He developed with Eric Maskin the first "folk theorem" for games in which players do not directly observe each other's decisions, with applications for learning in games. They argued that while learning theories cannot provide detailed descriptions of non-equilibrium behavior, they act as a useful tool in understanding which equilibria are likely to emerge.
With Eric Maskin, he created the first "folk theorem" for games where players don't directly see each other's choices. This theory is helpful for understanding how learning works in games and what kinds of outcomes might arise.