Reports Adam Liptak in the NY Times: "The United States has less than 5 percent of the world’s population. But it has almost a quarter of the world’s prisoners.
"Indeed, the United States leads the world in producing prisoners, a reflection of a relatively recent and now entirely distinctive American approach to crime and punishment. Americans are locked up for crimes — from writing bad checks to using drugs — that would rarely produce prison sentences in other countries. And in particular they are kept incarcerated far longer than prisoners in other nations.
"Criminologists and legal scholars in other industrialized nations say they are mystified and appalled by the number and length of American prison sentences." Read the rest, and ignore the typical anti-gun baloney.
NB: At the other end of the country spectrum, the tiny free-state of San Marino has the smallest percentage in prison. Maybe this is a better measure of relative freedom than those neocon Heritage or Koch Foundation studies.
As to why the US is the worst, I'll bet that the Republican public-works project of government-business (i.e. fascist) prisons is responsible, along with the Republican federalization of the criminal justice system, and the Republican step-up in the drug war under Nixon, Reagan, et al. (Thanks to Tom Lombardi.)