Stats on prison populations now (in the form of an economist article on private-sector prisons): http://www.economist.com/people/displayStory.cfm?story_id=4342571
Summary of Foucault's "Discipline and Punish":
"In Discipline and Punish: The Birth of The Prison, Michel Foucault examines the evolution, or devolution, of western society’s theories on punishment. He attempts to catalogue the various changes in penal structure, and their causes, throughout the last four hundred years, asking several important questions. What is it that has led the drive towards ‘humanizing’ punishment, and are the contemporary results of these theories any more successful than those of the seventeenth century? One of the most important conclusions he draws from these questions centers on the idea that, whereas the techniques of societies past focused on the body as the recipient of discipline, contemporary practices punish instead the ‘soul’ of the criminal, a soul which is itself a human creation, “born…out of methods of punishment, supervision, and constraint” (p. 29). Rather than encourage ‘humane’ punishments, this transition contributes instead to the resurgence and embedment of criminality within our society. But underneath this detailed analysis of the way we treat our condemned lies the assertion that conceptions of crime and its justified punishment are a direct result of the power relations in society, most importantly between those who rule and those who are ruled. He attempts to show that society not only molds its treatment of criminals to aid certain trends in state and economical development, but that society’s methods of punishment are themselves molded by these trends. The rise in certain methods of punishment thus mirrors the structure of general authority over the individual, be it in school, the military, or the factory. Foucault seems to conclude that the shift towards punishment of the soul, through its regulated and orderly discipline, is the result of the needs for efficiency and economy that developed with the growth of capitalistic and entrepreneurial enterprises within society." http://faculty.arts.ubc.ca/tiberg/Poli390-week4-Web/Olivier-Foucault.htm
obligatory wikipedia article: http://en.wikipedia.org/wiki/Prisons_in_the_United_States