Solitary confinement is 3 times as expensive as ordinary confinement, and there is where the problem lies.
No one wants to spend the kind of moolah to build individual cells for millions of people- in fact, penitentiaries which were built for individually occupied cells get double and triple bunked.
Further, keeping people in their cells all day requires a lot more staff, and all those guys guys paid. Bringing chow to the cells, escorting prisoners for individual showers, etc., is a lot more labor intensive.
Tens of thousands of prisoners across the United States spend months, years – and sometimes decades – locked alone in windowless concrete rooms the size of wheelchair accessible bathroom stalls for at least 23 hours a day, seven days a week. Prison officials, not judges or juries, decide both...
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