Stimulus over-selectivity refers to behavior being controlled by one element of the environment at the expense of other equally salient aspects of the environment. This is a common problem for many individuals, including those with autism spectrum disorders, and learning difficulties, and presents a considerable problem for information processing in many important situations involving complex cues and environments. Three experiments explored the strength and generality of the over-selectivity effect in non-clinical adults undertaking a cognitively demanding task, by training and testing participants on a two-component trial-and-error discrimination learning task. The over-selectivity effect was found in a variety of test conditions, including when the comparison test stimulus was neutral (Experiment 1), novel (Experiment 2), neutral with no conditioning history (Experiment 3), or when punished during training (Experiments 1, 2 and 3). Such results provide ubiquity to the phenomenon, making it important to investigate further.