The three chapters in Part I offer a framework for the conceptual and empirical data presentations from the JYLS and the FinnTwin studies that follow in Parts II, III, and IV. Chapter 1 by Pulkkinen offers an integrated introduction to the history, design, and conceptual framework of the Jyväskylä Longitudinal Study of Personality and Social Development. The JYLS is a long-term study that Pulkkinen began as her doctoral dissertation completed in 1969. Created as a model of individual differences in children's socioemotional behavior, the conceptual framework of the JYLS evolved into a more general perspective of individual differences in emotion regulation. And the more modest goals of investigating associations between children's emotion regulation and their adjustment expanded into a major longitudinal study that has tracked trajectories of education and occupation, family, health, and social integration from mid-childhood into mid-adulthood. A strength of the study is that its childhood assessments of compliant, constructive behavior and self-control permit tracking positive developmental outcomes into adulthood, and the Jyväskylä data show that predictive factors for positive outcomes accumulate over development, an interesting and important complement to the process of cumulative continuity sketched in the Introduction for negative, maladaptive behaviors.

Chapter 2 by Rose reviews basic principles of gene-environment (g–e) interaction and correlation. People select many of the environments in which they live their lives (g–e correlation), and age-to-age continuities in behavior are understood, in part, as consequences of active person-situation correlations.