Does "nearly everyone" in prison profess innocence?

Today's Post had a review of "Picking Cotton: Our Memoir of Injustice and Redemption", by Jennifer Thompson-Cannino and Ronald Cotton. Thompson-Cannino had pointed to Cotton, in a police line-up, as the man who raped her, and he went to prison for 11 years -- before it was proven that in fact he was not the perpetrator, and he was released. The two are now friends.

The review, by Kate Tuttle, is positive and sweet. But it starts with this: "Nearly everyone in prison protests their innocence, but Ronald Cotton was telling the truth" (the version currently on the website removes the word "their"). This strikes me as unlikely. Does anyone out there know of any research on this matter?