Balko puts American incarceration rates in perspective:
In 1970 one in 400 American adults was behind bars or on parole. As of 2008, the number was one in 100. Add in probation, and it's one in 31. The number of people behind bars for drug crimes has soared from 40,000 in 1980 to about half a million today. States today spend one of every 15 general fund dollars on maintaining their prisons. According to the King's College World Prison Population List (PDF), the U.S. is home to 5 percent of the world's population but nearly a fourth of its prisoners. Judging by these official numbers, America's incarceration rate leads the developed world by a large margin, although it's doubtful that authoritarian regimes such as China's are providing accurate data, especially about political prisoners. But among liberal democracies, the competition isn't even close: As of 2008, the U.S. incarceration rate was 756 per 100,000 people, compared to 288 for Latvia, 153 for England and Wales, 96 for France, and 63 for Denmark.
How did we get here?
As New York Times reporter Adam Liptak pointed out in a 2008 article, America's soaring incarceration rate may be largely due to the fact that we have one of the most politicized criminal justice systems in the developed world. In most states, judges and prosecutors are elected, making them more susceptible to slogan-based crime policy and an electorate driven by often irrational fear. While the crime rate has fallen dramatically since the early 1990s, polls consistently show that the public still thinks crime is getting worse.