Amihud (2002) - Illiquidity and Stock Returns: Cross-Section and Time-Series Effects

Overview

Amihud (2002) introduces the ILLIQ measure of stock illiquidity and shows that expected stock returns are positively related to illiquidity both across stocks and over time, establishing liquidity as a priced risk in the cross-section of returns.

The Measure

ILLIQ is the ratio of absolute daily return to daily dollar volume, averaged over the year. The intuition: a stock whose price moves a lot per dollar traded is costly to trade, because the trade itself moves the price. The measure is computable from daily data for decades back — a deliberate design choice that lets Amihud test liquidity over a long sample (NYSE stocks from 1964 to 1997).

The Cross-Sectional Result

Across stocks, expected returns rise with ILLIQ after controlling for size and other characteristics: investors demand compensation for holding assets that are expensive to exit. The effect is economically meaningful, not just statistically present — the most illiquid stocks earn returns that are measurably higher than the most liquid, after risk adjustment.

The Time-Series Result

Over time, expected market returns also respond to market-level illiquidity: when liquidity dries up, required returns rise, which means current prices fall — the mechanism behind the market liquidity shocks that accompany crises. The time-series test uses a stock's sensitivity to aggregate illiquidity (beta to ILLIQ shocks) rather than its own level, showing that the risk is systematic in both senses.

Why It Matters

ILLIQ remains the workhorse liquidity measure in empirical finance because it is computable, robust, and priced. For practitioners it connects market microstructure to asset pricing: expected returns embed liquidity premia, so portfolio construction should not treat a stock's cheapness in isolation from its tradability. For surveillance teams, the same measure quantifies exactly what market impact monitoring seeks to observe.

Key Takeaways

• Illiquidity is a priced risk, not a trading nuisance — expected returns compensate for it.
• Aggregate liquidity shocks move the whole market's required returns, which is how liquidity crises turn into price crashes.
• ILLIQ's design wins: one ratio, daily data, decades of history — computable for any listed market.