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 science on the other. Since then, a third culture, generally termed “social science” and comprised of fields such as sociology, political science, economics, psychology, and anthropology, has emerged. 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 culture are unique to it and do not apply to the others because the source of evidence for the term is special. 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.