Discussion paper

DP9958 Man or machine? Rational trading without information about fundamentals

We present a model of quantitative trading as an automated system under human supervision. Contrary to previous literature we show that price-contingent trading is the profitable equilibrium strategy of large rational agents in efficient markets. The key ingredient is uncertainty about whether a large trader is informed about fundamentals. Even when uninformed, he still learns more from prices than market participants who still wonder about whether he is informed. Therefore, he will trade a non-zero quantity based on past prices, whose direction ? trend-following or contrarian ? depends on parameters. When informed, he will trade on that information and disregard the algorithm. One implication is that future order flow is predictable even if markets are semi-strong efficient by construction.


Rossi, S and K Tinn (2014), ‘DP9958 Man or machine? Rational trading without information about fundamentals‘, CEPR Discussion Paper No. 9958. CEPR Press, Paris & London. https://cepr.org/publications/dp9958