Why more choices cause less cooperation in iterated prisoner's dilemma

The classic iterated prisoner's dilemma (IPD) has only 2 choices, cooperate or defect. However, most real-world situations offer intermediate responses, between full cooperation and full defection. Previous studies observed that with intermediate levels, mutual cooperation is less likely to emerge, and even if it does it is less stable. Exactly why has been a mystery. This paper demonstrates two mechanisms that sabotage the emergence of full mutual cooperation. First, to increase cooperation requires behavioral (phenotypic) diversity to explore different possible outcomes, and once evolution has converged somewhat on a particular degree of cooperation, it is unlikely to shift. Secondly, more choices allows a richer choice of stable strategies that are not simply cooperating with each other to exclude an invader, but which are symbiotic. Such non-symmetric and symbiotic players in the space of strategies act as roadblocks on the path to full cooperation.