  We present and study a Minority Game based model of a financial market where
adaptive agents -- the speculators -- interact with deterministic agents --
called producers. Speculators trade only if they detect predictable patterns
which grant them a positive gain. Indeed the average number of active
speculators grows with the amount of information that producers inject into the
market. Transitions between equilibrium and out of equilibrium behavior are
observed when the relative number of speculators to the complexity of
information or to the number of producers are changed. When the system is out
of equilibrium, stylized facts arise, such as fat tailed distribution of
returns and volatility clustering. Without speculators, the price follows a
random walk; this implies that stylized facts arise because of the presence of
speculators. Furthermore, if speculators abandon price taking behavior,
stylized facts disappear.
