The current four experiments examined the sunk cost effect—nonoptimal persistence following investment—in a laboratory‐based decision‐making task with adult humans. Subjects made repeated decisions about whether to persist in a course of action—a fixed‐ratio schedule whose response requirements varied unpredictably from one trial to the next—or to abandon it and escape in favor of a new trial with a potentially smaller fixed ratio schedule. Satisfying the ratio requirement produced a brief video clip from a preferred television program. In Experiment 1, sunk‐cost errors were less likely in subjects who had previously experienced markedly differential reinforcement for escape. In Experiment 2, stimulus changes correlated with changes in mean response requirement, and these changes reduced sunk‐cost errors in a small number of subjects. In Experiment 3, sunk‐cost errors occurred more frequently as the ratio of the mean response requirements for persistence and escape approached 1.0. The importance of this variable was further supported by the results of Experiment 4, in which features other than this ratio did not markedly alter performance. These four experiments identified some key determinants of whether humans commit the sunk‐cost error and confirmed the utility of video clips as reinforcers in experimental research with humans.