Despite decades of research on social development, the question of whether early interpersonal experiences play an important role in human development is just as controversial today as it was decades ago. In this chapter, we argue that there are at least three reasons why this debate has remained unsettled. First, outside of a few landmark longitudinal studies, relatively few investigations have prospectively charted the long-term correlates of experiences with primary caregivers in the first 3 years of life. Second, most analyses of datasets focused on the legacy of early experience have emphasized two-wave test-retest analyses. We contend that, to distinguish between enduring versus merely transient effects of early experience, it is instead necessary to study the pattern of associations that are observed across multiple assessment occasions. Third, despite decades of stinging critiques by behavior-genetic scholars, not enough attention has been paid in social developmental research to determining whether the robust correlations identified between parental behavior and child outcomes are genetically mediated. In this chapter, we discuss each of these issues, in turn, and review recent conceptual models and empirical data that help to address them.