Brain size in our lineage has tripled over the last two million years, reflecting the evolution of unprecedented mental and behavioral capacities. Evidence from paleontology, comparative biology, and psychology suggests that human encephalization was driven by some sort of autocatalytic, positive-feedback process, yet Fisher's (1930) theory of runaway sexual selection has been strangely overlooked as a possible explanation. This 600-page thesis presents many arguments and much data in support of the view that the encephalization of the human brain and the evolution of the human mind were driven primarily by runaway sexual selection elaborating the courtship behaviors of both male and female hominids, as both aesthetic displays and viability indicators. A more specific hypothesis is also proposed, in which hominid neophilia (preference for novelty and creativity) in mate choice favored mental capacities for generating various "protean" (adaptively unpredictable) courtship displays in the domains of language, music, art, sexual play, conceptual play, and ideology, thereby driving brain evolution. The mutuality of mate choice and the interactive nature of these courtship behaviors (e.g. conversational and musical dueting) explains the lack of sexual dimorphism in human brain size and mental capacities. Compared to other current theories of human brain evolution through hunter-gatherer foraging, social competition, or gene-culture coevolution, this new sexual selection theory is consistent with a wider range of theory and data, including: (1) the rapidity and uniqueness of human encephalization, (2) the lack of clear climatic, ecological, or technological correlates of encephalization, (3) the evolutionary theory of sexual selection, (4) the evidence for mate choice in animals, including primates and humans, (5) the elaboration of secondary sexual morphology in both male and female humans, and (6) the uniquely elaborated, universal human capacities for language, art, music, sexual play, conceptual play, and ideology, and their conspicuous use in courtship. In short, human psychological complexity and creativity evolved autocatalytically through 'psychological selection' in the form of selective mate choice. This theory allows for much tighter conceptual integration between the evolutionary biology of sexual selection, the evolutionary psychology of human courtship behavior, and the social sciences of human culture.
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