This chapter explains why the TSP framework elevates goal content focused on humans living and working together in cooperative groups above the many other personal goal themes evident in human goal striving, such as happiness, self-determination, and positive self-evaluations (as cataloged in the twenty-four-category Taxonomy of Human Goals presented in Chapter 3). Citing evidence from developmental and social psychology, experimental economics, social neuroscience, and the evolutionary human sciences, this chapter asserts that the core defining feature of humanity (from a motivational perspective) is not self-interest but social purpose. Consistent with this premise, readers will learn not only how social purpose evolved but how that achievement enabled humans to soar above all other species with respect to cultural and intellectual accomplishments. This chapter also directly tackles the common misconception (in Western cultures) that social purpose is merely “self-interest in disguise,” and why invalidating that fallacy is essential for continued human progress.