In this unique amalgam of neuroscience, genetics, and evolutionary psychology, Ryan argues that leftists and rightists are biologically distinct versions of the human species that came into being at different moments in human evolution. The book argues that the varying requirements of survival at different points in history explain why leftists and rightists have anatomically different brains as well as radically distinct behavioral traits. Rightist traits such as callousness and fearfulness emerged early in evolution when violence was pervasive in human life and survival depended on the fearful anticipation of danger. Leftist traits such as pro-sociality and empathy emerged later as environmental adversity made it necessary for humans to live in larger social groups that required new adaptive behavior. The book also explores new evolutionary theories that emphasize the role of the environment in shaping not only human political behavior but also humans' genetic architecture. With implications for the future of politics, the book explores how the niche worlds we build for ourselves through political action can have consequences for the evolution of the species. Proposing a new way of understanding human politics, this is fascinating reading for students and academics in psychology, the social sciences, and humanities, as well as general readers interested in political behavior.