Basically, the prison boom of the past 25 years seems to not so much have launched from zero as served as a substitute for a previous high rate of institutionalization in mental hospitals. Throughout the 1960s, we started emptying the mental hospitals. Then when crime went up, we started building tons and tons of prisons. Now it’s worth being clear that the striking visual doesn’t really prove anything. There are probably other statistics you could track that would show a similar time-series just by coincidence. But this is the kind of thing where it’s plausible to say it’s not a coincidence. I don’t personally know much about how mental hospitals were used in the 40s and 50s or have much knowledge of what they were like, but it would be interesting to try to learn more about that and see how much they acted as a kind of ersatz prison system.