This work tackles the question of why people's, and in particular sibling's, personalities differ. In doing so it tries to answer the fundamental quuestion: "What makes us the way we are?" The author challenges standard thinking on the role of parental influence on children. She argues, cogently and persuasively, that socialisation takes place in the peer group rather than in the family. Her controversial views are backed up by the findings of behavioural genetics (particularly on siblings, twins and adopted children) and lead to an evolutionary understanding of individual differences. For example, children almost always end up with the accents of their peers rather than their parents; children of culturally inept immigrants develop perfectly if they can learn from native-born peers. This book brings together behavioural genetics (the study of differences between people) and evolutionary theory (the study of what is universal in our species). The integration of these two approaches is a rare one in popular science writing, and one which will make a significant contribution to the science of our species.