In 1959 C. P. Snow delivered his now-famous Rede Lecture, “The Two Cultures,” a reflection on the academy based on the premise that intellectual life was divided into two cultures: the arts and humanities on one side and the natural sciences on the other. Since then, a third culture, generally termed “social science” and comprising the fields of sociology, anthropology, political science, economics, and psychology, has grown in importance. Jerome Kagan’s book describes the assumptions, vocabulary, and contributions of each of these cultures and argues that the meanings of many of the concepts used by each community are unique to its methods because the source of evidence contributes to meaning. The text summarizes the contributions of the social sciences and humanities to our understanding of human nature and questions the popular belief that biological processes are the main determinant of variation in human behavior.