One bar moving across the box to the left represents a person’s time in prison. The slower the bar, the longer the prison term.

Imagine if everyone in prison stayed there for one year. If the rate of people going to prison stays the same, then as new people enter, others leave—and the prison population stays stable.

Now imagine that some of these people begin to stay longer—say, 10 or 20 years. At first, this doesn’t have a large effect on the prison population. But over time, it does. When people stay longer, they start to stack up.

And when more people start serving more time, the combined effect is huge.

Our example here is fictional, but these trends are real for roughly 2.2 million people behind bars. In the United States, more people have been going to prison and staying there longer, mostly because of “tough-on-crime” policies that swept the country in the 1980s and ’90s. The prison population boomed as sentences got longer and release policies got more restrictive.