@gargath @Are0h I haven't seen the studies they're citing, but AFAIK there are a couple of things at work there.

Jails are in theory only occupied for a short term, so they don't have a lot of affordances that prisons do like visitation rights or prison commissaries, and usually everyone there pretty much shares one big cell. So there's a degree of physical coercion. Plea out and you can at least see your family and get a cell with only one other person.