A market maker quotes a bid-ask spread of $0.25 on a thinly traded small-cap biotech stock, compared to $0.01 on a major index ETF. The wider spread on the small-cap primarily reflects:
AHigher regulatory compliance costs for small companies
BGreater adverse selection risk — the probability of trading against someone with private information about the stock's true value is higher when fewer participants are monitoring it
CLower trading volume making automated systems expensive to operate
Adverse selection is the most theoretically central driver of spread differences across securities. A market maker's spread must be wide enough to recover losses from trades against informed counterparties (insiders, analysts with material nonpublic information) through profits on trades with uninformed counterparties. For a thinly traded stock that few analysts cover, any given counterparty is more likely to know something the market maker doesn't. The market maker widens the spread to compensate. For a heavily traded ETF with thousands of participants, the probability that any given order is informationally motivated is low, so a narrower spread suffices.
Question 2 Multiple Choice
A large pension fund needs to buy 3 million shares of a mid-cap stock (representing about 8% of its outstanding shares). According to market microstructure theory, what should the fund expect as it executes this order?
AIt will buy all shares at the current ask price, since that is what market orders execute at
BThe prices it pays will rise as it executes — its own order flow signals buying demand, causing market makers to update their quotes upward
CIt will receive a bulk discount since it is providing liquidity by taking a large position
DThe bid-ask spread will narrow as the fund's orders attract competing sellers
This is price impact — one of the most practically important concepts in market microstructure. When a large order enters the market, it consumes available liquidity at the best prices and then moves up the limit order book. More importantly, the market maker observes the sustained buying pressure and updates their quotes upward, recognizing it as an information signal. The fund ends up paying higher prices for later shares than for early shares. For large institutions, this price impact (also called execution cost) can dwarf explicit commissions and significantly affect realized returns.
Question 3 True / False
The bid-ask spread compensates market makers for real economic costs — including order processing, inventory risk, and adverse selection — not just transaction fees.
TTrue
FFalse
Answer: True
The bid-ask spread has a specific economic decomposition. Order processing costs cover clearing, settlement, and operational overhead. Inventory costs arise when market makers accumulate one-sided positions and are exposed to adverse price moves while seeking the offsetting flow. Adverse selection costs — the expected loss from trading against informed counterparties — are the most theoretically significant. Understanding this decomposition explains why spreads are not arbitrary: they are the price of immediacy, calibrated to the actual costs of providing it.
Question 4 True / False
In efficient markets, prices adjust instantaneously and costlessly to new information through a passive process that doesn't involve any friction or strategic behavior from market participants.
TTrue
FFalse
Answer: False
Market microstructure shows that price discovery is an active, friction-laden process, not a passive one. When an informed trader places an order, the resulting trade signals information to the market maker, who then updates quotes. This price impact — and the spread that compensates for adverse selection — is the mechanism through which information gets incorporated into prices. The process is neither instantaneous nor costless: there is a bid-ask spread (friction), price impact (cost to the informed trader), and strategic behavior (market makers updating quotes to protect themselves). The EMH's simplest form abstracts over this mechanism; microstructure fills it in.
Question 5 Short Answer
Explain why bid-ask spreads are systematically wider for less liquid, less-followed securities, using the concept of adverse selection.
Think about your answer, then reveal below.
Model answer: A market maker profits from buying at the bid and selling at the ask but loses money when trading against counterparties with private information about the stock's true value — because those counterparties know something the market maker doesn't and will trade only when it's to their advantage. The spread must be wide enough that profits from uninformed order flow offset these adverse-selection losses. For less liquid, less-followed stocks, the proportion of order flow that is informationally motivated (from insiders, concentrated analysts, corporate events) is higher, and less external monitoring keeps private information from being quickly distributed. Therefore, the expected loss per informed trade is higher and must be offset by wider spreads on all trades. For highly liquid stocks with many participants and continuous analyst coverage, information leaks quickly into public view, reducing the informational advantage of any single trader.
This is why market makers aren't simply greedier for small-cap stocks — they face a different adverse selection environment. A hedge fund that has done proprietary research on a small obscure company has a much larger informational edge than one trading a highly covered large-cap where dozens of analysts continuously process the same public information. The spread is the market maker's protection against this asymmetry, and its width is a rational response to the information environment.