In slot-machine play, near wins are stimuli that visually approximate winning stimuli but deliver no reinforcers. In two experiments, a categorical discrimination task was embedded in a concurrent chain to investigate how near wins affect preference for probabilistic versus certain food. Pecking in variable-interval initial links produced access to a fixed-ratio (FR) 1-FR 1-FR 1 chain. When all links were red, the chain was a “win” that produced access to food. A “clear loss” chain involved three green stimuli, and in a “near win,” key colors during successive FR 1 links were red, red, and green. In Experiment 1, the magnitude and probability of reinforcement were varied across conditions with and without near wins. Response allocation was sensitive to changes in reinforcer magnitude and probability. Generalized matching analyses revealed a consistent bias for probabilistic over certain outcomes, but only when they included near wins. Response rates on near-win trials were also intermediate to that of clear losses and wins. Near-win probability was varied across conditions of Experiment 2 and probability of near wins was positively associated with bias for probabilistic outcomes. The results from both experiments suggest that near wins encourage individuals to choose to gamble by functioning as conditioned reinforcers
