I. Introduction -- II. Intelligence as a General Ability -- III. Separable Aspects of Early Intelligence -- IV. Expressive and Receptive Lexical Ability as Aspects of Intelligence -- V. Nonverbal Aspects of Intelligence -- VI. The Present Study -- VII. Method -- VIII. Calculation of Variables -- IX. Relation Among Dependent Variables -- X. Group Differences -- XI. Individual Differences -- XII. Genetic and Environmental Influences -- XIII. Brief Summary of Results -- XIV. Limitations on Generalizations -- XV. The Development of Intelligence -- XVI. Genetic and Environmental Influences on Intelligence -- XVII. Conclusion -- App. B. Technique for Categorizing Patterns of Change -- Commentary: Infants' Cognitive Development: Trajectory and Segue / Adam P. Matheny, Jr. -- Statement of Editorial Policy