Complexity is a unified cognitive kind

Any information-processing system must deal with the complexity of its input. Yet, complexity arises in many forms: An image may be complex, a melody may be complex, and so too for linguistic or mathematical expressions. Are these disparate forms of complexity ‘unified’ in the mind, such that cognition represents a common quantity shared across qualitatively different types of information? Here, 11 experiments (N=1500) reveal domain-general representation of complexity. First, a reward-transfer task revealed that outcomes associated with complexity generalize across diverse stimulus classes, including shapes, dot-arrays, melodies, letter-strings, mathematical expressions, and tactile forms. Subsequent experiments demonstrated that such transfer occurs automatically (intruding on task-irrelevant judgments) and underwrites individual differences in higher-level judgments across domains; for example, participants who find simple shapes aesthetically pleasing also find simple melodies pleasing. These results implicate a capacity for type-independent representation of information density, consistent with a shared representational language across cognitive domains.