"Things You Wouldn't Know If We Didn't Blog Intermittently."
A related article that might be of interest:https://www.nap.edu/read/18613/chapter/4
Two factors need to be looked at:1) 209.9 million (1972)..318.6 million (2014)..Growth 108.7 million2) DNA science & Law enforcement methods have been vastly improved.
Ummmm... You forgot #3:https://www.washingtonpost.com/posteverything/wp/2015/04/28/how-for-profit-prisons-have-become-the-biggest-lobby-no-one-is-talking-about/?noredirect=on&utm_term=.4168f7ac8224http://thehill.com/blogs/pundits-blog/crime/321571-private-prisons-place-politics-and-profits-over-peoplehttp://money.cnn.com/2017/02/24/investing/private-prison-stocks-soar-trump/index.html
Let's look at your first factor: 50% increase in population from 1972 to 2014 should result in a 50% increase in prison population, which would bring the number incarcerated to... 450,000. Now you explain the other 1.85 million.
The War on Drugs at its finest.