Softening and Hardening in the Iterated Prisoner’s Dilemma

This article presents two methods to evaluate and draw distinction among general cooperative and aggressive strategic behavior in the iterated prisoner’s dilemma (IPD). The first method identifies classes of strategies, clusters them into subclasses based on their cooperative or aggressive disposition, and then comprehensively assess their behavior. The second method relies on applying transformative manipulations that soften or harden strategies of a given set and compare them with the results obtained with the initial set. Although examined classes of initial strategies are very different, the results are stable and convergent. While our findings agree with the classical analyses of the IPD, the results are qualified and precise. Furthermore, the reproducibility of prior results via new experimental methods offer experimental evidence. The proposed comparative strategy analysis method is a new instrument that can have broad implications for analyzing games beyond the prisoner’s dilemma.