The tension between cooperative and selfish impulses is a challenge for every society. But how is this problem perceived by individual participants in the context of a behavioral games experiment? We first assess individual differences in players’ propensity to cooperate or defect in a series of experimental games. We then use open-ended interviews with a subset of those players to investigate the various concepts (or ‘frames’) they use when thinking about self-interested and cooperative actions. More generally, we hope to raise awareness of player's perceptions of experimental environments to inform both the design and interpretation of experiments and experimental data.