Market Makers: Liquidity Providers and the Bid-Ask Spread

A market maker is a trader or firm that stands ready to buy and sell a security or contract at quoted prices, profiting primarily from the difference between bid (buy) and ask (sell) prices. The spread exists not as arbitrary markup but as compensation for three distinct economic functions: holding inventory risk, bearing adverse selection risk when prices move against accumulated positions, and providing the service of immediate execution to other traders. Understanding market-making mechanics illuminates how financial markets price liquidity itself.

Core Concept: The Dual Role

Market makers occupy a structural role distinct from other traders. While a speculator profits from directional price movements and an arbitrageur exploits temporary mispricings between venues, a market maker profits by completing both sides of transactions: buying from sellers and selling to buyers, often within seconds or microseconds, without necessarily holding directional conviction about the security's future price. In this sense, market makers are liquidity wholesalers. They stand between buyers and sellers, absorbing imbalances of order flow and stabilizing prices during periods when natural supply and demand might otherwise produce larger price gaps. Exchanges and regulators recognize this function and often require designated market makers (DMMs) on equity exchanges or impose market-making obligations on participants in certain derivatives contracts to ensure continuous order flow ^[1].

The market maker's revenue model is structural: it comes entirely from the spread, which is the gap between the price at which they buy (the bid) and the price at which they sell (the ask). A market maker who buys at 100.00 and sells at 100.05 captures 5 cents per round-trip transaction. On a security trading millions of shares per day, this compounds into meaningful revenue. Importantly, the spread is not a fee: it is transparent and embedded in the price a customer pays or receives ^[2].

How Spreads Arise: Economic Logic and Inventory Dynamics

Three factors drive spread width and justify why spreads are not arbitrary or exploitative, but rather market prices for liquidity services.

Adverse Selection Risk. Market makers do not know whether an order that arrives to buy or sell reflects public information (a news event) or is simply a customer rebalancing their portfolio. When a trader approaches a market maker to buy 10,000 shares immediately, the market maker risks that the buyer possesses information signaling the stock will rise, leaving the maker holding shares that are about to decline. This is adverse selection: the market maker's counterparty may be better informed. To protect against this risk, market makers widen spreads during periods of uncertainty or when information is asymmetric. During earnings announcements or macroeconomic data releases, spreads commonly widen because adverse selection risk spikes ^[1]. Academic research on market microstructure shows that bid-ask spreads contain a component proportional to the probability that the next move in price will be against the market maker's position ^[3].

Inventory Holding Cost. A market maker cannot simply buy once and sell once per day. Over the course of trading hours, they may accumulate net long or short positions. While holding these positions, they bear price risk: if they own 50,000 shares of stock and the stock declines 1%, they lose $50,000 in value. Market makers must compensate for this risk by widening spreads (taking a larger margin) or hedging dynamically, both of which are costly. The wider the spread, the greater the revenue per unit traded, allowing the maker to accept lower volumes and offset holding risk. Conversely, in highly liquid securities where inventory is easy to move, spreads narrow because holding risk is lower.

Immediacy and Operational Cost. A market maker provides the service of immediate execution. A limit order posted on an exchange book might take hours to fill; a market maker fills a customer's order instantly, often via electronic systems that operate 24/7. This immediacy has operational costs: technology infrastructure, compliance systems, regulatory fees, clearing and settlement costs, and capital requirements to back the firm's inventory. These costs are recovered through the spread.

Worked Example: Equities and Historical Context

Consider a market maker in an equity stock in 2024. The stock trades approximately 5 million shares per day across all venues. A typical bid-ask spread might be 1-2 cents for a stock with a price around $50. This spread is narrow relative to the stock price (0.02% to 0.04%), but represents a material economic outcome.

If a market maker quotes a bid of 49.995 and an ask of 50.005, the spread is 1 cent. If they execute 50,000 shares bought and 50,000 shares sold in a single day in balanced volume, they earn $500 gross revenue (50,000 shares × $0.01 per share). Against this must be deducted operational costs: technology spend, compliance staff, exchange fees, and the cost of capital to support the position. For a large market-making firm, these costs might consume 30-50% of spread revenue, leaving a competitive margin.

However, if a market maker receives a buy order for 50,000 shares before a sell order matches it, they hold 50,000 shares of long inventory. If the stock declines 50 cents before they can sell, they lose $25,000. This loss illustrates inventory risk. To guard against it, market makers either widen the spread to generate greater revenue per trade, or dynamically hedge by buying options or short-selling an offsetting position.

