Unstable Price Dynamics as a Result of Information Absorption in Speculative Markets

In speculative markets, risk-free profit opportunities are eliminated by traders exploiting them. Markets are therefore often described as “informationally efficient”, rapidly removing predictable price changes, and leaving only residual unpredictable fluctuations. This classical view of markets absorbing information and otherwise operating close to an equilibrium is challenged by extreme price fluctuations, in particular since they occur far more frequently than can be accounted for by external news. Here we show that speculative markets which absorb mainly self-generated information can exhibit both: evolution towards efficient equilibrium states as well as their subsequent destabilization. This peculiar dynamics, a generic instability arising from an adaptive control which annihilates predictable information, is realized in a minimal agent-based market model where the impacts of agents’ strategies adapt according to their trading success. This adaptation implements a learning rule for the market as a whole minimizing predictable price changes. The model reproduces stylized statistical properties of price changes in quantitative detail, including heavy tailed log return distributions and volatility clusters. Our results demonstrate that the perpetual occurrence of market instabilities can be a direct consequence of the very mechanisms that lead to market efficiency. Social systems self-organize. In consequence, collective behaviors can emerge that appear to pursue a common goal which is actually not present in the behavior of the single agents (1). The view that markets in fact operate in distinguished equilibrium states became influential in economics (2). Here, a fundamental hypothesis is that markets operate informationally optimal. That is, prices are assumed to ”fully reflect available information” (3), or at least come close to this limit (4), such that risk-free (arbitrage) profits cannot be made by (re-)using said information. If true, one of the implications of this ”Efficient Market Hypothesis” (EMH) is that resulting prices fluctuate randomly (5). Empirical findings in favor of the Efficient Market Hypothesis include the general absence of exploitable autocorrelations among price changes in financial markets (6). The magnitudes of price changes (”volatilities”), however, are correlated for long periods of time. That is,