Bid-Ask Spread Floor: Why Minimum Tick Sizes Transfer Wealth From Retail Limit Orders to Market Makers

In a physical auction house, a bidder who offers a fraction of a cent more than the current high bid is ignored because the auctioneer enforces a strict minimum bidding increment. In modern equity markets, however, that fractional bid is not only accepted off-exchange but serves as the primary mechanism by which intermediaries step ahead of public limit orders.

This structural discrepancy stems from the minimum tick size, a regulatory floor that dictates the smallest allowable price movement for publicly displayed stock quotes. For nearly two decades, that floor has been set at exactly one penny for most equities traded on national exchanges.

While intended to simplify the market and protect investors from microscopic price manipulation, the rigid penny increment created an unintended wealth transfer. It forces retail investors to queue at whole-cent intervals, while allowing institutional market makers to bypass those queues in private venues.

The mechanics of this transfer do not rely on high-frequency trading speeds or complex algorithms, but rather on a simple regulatory arbitrage. By understanding how the bid-ask spread floor operates, investors can see exactly why their limit orders often go unfilled even when the market appears to touch their price.

The Mechanics of the Penny Floor

The architecture of modern stock pricing is governed by Rule 612 of the Securities and Exchange Commission’s Regulation NMS. Implemented in 2005, the rule prohibits market participants from displaying, ranking, or accepting orders in pricing increments smaller than $0.01 for stocks priced above one dollar.[1]

This regulation effectively mandates a minimum bid-ask spread of one penny. If a buyer is willing to pay $50.00 for a share and a seller is asking $50.01, the spread is exactly one cent, and the stock is considered tick-constrained.[4]

For highly liquid stocks, natural supply and demand would often dictate a spread much tighter than a penny. However, because public exchanges cannot display sub-penny quotes, liquidity piles up at these artificial boundaries, creating massive queues of limit orders waiting for execution.[2]

When a retail investor places a limit order to buy 1,000 shares at $50.00, that order joins the back of the queue at the public exchange. The investor must wait for all prior orders at $50.00 to be filled before their trade can execute.[2]

The Sub-Penny Step-Ahead

The wealth transfer occurs because Rule 612 applies strictly to the public display and ranking of orders, but it does not prohibit off-exchange venues from executing trades at sub-penny prices. This loophole allows wholesalers and internalizers to operate by a different set of mathematical rules.[1]

When a marketable sell order arrives, an off-exchange market maker can intercept it by offering a price of $50.001. This price is technically better than the public best bid of $50.00, satisfying the regulatory requirement to provide price improvement to the seller.[4]

By improving the price by just one-tenth of a cent, the market maker completely bypasses the massive queue of retail limit orders waiting at $50.00. The intermediary captures the remaining nine-tenths of a cent of the spread when they subsequently sell the shares.[3]

The retail limit order, meanwhile, remains stuck on the public exchange. The investor provided the foundational liquidity that established the $50.00 floor, but the intermediary extracted the economic value of the trade for a fraction of a penny.[2]

The Cost of Adverse Selection

Missing out on a trade is only the first half of the wealth transfer mechanism. The second half involves a phenomenon known as adverse selection, which systematically penalizes the retail limit orders left waiting in the public queue.[2]

Because market makers selectively step ahead of retail orders to capture profitable spreads, they only leave the unprofitable trades for the public exchanges. If a sudden wave of selling pressure hits the market, the intermediaries simply step back and stop offering sub-penny improvements.[2]

When the intermediaries step back, the selling pressure crashes directly into the public limit order book. The retail investor’s order at $50.00 is finally executed, but only because the market is in the process of moving sharply lower.[2]

The retail investor buys the stock at $50.00 just as the price drops to $49.95. They suffer an immediate paper loss, having absorbed the toxic liquidity that the institutional market makers deliberately avoided.[3]

This dynamic means retail limit orders face a structural disadvantage. They miss out on executions when the price remains stable or moves in their favor, and they only receive fills when the market is actively moving against their position.[2]

Quantifying the Wealth Transfer

The economic scale of this structural disadvantage is substantial when aggregated across the entire equity market. Academic analyses of the SEC’s Tick Size Pilot program revealed that artificial penny constraints systematically degrade execution quality for retail liquidity providers.[1]

When normalizing the non-execution costs against the sub-penny spread capture across a standardized 10,000-share block, the data reveals a stark asymmetry. Market makers routinely capture $80 to $90 of the spread value, while retail limit orders absorb the opportunity costs.[3]

This dynamic effectively functions as a tax on public liquidity provision. The retail investor assumes the risk of displaying a firm price to the market, but the economic reward for that risk is intercepted by intermediaries operating in dark pools or wholesale networks.[3]

The wealth transfer is most pronounced in highly liquid, low-priced stocks where the one-cent tick size represents a large percentage of the share price. In these tick-constrained environments, the queue at the penny increment can stretch to millions of shares.[4]

Regulatory Reforms and the Half-Penny Shift

Recognizing the market distortions caused by the rigid penny floor, the Securities and Exchange Commission recently moved to modernize the quoting infrastructure. In adopting unanimous amendments to Rule 612 in September 2024, the SEC explicitly noted that the one-penny minimum increment was "outdated, and too wide for today’s markets."[1]

Under the new framework, which reaches full compliance in November 2025, stocks with a time-weighted average quoted spread of $0.015 or less will be assigned a minimum tick size of $0.005. This half-penny increment aims to alleviate the massive queues in tick-constrained securities.

By allowing public exchanges to quote in half-penny increments, the SEC intends to foster tighter spreads and reduce the structural advantage held by off-exchange wholesalers. Retail limit orders will be able to compete more aggressively for executions without relying on sub-penny internalizers.[3]

However, the amendments notably stopped short of banning sub-penny executions entirely. Market makers will still be permitted to offer price improvement at increments finer than the new half-penny tick, preserving a narrower version of the step-ahead mechanism.

The shift to a half-penny tick represents the most significant change to equity market microstructure in two decades. It acknowledges that the artificial constraints of the past transferred billions in spread value away from the public order book.

As the market prepares for the 2025 implementation, the fundamental math of liquidity provision remains unchanged. Until public limit orders can compete on the exact same pricing increments as private intermediaries, the structural wealth transfer will persist, albeit at a smaller scale.[3]

Institutional Impact and Maker-Taker Fees

Retail investors are not the only participants penalized by the bid-ask spread floor. Institutional asset managers, pension funds, and mutual funds also rely heavily on public limit order books to accumulate or distribute large block positions over time.[2]

When an institutional algorithm places passive limit orders to buy 100,000 shares, it faces the exact same queueing disadvantage at the penny increment. The institution's orders are routinely bypassed by sub-penny internalizers, extending the time required to complete the trade.[4]

This delay exposes the institution to greater market risk and information leakage. The longer their limit orders sit unexecuted in the public queue, the higher the probability that other market participants will detect their buying interest and drive the price upward.[2]

Furthermore, the economics of the penny tick are deeply intertwined with the maker-taker fee models used by public exchanges. Exchanges charge access fees to participants who take liquidity, while paying rebates to those who provide it through limit orders.[1]

The SEC's 2024 reforms also addressed these access fees, reducing the caps to align with the new half-penny tick sizes. By shrinking both the tick size and the access fees, regulators hope to compress the total friction costs embedded in every equity transaction.

Ultimately, the bid-ask spread floor demonstrates how seemingly minor technical rules can dictate the flow of capital. A rigid one-cent boundary, designed for simplicity, inadvertently engineered a permanent structural advantage for those equipped to trade in the fractions beneath it.[3]