Partial Adjustment or Stale Prices? Implications from Stock Index and Futures Return Autocorrelations

Abstract

This paper investigates the relation between returns on stock indices and their corresponding futures contracts in order to evaluate potential explanations for the pervasive yet anomalous evidence of positive, short-horizon portfolio autocorrelations. Using a simple theoretical framework, we generate empirical implications for both microstructure and partial adjustment models. These implications are then tested using futures data on 24 contracts across 15 countries. The major findings are (i) return autocorrelations of indices tend to be positive even though their corresponding futures contracts have autocorrelations close to zero, (ii) these autocorrelation differences between spot and futures markets are maintained even under conditions favorable for spot-futures arbitrage, and (iii) these autocorrelation differences are most prevalent during low volume periods. These results point us towards a market microstructure-based explanation for short-horizon autocorrelations and away from explanations based on current popular behavioral models.