Sexual Dimorphism- Various biological studies on the subject of sex differences have been published. The subjects of these studies are not only humans, but also other mammals, birds, amphibians, insects, extending to ostracoda in the Paleozoic era. Moreover, original methods in individual studies have been used. This book provides convincing reasons which explain sex differences. The book also shows that, even if considered that some living things do not have sex differences, in reality they do. The somewhat different data on sex differences in this book will offer new ideas not only to life scientists and biologists, but social and cultural scientists as well.