  This paper considers mean field games in a multi-agent Markov decision
process (MDP) framework. Each player has a continuum state and binary action,
and benefits from the improvement of the condition of the overall population.
Based on an infinite horizon discounted individual cost, we show existence of a
stationary equilibrium, and prove its uniqueness under a positive externality
condition. We further analyze comparative statics of the stationary equilibrium
by quantitatively determining the impact of the effort cost.
