This popular behavioral endocrinology text provides detailed information on what hormones are, how they affect cells, and how such effects can alter the behavior of animals, including humans. Presenting a broad continuum of levels of analysis, from molecular to evolutionary, the book discusses how genes work, the structure of cells, the interactions of endocrine organs, the behavior of individuals, the structure of social hierarchies, and the evolution of mating systems. The second edition, while maintaining the strengths of the first edition, has been thoroughly revised to reflect recent developments in genetics and molecular biology and related social concerns. It contains four new chapters: on the use of molecular biology techniques in behavioral endocrinology, on psychoneuroimmunology, on hormonal influences on sensorimotor function, and on cognitive function in nonhuman animals.