The Two Sides Of Liquidity: Supply And Demand

A trader wants to sell 20,000 shares of a mid-cap stock right now. The order book shows only 3,000 shares resting on the bid at the best price. Where does the rest of the liquidity come from? Not from a warehouse of pre-existing shares — it comes from someone deciding, in real time, that they are willing to buy at a worse price than the current bid. Liquidity is not a stockpile; it is a willingness, and that willingness has a price.

It helps to think of liquidity the way an economist thinks of any market: as supply and demand, except the good being traded is immediacy itself. On the supply side sit market makers and other liquidity providers who post resting limit orders. They are effectively selling a service — "trade with me now" — and they charge for it through the bid-ask spread. On the demand side sit traders who need to transact urgently: a fund covering a redemption, a hedger closing a position before a number, an algorithm exiting a stop. They are buying immediacy, and they pay for it by crossing the spread or by moving the price when the resting size runs out.

The key insight is that supply is not static. A market maker's willingness to supply liquidity depends on their own inventory and on how confident they are that the person hitting their quote isn't holding better information than they are. If a market maker senses that a wave of selling is informed — that the seller knows something the market doesn't — they pull their bids rather than keep absorbing shares at a price that is about to fall further. That single behavioral fact is why liquidity supply shrinks exactly when demand for it spikes: a selloff makes market makers more worried, not less, so they widen and thin out right when everyone wants to sell.

A worked example

Suppose a market maker is quoting a stock at $50.00 bid / $50.02 ask, 2,000 shares on each side, and normally refreshes the book within 400 milliseconds after a fill. A large sell program starts working the bid. After the first 2,000 shares are absorbed, the market maker's revised quote isn't $50.00 again — it's $49.97 bid, and only 800 shares deep, because two consecutive large sells have raised the probability (in the market maker's model) that this is informed flow rather than random noise. After another 2,000 shares clear that level, the bid steps down to $49.90 with 500 shares. The seller who needed 20,000 shares out has, in effect, walked down a demand curve that got steeper with every fill — not because the stock's "true value" fell by that much, but because the market's willingness to supply liquidity fell faster than the seller's demand for it.

Contrast that with a calm market: the same 20,000-share sell order, spread evenly over two hours with no other selling pressure, might only walk the price down 3-4 cents total, because market makers have time to reload their quotes between clips and never build up the suspicion that they're being adversely selected.

Where this shows up

• Execution algorithms are built entirely around this asymmetry: the whole point of a schedule like TWAP or VWAP is to look like patient, uninformed demand so liquidity suppliers don't pull away.
• Adverse selection models (Kyle, Glosten-Milgrom) formalize the market maker's side of this: quote wider or thinner the more informed you believe the flow to be.
• Crisis liquidity dynamics: the 2008 and 2020 selloffs both featured this feedback loop at a market-wide scale — falling prices triggered forced selling, which market makers read as informed, so they withdrew supply exactly when demand for liquidity was highest, amplifying the move.

In interviews

If asked "what is liquidity," resist the urge to define it as book depth. Define it as a market for immediacy with two sides, note that the supply side reacts to inferred information in the demand side, and give the selloff example: liquidity is thinnest exactly when everyone wants it most. That single observation — supply and demand for liquidity are negatively correlated in stress — is the one interviewers are usually fishing for, and it sets up Cascading Liquidity Withdrawal In A Selloff and Measuring Resilience: The Liquidity Half-Life as natural follow-ons.