Market Makers: Liquidity, Compensation, and the Bid-Ask Spread

A market maker is a securities firm that quotes both buy and sell prices for an asset and stands ready to trade at those prices, supplying liquidity to the market. Market makers are compensated through the bid-ask spread, which reflects their cost of providing immediacy and bearing inventory and information risks. Understanding the spread requires examining what liquidity costs and why intermediaries require payment to absorb temporary imbalances in order flow.

What Is a Market Maker?

A market maker simultaneously maintains an inventory of securities and quotes two-sided prices: the bid price (the price at which it will buy) and the ask price (the price at which it will sell). The bid is always lower than the ask. This willingness to trade at posted prices on both sides of the market distinguishes market makers from other market participants who simply submit limit orders and wait^[1]. Market makers exist in virtually all financial markets: equities, options, futures, bonds, and foreign exchange.

The market maker's fundamental role is to provide liquidity, meaning the ability for other traders to execute large trades quickly without causing the price to move sharply. Without market makers, a trader wishing to sell a block of shares might have to search for individual buyers, negotiate prices with each, and face long delays. Instead, the market maker steps in as a counterparty, committing capital and inventory to fill orders immediately^[1].

In US equity markets, market makers operate under SEC and FINRA rules that define their obligations. Principal among these is the requirement that market makers maintain continuous two-sided quotations in assigned securities during normal trading hours, providing a measure of depth and resilience to the market^[2]. Market makers do not simply facilitate trades; they take principal risk by holding inventory, meaning they own the securities they buy and are exposed to price changes before they resell them.

The Bid-Ask Spread and Revenue

The bid-ask spread is the difference between the ask price (the price at which the market maker will sell to you) and the bid price (the price at which it will buy from you). For example, if a stock has a bid of $100.00 and an ask of $100.05, the spread is $0.05. This spread is the market maker's primary source of revenue^[1]. When a trader buys at the ask, the market maker receives the ask price and must then resell at the bid price (or hold inventory). When a trader sells at the bid, the market maker pays the bid and must later resell the shares at the ask.

In liquid securities like large-cap equities or major currency pairs, spreads are tight, often measured in fractions of a cent. In illiquid securities or during volatile periods, spreads widen sharply. This variation is not arbitrary; it reflects different costs and risks facing the market maker in different contexts.

The spread compensates the market maker for three distinct economic costs: immediacy, inventory risk, and adverse selection risk^[3].

First, immediacy is valuable because traders value the ability to execute instantly. A trader who needs to sell shares immediately will pay the bid price rather than wait for a buyer at a higher price. The ask-bid difference is partly payment for providing this service.

Second, inventory risk arises because the market maker accumulates positions while waiting to resell. If a market maker buys 10,000 shares at $100 and the stock falls to $99.50 while searching for a buyer, the market maker loses $5,000. This risk is real and material, especially in volatile markets. The spread must be wide enough to compensate for the possibility of adverse price moves during the holding period^[3].

Third, and most subtle, is adverse selection risk. Market makers face a structural disadvantage: informed traders are more likely to trade when they possess information that the market maker does not. If an investor sells a large block quickly, the market maker may suspect that person knows the price will fall, and thus quotes a lower bid to protect itself. Conversely, informed buyers drive prices up by buying aggressively. Over time, market makers lose money to informed traders (who trade when they have an edge) and make money from uninformed traders (whose trades are random with respect to future price moves). The spread is set to ensure that the flow of uninformed trades is large enough to offset losses to informed traders^[3].

Mechanical Example with Historical Context

Consider the market for Apple Inc. Common stock (AAPL) on January 2, 2025. On that date, AAPL shares traded at approximately $238.00. The bid-ask spread for AAPL is typically very tight, often $0.01 per share, because Apple is one of the most liquid stocks in the world, with billions of shares traded daily. This tight spread reflects the low cost of providing liquidity for such a heavily traded security: competition among market makers is intense, inventory risk is small (since the market maker can quickly resell), and the probability of informed trading on publicly available Apple news is proportionally small compared to the volume of uninformed flows.

