Option prices and model-free measurement of implied herd behavior in stock markets

In this paper, we introduce two classes of indices which can be used to measure the market perception concerning the degree of dependency that exists between a set of random variables, representing different stock prices at a fixed future date. The construction of these measures is based on the theory of comonotonicity. Both types of herd behavior indices are model-free and risk-neutral, derived from available option data. Depending on its particular definition, each index represents a particular aspect of the market sentiment concerning future co-movement of the underlying stock prices.