This book presents the results of an ethno-psychological study of Mexican siblings which aims to contribute to the study of the relationships between brothers and sisters from the perspective of social psychology. Building upon the Mexican school of ethnopsychology, the study presented in this volume shows how psychosocial processes shape the relationships between siblings and how these relationships affect the development of boys and girls in emerging adulthood (between 18 and 25 years).  The relationship between siblings is the first laboratory through which anyone who has a brother or sister learns to establish social relationships. Taking this into account, the study presented in this book analyzed 1240 emerging Mexican adults to identify the most important psychosocial variables that contribute to the establishment and development of sibling relationships. The results of the study show the central role played by attachment styles in the establishment of this relationship and how both positive aspects – such as closeness, similarity-complementarity and complicity – and negative aspects – such as favoritism, jealousy, conflict and aggression – of the relationships between siblings are essential parts of the developmental processes though which boys and girls learn how to establish social relationships. Brothers and Sister in Emerging Adulthood: An Ethno-Psychological Study of Mexican Siblings will be of interest to social, developmental and family psychologists interested in understanding the psychosocial mechanisms behind the most durable relationship in the life of anyone who has a brother or sister.