  Collective phenomena with universal properties have been observed in many
complex systems with a large number of components. Here we present a
microscopic model of the emergence of scaling behavior in such systems, where
the interaction dynamics between individual components is mediated by a global
variable making the mean-field description exact. Using the example of
financial markets, we show that asset price can be such a global variable with
the critical role of coordinating the actions of agents who are otherwise
independent. The resulting model accurately reproduces empirical properties
such as the universal scaling of the price fluctuation and volume
distributions, long-range correlations in volatility and multiscaling.