Historically, spreads were much wider before electronic trading automated market-making. In the 1980s, bid-ask spreads on NYSE stocks were typically 12.5 cents to 50 cents or more for stocks in the $20-$100 range, reflecting the cost of human floor traders and slower information processing ^[4]. The rise of electronic communication networks (ECNs) and automated market makers has compressed spreads dramatically, lowering transaction costs for end investors. The SEC's regulation of market-maker obligations, particularly through Rule 10b-7 and related frameworks, reinforced the link between spreads and the quality of market functioning ^[1].

Limitations and Critical Considerations

Several limitations constrain this framework:

Spreads Do Not Always Reflect Fundamental Risk. During market stress or when information is highly asymmetric, spreads can widen sharply beyond the economic compensation justified by immediate risks. Flash crashes and liquidity evaporations demonstrate that spreads may reflect panic and not just rational risk pricing. In March 2020, during the initial COVID-19 volatility, spreads on even the largest-cap U.S. Equities widened to levels not seen in years, suggesting market makers withdrew capital rapidly and spreads became rationed scarcity pricing rather than efficient risk compensation ^[2].

Concentration and Market Power. In less liquid securities or derivatives markets, a small number of market makers may dominate, conferring pricing power. A market maker can theoretically quote unreasonably wide spreads if no competitor will undercut them. Competition is the primary check on spread width, but not all markets are equally competitive. Swap markets, certain fixed-income bonds, and commodity futures contracts often have thinner competition and wider spreads than highly liquid equities.

Spreads Extract Value from Passive Investors. While spreads are economically justified, they represent a cost borne by traders, especially passive investors who must buy or sell regardless of price. Someone who must raise cash by selling 1 million shares pays the full width of the spread. A retail investor placing a market order bears the spread cost while the market maker captures it. This is not unfair in a legal sense, but it highlights that spreads are fundamentally a transfer of wealth from less-patient to more-patient traders.

Technology and Latency. The rise of high-frequency trading and algorithmic market-making has made spreads narrower, but at the cost of concentrating technology and speed advantages among large firms. Smaller market makers or traditional firms cannot compete effectively, reducing diversity in market-making and potentially concentrating systemic risk.

Key Definitions

Adverse selection: The risk that a counterparty to a trade possesses information not known to the market maker, making the transaction unprofitable or disadvantageous for the maker in expectation.

Bid-ask spread: The difference between the highest price a buyer will pay (bid) and the lowest price a seller will accept (ask) for a security at a given moment.

Inventory risk: The risk that the value of securities held by a market maker will decline before they are sold, resulting in a loss.

Immediacy: The service of executing a trade without delay, providing certainty of execution and timing.

Liquidity: The ease and speed with which a security can be bought or sold in large quantities without materially affecting its price.

Market maker: A firm or trader that continuously quotes both bid and ask prices and stands ready to buy from and sell to other traders, earning profit from the spread.

Order flow: The stream of buy and sell orders that arrive at a market or exchange.

References

• U.S. Securities and Exchange Commission, "Market Makers and Affiliated Persons" (Rule 10b-7 and related regulations), SEC.gov (updated). Https://www.sec.gov
• Federal Reserve, "Financial Stability and the COVID-19 Crisis," Financial Stability Reports (2020-2021). Https://www.federalreserve.gov
• Madhavan, A., and Sofianos, G., "An Empirical Analysis of NYSE Specialist Trading," Journal of Financial Economics, Vol. 48, No. 2 (1998). Doi.org/10.1016/S0304-405X(98)00008-2
• CME Group, "Market-Making and Liquidity Provision in Futures Markets," CME Group Education. Https://www.cmegroup.com
• Investopedia, "Bid-Ask Spread," Investopedia (2024). Https://www.investopedia.com/terms/b/bid-askspread.asp
• Hasbrouck, J., and Saar, G., "Low-Latency Trading," Journal of Financial Markets, Vol. 16, No. 3 (2013). Doi.org/10.1016/j.finmar.2012.09.003

Educational research on historical data only. Not investment advice, not a signal, and never a performance promise. Past results do not predict future performance. Every reference is link-verified before publication and every paper is re-audited weekly against the library's editorial standard.

Last reviewed by the PropLedger research pipeline: 2026-09-27. Educational research on historical data, not financial advice.