THE 'iterated prisoner's dilemma' is now the orthodox paradigm for the evolution of cooperation among selfish individuals. This viewpoint is strongly supported by Axelrod's computer tournaments, where 'tit for tat' (TFT) finished first¹. This has stimulated interest in the role of reciprocity in biological societies^1–8. Most theoretical investigations, however, assumed homogeneous populations (the setting for evolutionary stable strategies^9,10) and programs immune to errors. Here we try to come closer to the biological situation by following a program⁶ that takes stochasticities into account and investigates representative samples. We find that a small fraction of TFT players is essential for the emergence of reciprocation in a heterogeneous population, but only paves the way for a more generous strategy. TFT is the pivot, rather than the aim, of an evolution towards cooperation.