Why Bid-Ask Spreads Widen Under Stress

The difference between the price at which something can be bought and the price at which it can be sold is a cost paid on every transaction. That cost varies with conditions.

Who is on the other side of the quote

Quotes are supplied largely by firms that stand ready to buy and sell continuously, earning the spread as compensation for providing that service.

Such a firm takes on inventory it did not choose, holding a position until an offsetting trade arrives, and bears the price risk in between.

The spread must cover that inventory risk, the risk of trading against someone better informed, and the firm's costs of operating.

What stress does to those risks

When prices are moving rapidly, the risk of holding inventory rises sharply, since the position can lose value before it can be offset.

The probability of trading against someone with better information also rises during news-driven moves, which raises the price of quoting.

Firms respond by widening quotes and reducing the size they will transact, both of which raise the effective cost to anyone needing to trade.

Depth matters as much as spread

A quoted spread describes the price for a small order. Larger orders consume the available size and execute progressively worse.

Under stress, quoted size falls even where the spread appears normal, so the visible quote understates the cost of a meaningful transaction.

This is why measured trading costs during volatile periods far exceed what quoted spreads alone would suggest.

Where this bites hardest

Securities that trade thinly in calm conditions can become extremely expensive to trade in stressed ones, since there was little depth to begin with.

Certain bond markets, small companies and niche exchange-traded products all fall into this category, and they are frequently held precisely for diversification.

The result is that the assets expected to help in a difficult period can be the most costly to access during one.

The implication for planning

Liquidity should be assessed under the conditions in which it will be needed, not the conditions in which it is being examined.

Holding a portion of a portfolio in instruments that remain readily tradable reduces the chance of being forced to transact in the expensive part.

Limit orders control the price paid but not whether the trade occurs, which is the tradeoff facing anyone transacting in a disorderly session.