Why are humans so clever? The 'Social intelligence' hypothesis explores the idea that this cleverness has evolved through the increasing complexity of social groups. Our ability to understand and control nature is a by-product of our ability to understand the mental states of others and to use this knowledge to co-operate or deceive. These abilities have not emerged out of the blue. They can be found in many social animals that co-operate and compete with one another, birds as well as mammals. This book brings together contributions from an impressive list of authorities in the field, appropriately concluding with a chapter by Nick Humphrey (one of the pioneers in this field). This volume examines social intelligence in many different animal species and explores its development, evolution and the brain systems upon which it depends. Better understanding and further development of social intelligence is critical for the future of the human race and the world that we inhabit. Our problems will not be solved by mere cleverness, but by increased social co-operation.