A Simple Automated Market Maker for Prediction Markets

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There is wide scope for reliance on automated “robot” market makers in prediction markets and market simulation games in experimental economics and behavioral finance. The market maker presented here is an alternative to the well-known but less easily understood Hanson market maker. It has the advantage of being easy to derive and makes a good mathematical introduction to the logic of automated bid and ask price–setting in prediction markets. Its main advantage is that the opening security price can be set arbitrarily between zero and one, so as to match the market maker’s prior beliefs. A weakness of the Hanson market maker is that it opens automatically with a uniform prior distribution. In many real-world applications, this is unrealistic and prone to cause the market maker unnecessary trading losses (on average). Common practice, such as in betting markets and over-the-counter financial markets for binaries, is to set opening prices based on expert subjective probabilities.