  Spin models of markets inspired by physics models of magnetism, as the Ising
model, allow for the study of the collective dynamics of interacting agents in
a market. The number of possible states has been mostly limited to two (buy or
sell) or three options. However, herding effects of competing stocks and the
collective dynamics of a whole market may escape our reach in the simplest
models. Here I study a q-spin Potts model version of a simple Ising market
model to represent the dynamics of a stock market index in a spin model. As a
result, a self-organized gain-loss asymmetry in the time series of an index
variable composed of stocks in this market is observed.
