  Mean field games (MFG) are dynamic games with infinitely many infinitesimal
agents. In this context, we study the efficiency of Nash MFG equilibria:
Namely, we compare the social cost of a MFG equilibrium with the minimal cost a
global planner can achieve. We find a structure condition on the game under
which there exists efficient MFG equilibria and, in case this condition is not
fulfilled, quantify how inefficient MFG equilibria are.
