In social relationships-whether between mates, parents and offspring, or friends-we find much of life’s meaning. But in these relationships, so critical to our well-being, might we also detect the workings, even directives, of biology? This book, a rare melding of human and animal research and theoretical and empirical science, ventures into the most interesting realms of behavioral biology to examine the intimate role of endocrinology in social relationships.The importance of hormones to reproductive behavior-from breeding cycles to male sexual display-is well known. What this book considers is the increasing evidence that hormones are just as important to social behavior. Peter Ellison and Peter Gray include the latest findings-both practical and theoretical-on the hormonal component of both casual interactions and fundamental bonds. The contributors, senior scholars and rising scientists whose work is shaping the field, go beyond the proximate mechanics of neuroendocrine physiology to integrate behavioral endocrinology with areas such as reproductive ecology and life history theory. Ranging broadly across taxa, from birds and rodents to primates, the volume pays particular attention to human endocrinology and social relationships, a focus largely missing from most works of behavioral endocrinology.