Bryan Caplan directs us to this claim:
More prisoners reported abuse by staff than abuse by other prisoners: 2.9 percent of respondents compared with about 2 percent.
Some of these are likely false charges but this is startling nonetheless. Keep in mind there are more prisoners than staff by some number so the relative likelihood of staff abuse must be high in per capita terms. Bryan sums it up:
The lesson: You might think that no one would be more inclined to sexual abuse than criminals in unisex confinement. But if you give authority to relatively normal people who can leave the prison anytime they like, they're worse.