, the largest equity market maker by volume, reportedly handles roughly 25% of all U.S. equity trading volume. Virtu Financial, another major player, has disclosed profitability on nearly every trading day since its founding.
But market making is not risk-free. The market maker accumulates inventory, shares it has bought but not yet sold, or short positions from shares it has sold but not yet bought back. That inventory is exposed to price movements. If a market maker buys 10,000 shares of a stock at $50.00 and the price drops to $49.50 before it can sell, it loses $5,000. Managing this inventory risk is the central challenge of the business.
Designated vs. Competitive Market Makers
The structure of market making varies by exchange and security type.
On the NYSE, designated market makers (DMMs) are assigned to specific stocks. Firms like Citadel Securities and GTS operate as DMMs, maintaining continuous quotes and providing liquidity during the opening and closing auctions. DMMs have obligations: they must maintain a fair and orderly market, which means quoting during times of stress when pulling back would be easier. In exchange, they receive certain informational advantages, including the ability to see the order book and participate in the opening auction process.
On Nasdaq and most electronic exchanges, multiple market makers compete for each stock. There is no single designated firm. Instead, dozens of market makers post competing quotes, and orders route to whoever offers the best price. This competitive model generally produces tight spreads in heavily traded stocks, where many firms compete for the same flow.
In the options market, market makers face additional complexity. They must quote prices across dozens of strike prices and expiration dates for each underlying stock. A single stock might have 500 actively traded option contracts, each requiring a bid and ask. Options market makers use sophisticated models based on the Black-Scholes framework to price these contracts and manage the Greek risk exposures (delta, gamma, theta, vega) across their entire book.
How Market Makers Manage Risk
Inventory management is where the skill in market making lies. A market maker does not want to hold large directional positions. It wants to buy and sell roughly equal quantities, earning the spread while keeping net exposure near zero.
Several techniques accomplish this:
Quote adjustment. If a market maker has accumulated a long position (bought more than it has sold), it will lower its bid price slightly to discourage further buying and raise its ask price to encourage selling. This gradual adjustment nudges the inventory back toward neutral.
Hedging. Market makers routinely hedge their exposure using correlated securities. A market maker with a large long position in Nvidia might short the Philadelphia Semiconductor Index ETF (SOXX) to offset the sector risk while retaining only the stock-specific exposure.
Portfolio-level risk management. Large market-making firms operate across thousands of securities simultaneously. At the portfolio level, long positions in some stocks partially offset short positions in others. Risk models calculate net exposure by sector, factor, and volatility regime, and the firm adjusts its quoting behavior accordingly.
Speed. The faster a market maker can update its quotes in response to new information, the less risk it takes. This is why market-making firms invest billions in technology infrastructure: low-latency connections to exchanges, colocation services that place servers physically close to exchange matching engines, and custom hardware including field-programmable gate arrays (FPGAs) that process data faster than general-purpose computers.
The Bid-Ask Spread as an Economic Signal
The size of the bid-ask spread reflects the cost of immediacy. A tight spread (one cent in a large-cap stock) means the cost of buying and immediately selling is negligible. A wide spread (fifty cents in an illiquid small-cap) means the round-trip cost is significant.
Several factors determine spread width:
Trading volume. Heavily traded stocks attract more market makers competing for flow, which compresses spreads. Apple, with average daily volume exceeding 50 million shares, has a spread measured in fractions of a cent. A small-cap stock trading 50,000 shares per day might have a spread of five to ten cents.
Volatility. When a stock's price is moving rapidly, market makers widen their spreads to compensate for the increased risk that the price will move against them before they can offset their position. During earnings announcements, the spread on even large-cap stocks can widen noticeably.
Information asymmetry. Market makers face an adverse selection problem. Some of the orders they receive come from informed traders, institutional investors or algorithmic strategies that have a better estimate of the stock's fair value. When a market maker trades with an informed counterparty, it loses. To compensate for these losses, market makers widen spreads. Stocks with high institutional ownership and active hedge fund interest tend to have wider spreads than stocks with primarily retail flow, all else equal.
Tick size. The minimum price increment (tick size) in U.S. equity markets is one cent for stocks above $1. This sets a floor on the spread for most stocks. The SEC has debated reducing the tick size for heavily traded stocks, which could tighten spreads further, and has also tested wider tick sizes for less liquid stocks through pilot programs.
Liquidity Is Not Constant
One of the most important things to understand about liquidity is that it can disappear rapidly.
