One big thing this article misses is that crime has been falling worldwide by a similar rate as in America (http://www.economist.com/news/briefing/21582041-rich-world-s...).

So prisons do disproportionately imprison the lower classes and far far too many people are put in disgusting prisons to be tortured and treated like subhumans in the US but that doesn't necessarily mean it has caused the big drop in crime.