  We consider the adaptive learning rule of Harley (1981) for behavior
selection in symmetric conflict games in large populations. The rule uses
organisms' past, accumulated rewards as the predictor for the future behavior,
and can be traced in many life forms from bacteria to humans. We derive a
partial differential equation (PDE) that describes the stochastic learning in a
population of agents. The equation has simple structure of the `conservation of
mass'-type equation in the space of stimuli to engage in a particular type of
behavior. We analyze the solutions of the PDE model for typical 2x2 games. It
is found that in games with small residual stimuli, adaptive learning rules
with faster memory decay have an evolutionary advantage.
