There's a great piece on imprisonment in American over on boingboing.
It’s important to understand just how extreme the situation is. We now incarcerate a larger proportion of our citizens, and a larger absolute number of them, than any other nation in the world. The United States has less than 5 percent of the global population yet has almost one-quarter of all the world’s prisoners. (Source: New York Times, April 23, 2008.) The Land of the Free has become the land of the confined.
Now here’s the interesting part. From 1925 through 1975, the American incarceration rate remained around 110 prisoners per 100,000 population, not far from the current world median of approximately 125. (Source: New York Times, as above.) What happened since then? How could the rate increase by a factor of 7 during just three decades?
I can suggest an answer in three words: Money, fear, and politics.
Check it out. It's an eye opener.