Aggregation and Dissemination of Information in Experimental Asset Markets in the Presence of a Manipulator

Abstract

We study with the help of a laboratory experiment the conditions under which an
\nuninformed manipulator - a robot trader that unconditionally buys several shares of a
\ncommon value asset in the beginning of a trading period and unwinds this position later
\non - is able to induce higher asset prices. We find that the average contract price is
\nsignificantly higher in the presence of the manipulator if, and only if, the asset takes the
\nlowest possible value and insiders have perfect information about the true value of the
\nasset. It is also evidenced that the robot trader makes trading gains; i.e., independently
\non whether the informed traders have perfect or partial information, it earns always
\nmore than the average trader. Finally, not only uninformed subjects suffer from the
\npresence of the robot trader, but also some of the imperfectly informed insiders have
\nlower payoffs once the robot trader is added as a market participant.