One should probably subtract off a more average incarceration rate first. There is, alas, a subset of people who are behind bars because if they aren't, they truly will net an even larger loss to society.

The truly minimum rate is really low, though, at least for some societies. For example, if the U.S. imprisoned at the same rates as Norway (0.07%), it would have around 250,000 total prisoners. And even there the number is arguably higher than the absolutely necessary minimum. If you dig into who's imprisoned in Norway and why: 1) a full 20% of the prison population are there for administrative offenses like nonpayment of fines; and 2) most of the rest are there for nonviolent drug offenses.

I wonder how many non-violent criminals are arrested for every 'truly dangerous' menace to society.

From what I've read and heard, one of the biggest contributions to prison population growth has been the advent of the predicate felony ("three-strikes") laws in recent decades. These days, incarceration is like compounding interest. Get locked up once, and the next time you so much as look the wrong way at the wrong time, you're back in prison serving an even more severe sentence. In this way, someone with a string of nonviolent offenses does the same time as a violent criminal.

Prisons also serve to harden the imprisoned -- helping to ensure our country's record-high recidivism rates. Prison conditions are brutal. Gangs run daily life, and failure to join a gang can mean rape, severe injury, or even death. Of course, joining a prison gang means an indoctrination into the world of hard-core violence and reprisal. It's pretty hard to go to prison a relatively nonviolent person and emerge the same way.

Another big trend over the last 30-odd years has been a marked shift in society's -- and our justice system's -- philosophical view of the prison itself. Many years ago, prisons were seen as places to reform criminals -- not just to lock them away for a certain number of years, but to adapt them to a successful return to society upon completion of those years. These days, reform and adaptation aren't on the agenda. (Hell, good luck trying to find a stable job in post-release society with a conviction on your record). Modern prison is purely about locking people away and treating them as hopeless causes. More often than not, that hopelessness becomes a self-fulfilling prophecy.

To be fair, some people in our society really are lost causes. There are violent psychopaths rotting in prison who should probably stay there and rot. But today's criminal justice system paints all offenders with that brush. Perhaps I'm a softie, but I choose to believe that a significant fraction of the 6 million is not beyond redemption.

> From what I've read and heard, one of the biggest contributions to prison population growth has been the advent of the predicate felony ("three-strikes") laws in recent decades.

Actually, three-strikes laws and the like aren't new, they're returning. They were abolished in the 60s.