Here's the most startling thing I saw on a tour of Alcatraz, San Francisco's infamous prison

This weekend, I did something I hadn't done in more than 20 years: I booked a tour of Alcatraz, the infamous former federal prison on an island in San Francisco Bay.

Alcatraz operated as a prison from 1934 to 1963. It was abandoned for a while, then occupied by a group of Native American protestors from 1969 to 1971. After that, it was taken over by the US National Park Service and turned into a historical monument. It now draws more than 1.3 million visitors per year.

The main attraction is a self-guided prison tour. You put on earphones and listen as former guards and prisoners explain its history.

The saddest part to me was a former prisoner talking about hearing partygoers at the San Francisco Yacht Club, just a few miles across the bay, every New Year's Eve. I also learned that there was one successful escape from the prison - three prisoners created dummies of themselves and cut through the walls of their cells with spoons - but the escapees were never heard from or seen again, and they presumably drowned in the icy cold bay.

But the most startling thing I saw came after the self-guided tour had finished. On the stairway down from the old dining hall to the spot where you return your headphones was this poster:

The top half shows the number of inmates in the United States in each year (black and red bars), and the percentage increases in each year (light blue bar). The bottom is simpler, and simply shows how the United States has a much higher percentage of people incarcerated than other countries.

Things that stood out:

• The rate of incarceration began increasing in the late 1970s, and there were several big jumps in the 1980s.
• There was a 51% increase in incarceration rates between 2003 and 2004.
• The rate of incarceration had a couple of sustained decreases - during World War II (not surprising), and again in the mid-1950s.
• The rate of incarceration has gone down slightly in the last couple years.

There's a lot of debate about what's behind the big increase in recent years.

Stricter drug laws and mandatory sentencing are often cited as reasons, but Fordham professor John Pfaff has an interesting alternate explanation. From 1975 to 1991, he said, the rise in sentencing dovetailed with the rise in crime. More crime, more prisoners. But after 1991, he said, district attorneys became much more aggressive about filing felony charges. So this seems to suggest broader political pressure to be tough on crime.

If you've been following this issue at all, none of this will surprise you.

But it's stark to see it laid out like this in a poster in a US National Park visited by more than 1 million people per year.