Limited profit in predictable stock markets

TL;DR

This paper challenges the idea that predictable stock markets cannot yield profits, showing that market efficiency does not eliminate all arbitrage opportunities and that collective dynamics influence profit potential.

Contribution

It introduces a new measure of market inefficiency based on the maximal profit an ideal trader can extract, revealing limited profits that decay with more agents.

Findings

Predictability alone does not imply market inefficiency.

Market profits decrease as the number of agents increases.

Markets can be both predictable and highly efficient.

Abstract

It has been assumed that arbitrage profits are not possible in efficient markets, because future prices are not predictable. Here we show that predictability alone is not a sufficient measure of market efficiency. We instead propose to measure inefficiencies of markets in terms of the maximal profit an ideal trader can take out from a market. In a stock market model with an evolutionary selection of agents this method reveals that the mean relative amount of realizable profits $P$ is very limited and we find that it decays with rising number of agents in the markets. Our results show that markets may self-organize their collective dynamics such that it becomes very sensitive to profit attacks which demonstrates that a high degree of market efficiency can coexist with predictability.