% of population in prison: which are the factors?

[Egeides]

I just got pretty astounded by learning that "The Unites States prison population is 2.1 million ? over twice the number in 1988, when the first President Bush was elected." I read the rest of the article (http://www.opendemocracy.net/themes/article-3-1777.jsp) and it's an interesting phenomena to see that some societies tend to get a higher percentage of their population in prison (excluding political prisoners, we're really talking of emprisonment for going against the criminal law here). This can be observed with Rome and man other civilizations.

This thread is not to discuss whether prison for drug-linked forfeits is okay or not, it's really to see what is this phenomenon in society, what are its causes and whattever may pertain to it. Personally, I'm a bit unsure but have a possible answer:

A society as a whole has a "fear-o-meter", a "acceptance-of-difference-o-meter" and so on. In certain circumstances it goes on the negative side and thus permits the society and politicians to act correspondingly.