Richard E. Tremblay, PhD, is Professor of Pediatrics, Psychiatry, and Psychology at the University of Montreal and Director of the Inter-University Research Unit on Children's Psychosocial Maladjustment. For over 20 years, he has conducted a program of longitudinal and experimental studies addressing the physical, cognitive, emotional, and social development of children from conception onward to understand the development and prevention of antisocial behavior. Willard W. Hartup, EdD, is Regents' Professor Emeritus and former Director of the Institute of Child Development at the University of Minnesota. Dr. Hartup has spent many years researching friendship and peer relations in child development, antipathies and their significance, and conflict and aggression in childhood and adolescence. John Archer, PhD, is Professor of Psychology at the University of Central Lancashire, Preston, United Kingdom. His research is concerned with human aggression, grief and loss, and sex differences. Dr. Archer is also the author of several books, including Sex and Gender (with Barbara Lloyd) and The Nature of Grief; numerous book chapters; and over 100 articles in refereed journals covering psychology, medicine, and biology. In recent years, he has published a number of meta-analytic reviews on topics connected with sex differences in aggression.