Flexible behavior or flexible methods? A cross-taxon review of experimental designs in reversal learning

Behavioral flexibility—the ability to adapt behavior in response to changing conditions—is widely recognized as a key feature of animal cognition. It is often measured using reversal learning tasks, where individuals must inhibit a previously rewarded response and adopt a new one after contingencies shift. Despite its widespread use, the comparability of these tasks across species remains unclear. This paper establishes a foundation for resolving this issue by examining how reversal learning has been designed and implemented across taxa. We conducted a systematic review of 206 empirical studies (2014–2023) spanning eight major taxonomic groups: invertebrates, fishes, amphibians and reptiles, birds, rodents, non-human primates, humans, and other mammals. For each study, we extracted variables related to taxon coverage, sampling, learning and reversal criteria, cue types, and outcome measures. Analyses included nonparametric tests to assess group-level differences, linear discriminant analyses to explore multivariate structure, and model-based robustness checks. We identified three methodological obstacles to understanding reversal learning across diverse taxa. First, the distribution of research is highly imbalanced: birds, rodents, and humans accounted for most of the populations in the reviewed work. When considering species coverage, most animal diversity—especially invertebrates, fishes, and amphibians and reptiles—remains virtually untested, with less than 1% of described species included per taxon. Second, research is taxonomically fragmented: 99% of studies focus on a single group, limiting opportunities for direct comparison. Third, and most critically, methodological standards diverge dramatically across taxa. Humans are consistently held to the strictest learning criteria, while other taxa most often use lower thresholds. The number of reversal phases differ more than threefold among taxa. Nearly all studies of amphibians and reptiles, fishes, and invertebrates use single-reversal designs, whereas multi-reversal protocols are much more common in humans and non-human primates. Sample sizes, evaluation window lengths, cue types, and outcome metrics also display taxon-specific patterns. These systematic differences in experimental design introduce structural asymmetries that complicate cross-taxon comparisons, blurring the line between true cognitive variation and methodological artifacts. Although research to date has advanced our understanding, further progress will depend on greater methodological coordination and broader taxonomic coverage. Emerging large-scale collaborations are beginning to address these gaps, offering a promising path toward a more robust and equitable science of behavioral flexibility.