This book analyzes the MacArthur Longitudinal Twin Study, a collaborative study by leading developmental scientists and behavioral geneticists on the transition from infancy to early childhood. Part 1 of the book describes the twin method and procedures used and introduces the analytic strategies. Parts 2 through 4 present results related to temperament, emotion, and cognition, respectively. Part 5 deals with environmental contributions and cross-domain integrations. Part 6 reviews major themes and conclusions. The chapters are: (1) "The Dynamics of Development in a Unique Multidisciplinary Collaboration" (Emde); (2) "The Twin Method: What We Can Learn from a Longitudinal Study" (Hewitt, Emde, and Plomin); (3) "The Conduct of the Study: Sample and Procedures" (Robinson, McGrath, and Corley); (4) "The Structure of Temperament" (Kagan); (5) "Extremes Analyses of Observed Temperament Dimensions" (Manke, Saudino, and Grant); (6) "Parental Ratings of Temperament in Twins" (Saudino and Cherny); (7) "Sources of Continuity and Change in Observed Temperament" (Saudino and Cherny); (8) "Behavioral Inhibition and Related Temperaments" (Kagan and Saudino); (9) "Emotional Development in the Twin Study" (Robinson and Emde); (10) "Reactions to Restraint and Anger-Related Expressions during the Second Year" (Emde and others); (11) "Empathy and Prosocial Patterns in Young MZ and DZ Twins: Development and Genetic and Environmental Influences" (Zahn-Waxler and others); (12) "Dispositional Cheerfulness: Early Genetic and Environmental Influences" (Robinson, Emde, and Corley); (13)"Cognitive Development, Cognitive Abilities, and Modularity" (Fulker and Plomin); (14) "Experimental Assessment of Specific Cognitive Abilities during the Second Year of Life" (Wilson and others); (15) "Continuity and Change in General Cognitive Ability from 14 to 36 Months" (Cherny and others); (16) "Language and Cognition" (Young and others); (17) "Context and Specificity of Individual Differences and Their Origins" (Emde); (18) "Mothers' Perceptions of Differential Treatment of Infant Twins" (McGuire and Roch-Levecq); (19) "Relationship Context as a Moderator of Sources of Individual Differences in Empathic Development" (Robinson, Zahn-Waxler, and Emde); (20) "The Development of Observed Shyness from 14 to 20 Months: Shyness in Context" (Cherny and others); (21) "Perinatal Effects on General Cognitive Ability, Weight, Height and Temperament" (Martin and Hershberger); (22) "Height, Weight, and Body Mass Index" (Chambers and others); (23) "Temperament, Mental Development, and Language in the Transition from Infancy to Early Childhood" (Kubicek, Emde, and Schmitz); (24) "Early Predictors of Problem Behavior at Age Four" (Schmitz and others); and (25) "An Experiment in Collaborative Science" (Plomin and others). Each chapter contains references. (KB)