This work is largely based on what has been a mammoth-one person called it a "heroic" -research project. Both fieldwork and data analyses were laborious and time-consuming, and the work could not have come to fruition without the cooperation of many people. Above all, I owe a debt of gratitude to the mothers and fathers who recognized the importance of such an investigation in building a secure knowledge base concerning human development and who kindly allowed us to come into their homes. The children, at 2 V2, did not have such an appreciation, but naturally I am very grateful to them for the star roles they played in the work. I have to thank all my collaborators for their help in various aspects of the research: Walter Zwirner was statistical consultant to the project, and Pat Olsen and Arlene Grineau were the chief research assistants-! owe particular thanks to them. Others who helped generously with data collection or data analysis (including program writing) were Pat Bachor, Valerie Becker, Rob Black, Doreen Darby, Judy Eser, Con Ferris, Susan Horsley, "Jagan," Ann Johnson, Wayne Miller, Sambhu Nath, Deanna Piwowar, Bruce Roe, Ken Ryba, Laurel Saville, Cecilia Schnurr, Terry Taerum, Debbie Twaddle, and John Wrenshall. Sherry Pitcher kindly prepared the index. Dorice Conway and Reginald Sauve collaborated in the analysis of identical-fraternal twin differences (Chapter 4); Nicholas Martin and Lindon Eaves were chiefly responsible for the biometric-genetic analysis of the data (Chapter 9).