This book looks at 'altruism' -- but carefully distinguishes between psychological altruism (at a mental, intentional level) and evolutionary altruism (at the genetic level), what is perhaps better described now as "costly helping." Sober and Wilson dissect causes superbly, profiling the interaction between "selfishness" at one level with benefits to a cooperative collective on another. They ultimately support a major role for group selection, although without having fully considered the role of social selection and sanctioning, which have become more prominent since they published their book.