The extraversion continuum, one of several basic higher-order personality dimensions, has been under intense study since the inception of personality psychology. However, the specific causal origins of individual differences in extraversion remain almost entirely mysterious. Recent theories and some findings suggest that personality variation can be orchestrated directly by specific genetic polymorphisms. Few studies, however, have examined an alternative hypothesis that personality traits are facultatively calibrated to variations in other phenotypic features, and none have considered how these distinct mechanisms may interact in personality determination. The current investigation proposes: (1) that the maintenance of individual differences in extraversion over evolutionary history reflects reproductive cost-benefit trade-offs that have occurred at different levels along its continuum; (2) that an individual’s relative standings on association value (i.e. one’s ability to benefit others as an associate) and formidability (i.e. one’s ability to inflict costs on others), respectively, predict the cost-benefit ratio of extraverted strategies; and therefore (3) that extraversion should be facultatively calibrated to variations in these phenotypic characteristics. Chapter II demonstrates that possession of cues to association value and formidability—physical attractiveness and physical strength—does indeed predict individual differences in extraversion, and that these patterns of multi-trait association are unlikely to be explained by the androgen receptor gene polymorphism (despite the fact that this polymorphism had effects on both men’s strength and extraversion). In Chapter III, this logic is extended to show that the multi-dimensional personality coordination occurring within an extraversion-related “personality syndrome” (encompassing, e.g., extraversion, attachment style, emotionality) likely reflects the fact that each of the constituent trait dimensions is facultatively calibrated to association value and/or formidability. In Chapter IV, it is argued that one’s relative association value and/or formidability may shift across behavioral audiences of distinct adaptive significance, and that one’s level of extraversion should therefore be adaptively tailored to the current behavioral audience. Consistent with this, it is shown that people exhibit variable “audience-specific extraversion signatures”, which may be calibrated independently within individuals. Together, these studies provide support for a novel theory of extraversion, and may serve as a template for investigating personality origins from an evolutionary psychological perspective.