Preferences for pitch sequence complexity are consistent with an inverted U-shape in humans and parrots

A well-established inverted U-shape relationship links stimulus complexity and human music preference, with preferences peaking at intermediate levels. We investigated whether preference for intermediate complexity is unique to humans or shared with other species. We compared the acoustic preferences of humans and budgerigars, a species known for their vocal learning and pattern recognition abilities, using a three-option place preference paradigm. Participants were presented with choices between stimuli of different, computationally defined complexity: pitch sequences with tones arranged either randomly (high complexity), repeating patterns with changes (medium) or fully repetitive patterns (low). Our results demonstrate that, without specific goals or external rewards, both humans and budgerigars spontaneously exhibit a nonlinear (intermediate) preference profile, indicating a pattern consistent with an inverted U-shape relationship. Because a U-shape relationship may reflect a strategy to seek informative sources maximally useful for learning-that is, for building and refining internal models of the environment-this preference in the acoustic domain may support vocal learning. However, further testing could determine whether it also exists in vocal non-learning species. Because vertebrates show inverted U-shape responses in a variety of non-auditory contexts, our findings may stem from general processes, but potentially depend on vocal learning to translate to acoustic contexts.