Indirect reciprocity only works when donors discriminate between individuals that have or have not helped others in the past. To understand the role of indiscriminate altruists, we add to the population of defectors and discriminators a fraction z of cooperators, who always give help irrespective of the their co-player's score.

Nowak, M. A., & Sigmund, K, Evolution of Indirect Recip…, loc. 114