There are over 1.5 million people in American jails and prisons. Why are they there? Take a look at the latest numbers from the Bureau of Justice Statistics.

Here is the offense breakdown for state-level incarceration for 2012, which continues to outnumber federal incarceration by a factor of 6:1 or so:

Here’s the federal breakdown for 2012:

Most libertarians blame the massive U.S. incarceration rate on the War on Drugs. At the federal level, this is hard to deny. But the federal level remains a small slice of the pie. At the state level, drug offenses are only 16% of the story.

In absolute terms, of course, that’s tons of people. Drug offenders outnumber murderers. They outnumber rapists. They outnumber robbers. Indeed, they are only slightly outnumbered by all property criminals combined. But at first glance, blaming the War of Drugs for mass imprisonment runs afoul of basic facts.

What about at second glance? Econ 101 warns us against using mere accounting to resolve causation. In the absence of the War on Drugs, many non-drug offenses would never have been committed. Without prohibition, gang-related violence – and related weapons charges (subsumed under “Public-order” at the state level) – would plummet. Habit-related property crimes would probably do the same, albeit to a smaller degree. Theoretically, drug offenders might simply switch into other illegal activities once drugs were legal, but it’s hard to believe this effect would be sizable.

My best guess: Five years after the end of the War on Drugs, half of the prison-years handed out for non-robbery violent crime would be gone, along with 20% of the prison-years for robbery and property crimes, and 75% of the prison-years for weapons charges. That’s roughly half of all prison-years at the state level, and two-thirds of all prison-years at the federal level.

Got a better guess? Please share in the comments – and please show your work.