Most explanations of the unprecedented increase in American incarceration rates are inadequate. Crime rate increases, more punitive public attitudes, postmodernist angst, and cynical politics are all only part of the explanation. Those things characterize all Western countries; in some of these countries, imprisonment rates have long been stable or declining, and, where they are rising, absolute levels and rates of increase are dwarfed by those in America. The scale of the phenomenon is distinctly American. It arises partly from American moralism and partly from structural characteristics of American government that provide little insulation from emotions generated by moral panics and long-term cycles of tolerance and intolerance.