The hard–easy effect is a cognitive bias where individuals overestimate success in difficult tasks and underestimate it in easy ones. It is typically identified via calibration curves, where subjective confidence is plotted against the actual proportion of correct responses.The sources explore this phenomenon across diverse domains: animal learning (including brightness, auditory, and flavor discrimination in rats and pigeons), human cognitive tasks (mental arithmetic, general knowledge, and memory review), and complex decision-making under acute stress. These materials evaluate competing interpretations, such as selective attention, stimulus generalisation, and ecological models.Final learnings highlight the ubiquity of the effect, which manifests across various sensory modalities and regardless of individual judge types. In animal psychology, progressive training (transitioning from easy to hard versions of a task) often facilitates learning more effectively than training on hard tasks alone. In human contexts, acute stress and time pressure significantly impair decision quality, leading to higher error rates even at lower complexity levels. Methodologically, some sources suggest the effect might be a statistical artifact resulting from scale-end effects, linear dependency, or biased item selection in experiments. However, other studies maintain that it is a robust indicator of how miscalibrated confidence impacts performance and self-regulated learning. Ultimately, the subject illustrates the complex interplay between perceived difficulty, actual accuracy, and environmental context.