Why do we like to stay with our friends? Modelling the evolutionary dynamics of interpersonal commitment

Why are people inclined to build friendships and maintain durable, nonreproductive relationships? Previous computational modeling work on the evolution of commitment in repeated exchange showed that being largely unconditionally cooperative in committed relationships is more viable than conditional cooperation based on fair reciprocity (e.g. Tit for Tat). But this work did not conduct a sufficiently strict test of the viability of commitment because it did not allow for the spontaneous emergence of "smart cheaters" who take advantage of the tolerance of commitment players. We extend previous studies with the possibility of randomly mutating strategies under evolutionary pressures, and give a much larger coverage of an infinite strategy space. Our results show the lack of evolutionary stable strategies in our dilemma setting. We find that emerging populations alternate between temporarily stable states that can be characterized as either cooperative or opportunistic. Using the simulated results as input to statistical analysis, we show that the viability of strategies increases with increasing levels of interpersonal commitment, and that the effect of interpersonal commitment on viability is larger than the effect of fairness.