  We study the dynamics of a system composed of interacting units each with a
complex internal structure comprising many subunits. We consider the case in
which each subunit grows in a multiplicative manner. We propose a model for
such systems in which the interaction among the units is treated in a mean
field approximation and the interaction among subunits is nonlinear. To test
the model, we identify a large data base spanning 20 years, and find that the
model correctly predicts a variety of empirical results.
