> In 1970 the proportion of Americans behind bars was below one in 400, compared with today’s one in 100. Since then, the voters, alarmed at a surge in violent crime, have demanded fiercer sentences. Politicians have obliged. New laws have removed from judges much of their discretion to set a sentence that takes full account of the circumstances of the offence. Since no politician wants to be tarred as soft on crime, such laws, mandating minimum sentences, are seldom softened. On the contrary, they tend to get harder.
On top of that, you also have an entrenched set of special interests who benefit from the status quo (police unions, prison guard unions, private prisons, etc.), so the pressure on politicians is from two sides.
So how do you "solve" a problem that special interests, along with a sizable majority of the voting population, have no interest whatsoever in solving?
Without a massive culture shift, you don't.
First: since you go to prison after the crime, the inmate population size should be a trailing indicator of the crime rate. More serious crimes affect the prison population for a long period, since their sentences are longer.
Second: take a look at these graphs, and how dramatically they shoot up around 1965:
http://www.wolframalpha.com/input/?i=murder+USA
Another factor is greatly improved reporting. Rape in particular used to be very poorly reported, and this has improved a lot as the social stigma against reporting it lessened. The shape long rise from the 60s to 1990 was certainly strongly affected by that.
A number of reasons exist for the recent drop in very violent crime that the murder graph shows. Demographics are surprisingly only a small part of it. Most of it was the result of public health efforts that reduced the incidence of lead poisoning. See http://www.washingtonpost.com/wp-dyn/content/article/2007/07... for more. A much smaller factor which many have heard of was the legalization of abortion. While this has an effect, and was popularized in Freakonomics, the effect was much less than the benefit of reduced lead poisoning.