In this textbook, Simon LeVay and Janice Baldwin aim to help students understand the diversity of human sexual expression as well as the diversity of perspectives from which sexuality can be studied. Well-known for its high-quality presentation of biological aspects of sexuality, the book also devotes rich coverage to the insights gained from cognitive science, social psychology, sociology, feminism, and cross-cultural studies, along with both moral and political discourse on sexual themes. The Fourth Edition provides up-to-date coverage of all topics, ranging from gay marriage in New York to the latest developments in contraceptive technology and exciting findings on pre-exposure prophylaxis against HIV. Still the leader in terms of its biological coverage, the Fourth Edition also has expanded coverage of many other issues, ranging from sex-therapy exercises to learning theories of gender and paraphilias.