From the New York 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. The United States has, for instance, 2.3 million criminals behind bars, more than any other nation, according to data maintained by the International Center for Prison Studies at King’s College London.
China, which is four times more populous than the United States, is a distant second, with 1.6 million people in prison.
Thank the war on drugs, the easy availability of guns, and three strikes laws.