The US leads the world in incarceration, but most of its prisoners are in state prison. And a majority of state prisoners are serving sentences for violent crimes. That means that while states have made some progress in recent years in reducing their prison populations, it's going to be extremely difficult for that trend to continue indefinitely.

This infographic, from the Marshall Project, shows how hard it is to reduce the prison population substantially just by targeting nonviolent, low-level offenders:

In September, Vox offered a few reasons why recent criminal-justice reforms haven't made that much of a dent in the prison population. One reason is that they often targeted people who weren't going to prison to begin with, rather than tackling the question of what to do with violent offenders.

The good news is that lawmakers don't have to stop putting violent offenders in prison to reduce the incarceration rate substantially. They just have to stop putting them in prison for quite so long: