Why are humans sometimes generous to each other? This remains a central question in social psychology, evolutionary biology, experimental economics, sociology, and other disciplines: if generous individuals provide benefits to others at a cost to themselves, then these costs imply that generosity will not evolve, not be learned, and/or not be chosen rationally. Despite these costs, humans often do help others at a cost to themselves. This book suggests that generous individuals can benefit in many ways for their acts, such that these reputational benefits can help explain why co-operative sentiment evolved and/or is learned through social reinforcement.