During normal market conditions, electronic market makers provide abundant liquidity in most stocks. Order books are thick, spreads are tight, and large orders can execute with minimal price impact. But during market dislocations, liquidity can evaporate. Market makers, acting rationally, widen their spreads or pull their quotes entirely when the risk of holding inventory becomes too high.
The flash crash of May 6, 2010, demonstrated this vividly. In a span of minutes, the Dow Jones Industrial Average dropped nearly 1,000 points before recovering. During the worst of the decline, market makers withdrew from many stocks, and some trades executed at absurd prices: Accenture traded at one cent per share, and Sotheby's traded at nearly $100,000 per share. The episode revealed that electronic market making, while efficient under normal conditions, lacked the obligation to stay in the market that human specialists on the NYSE floor had traditionally maintained.
In response, regulators implemented Limit Up-Limit Down (LULD) bands that prevent trades from executing at prices too far from recent averages. They also introduced more stringent obligations for designated market makers. But the fundamental reality remains: liquidity is a fair-weather friend. It is there when conditions are calm and tends to thin out precisely when investors need it most.
The Economics of Payment for Order Flow
A significant portion of retail equity orders in the United States do not reach a public exchange. Instead, they are routed to wholesale market makers, primarily Citadel Securities and Virtu Financial, who execute the orders internally.
These firms pay brokers for the right to execute retail order flow. This is payment for order flow (PFOF), and it has been the economic engine behind commission-free trading. Robinhood, E*Trade (now Morgan Stanley), and TD Ameritrade (now Schwab) all received substantial revenue from PFOF arrangements.
The economics work because retail order flow is, on average, less informed than institutional flow. Retail investors are not typically trading on material nonpublic information or sophisticated quantitative models. This makes the flow less risky for market makers to internalize, so they can offer price improvement, filling orders at prices slightly better than the public NBBO, while still profiting from the residual spread.
The debate centers on whether this arrangement genuinely benefits retail investors or creates a conflict of interest where brokers route orders to maximize their own revenue rather than their customers' execution quality. Academic research has produced mixed results. Some studies show that retail investors receive meaningful price improvement from wholesalers. Others argue that the fragmentation of order flow away from public exchanges harms overall price discovery.
The SEC under Chair Gary Gensler proposed reforms that would have required retail orders to be exposed to auction competition before a wholesaler could internalize them. The proposal generated significant industry pushback and had not been finalized by the time Gensler departed. The regulatory trajectory remains uncertain.
Measuring Liquidity
Investors can assess a stock's liquidity using several metrics:
Average daily volume (ADV): The number of shares traded per day, averaged over a period (typically 20 or 30 days). Higher ADV generally means better liquidity.
Bid-ask spread: Available on any Level I quote. Tighter spreads indicate more competitive market making and lower transaction costs.
Market depth: Level II quotes show the number of shares available at each price level beyond the best bid and ask. Deep order books mean that large orders can execute without moving the price far.
Price impact: How much the price moves in response to a given order size. A stock where a $1 million order moves the price 0.1% is far more liquid than one where the same order moves the price 2%.
Turnover ratio: Trading volume divided by shares outstanding. A turnover ratio of 1.0 means the entire float trades once per year. Higher turnover suggests more active trading and better liquidity.
For most investors buying a few hundred shares of a large-cap stock, liquidity is not a concern. Where it matters is in small-cap and micro-cap stocks, in options with low open interest, during periods of market stress, and for any order large enough to represent a meaningful fraction of daily volume.
Why Liquidity Matters for Long-Term Investors
Some investors dismiss liquidity as a concern only for day traders. This is a mistake. Liquidity affects the price received when entering a position, the price received when exiting, and the ability to exit at all during periods of stress.
Academic research has consistently found that less liquid stocks must offer higher expected returns to compensate investors for the additional cost and risk of trading them. The Amihud illiquidity ratio, developed by Yakov Amihud, measures the daily price impact per dollar of trading volume and has been shown to predict cross-sectional stock returns. Less liquid stocks earn a premium, on average, but that premium comes with the genuine risk of being unable to sell at a fair price when it matters most.
Market makers are the plumbing that makes liquidity possible. They do not act out of altruism. They are profit-seeking firms that earn the spread in exchange for bearing inventory risk and providing immediacy. But the service they provide, the constant availability of a buyer when someone wants to sell and a seller when someone wants to buy, is what transforms stock ownership from a long-term illiquid commitment into a continuously tradable instrument. Without market makers, the stock market would not function as investors know it.
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