Now consider a hypothetical thinly traded stock, "XYZ Corp," which trades only 50,000 shares per day. If a market maker quotes XYZ at a bid of $50.00 and an ask of $50.20, the spread is $0.20 per share or 0.4 percent. Why so much wider than AAPL's spread?

The market maker faces greater inventory risk: if it buys 50,000 shares at $50.00, it owns the entire day's volume and must find buyers slowly. If the price drifts down to $49.90 while it accumulates inventory, it loses $5,000 on a relatively large position. Also, the market maker has less certainty about fundamental value in a thinly traded stock, increasing the weight it places on adverse selection risk. When someone wants to dump 10,000 shares of XYZ quickly, the market maker worries the seller knows something. The $0.20 spread is the market maker's insurance premium against these risks.

A trader selling 50,000 shares of XYZ pays the bid, receiving $50.00 per share, or $2.5 million. The market maker stands ready to provide this immediacy. Without it, the seller would post a limit order and wait, potentially for hours or days.

Why Markets Allow and Require Spreads

The spread exists because neither exchange authorities nor economic logic permits market makers to operate without compensation. The CFTC and SEC could theoretically mandate that market makers post zero spreads, but this would eliminate their incentive to show up, and market-making supply would vanish^[2]. Liquidity is not free; it requires firms to tie up capital, bear risk, and invest in technology. If spreads were prohibited, most securities would trade far less frequently, and the prices that existed would be less reliable.

The spread is also necessary for financial stability. During times of stress, when informed traders dominate and the direction of prices is highly uncertain, spreads must widen to compensate market makers for the spike in adverse selection risk. If spreads were capped by regulation, market makers would withdraw liquidity during precisely the moments when liquidity is most needed, amplifying crashes and dislocations.

Limitations

This framework has important limitations. First, modern electronic markets have partially eroded the traditional market maker model. High-frequency trading firms act as de facto market makers in many equities, quoting continuously on exchanges but with far lower inventory commitment and much shorter holding periods than traditional dealers. Whether these firms should be called "market makers" in the classical sense is conceptually unclear, and their economic role remains contested^[3]. Academic research on whether their presence improves or degrades market quality has reached mixed conclusions, depending on the metric and time period studied.

Second, the theory predicts that spreads should widen with volatility and inventory risk, but empirical measurement of the precise contribution of each component, immediacy, inventory cost, and adverse selection, remains difficult. Studies using order flow data can estimate these components, but the estimates are model-dependent and vary significantly depending on the specification^[3].

Third, this paper assumes market makers quote spreads to cover expected costs. In practice, market makers may sometimes quote tighter spreads than cost-minimizing levels (accepting losses) to gain market share or during competitive intensity, or wider spreads during low-liquidity windows. The spread is not always an equilibrium price; it is sometimes a strategic choice.

Finally, regulations that cap spreads or impose costs on market making (such as transaction taxes or tick size restrictions) can reduce the supply of liquidity and widen effective spreads for ordinary traders, creating a false economy. This possibility highlights the risk of regulating market-making fees without understanding their economic function.

Key Definitions

Bid: The price at which a market maker will buy from a trader.

Ask (or Offer): The price at which a market maker will sell to a trader.

Bid-ask spread: The difference between the ask and bid prices, measured in currency units or basis points; the primary revenue source for market makers.

Inventory risk: The cost borne by a market maker from holding securities while waiting to resell them, due to the possibility that prices move unfavorably.

Adverse selection (or adverse selection risk): The cost to a market maker from trading with informed counterparties who possess non-public information or better price predictions.

Immediacy: The service of allowing a trader to execute a transaction instantly at posted prices rather than waiting for a match with another trader.

Liquidity: The ability to buy or sell a large quantity of a security quickly without substantially moving its price.

References

1. Stoll, H.R. "Market Microstructure." Handbook of the Economics of Finance (2003). Doi.org/10.1016/S1574-0102(03)01005-8

2. U.S. Securities and Exchange Commission, "Market Makers." https://www.sec.gov/investor/pubs/tradingsecurities.htm (primary)

3. Hasbrouck, J. "Empirical Market Microstructure." Oxford University Press (2007).

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Last reviewed by the PropLedger research pipeline: 2026-10-11. Educational research on historical data, not financial advice.