Boone et al. (Boone, C., De Brabander, B., & van Witteloostuijn, A. (1999a). Locus of control and strategic behaviour in a prisoner's dilemma game. Personality and Individual Differences. 27, 695-706; Boone, C., De Brabander, B., & van Witteloostuijn, A. (1999b). The impact of personality on behaviour in five Prisoner's Dilemma games., Journal of Economic Psychologic 20, 343-377) showed that subjects with an internal locus of control were, on average., more cooperative in a prisoner's dilemma (PD) game than subjects with an external locus of control. They conjectured that this finding should not be interpreted as evidence for stable differences in cooperative behaviour between internals and externals. Specifically. they suggested that it is the capacity to adapt to different circumstances over time that distinguishes internals from externals. In the present study we want to investigate the validity of this proposition. We argue that in a PD setting individuals gradually learn to understand the subtle interplay between cooperation and self-interest. Repetition and learning breed cooperation because people learn to understand that cooperation is instrumental in obtaining long-run profit. There is. however, good reason to believe that individuals differ as to the speed of learning to cooperate. We hypothesise that internals are more astute in learning to cooperate in a PD game because they are more endowed with the cognitive faculties necessary for quick learning than externals. Our empirical findings indeed reveal that externals play less cooperatively, on average., in the first part of a series of PD games. However. this difference gradually disappears. In fact. the experiment suggests that learning and repetition reduce the impact of individual differences. By way of appraisal. implications for further research are further discussed.