From mitochondria to meerkats, the natural world is full of spectacular examples of social behaviour. In the early 1960s W. D. Hamilton changed the way we think about how such behaviour evolves. He introduced three key innovations - now known as 'Hamilton's rule,' 'kin selection,' and 'inclusive fitness' - and his pioneering work kick-started a research programme now known as social evolution theory. His work has been enormously influential, but remains the subject of fierce controversy. This is a book about the philosophical foundations and future prospects of social evolution theory. In Part I, 'Foundations', Jonathan Birch provides a careful exposition and defence of Hamilton's ideas, with a few modifications along the way. In Part II, 'Extensions', Birch shows how these ideas can be applied to phenomena including cooperation in microorganisms, cooperation among the cells of a multicellular organism, and culturally evolved cooperation in the earliest human societies. Birch argues that real progress can be made in understanding microbial evolution, evolutionary transitions, and human evolution by viewing them through the lens of social evolution theory, provided the theory is interpreted with care and adapted where necessary. This book, the first book-length philosophical study of Hamilton's ideas, places social evolution theory on a firm philosophical footing and sets out exciting new directions for further work. It is essential reading for philosophers of science, evolutionary biologists, and evolutionary social scientists. -- from dust jacket.