>>7 ratio
Unknown, unknowable, and imponderable. Complete information is rarely available and when asked, the accused invariably claim to be innocent, misunderstood, insane, incapable of knowing right from wrong, to have found Jeebus, etc. Appealing to science hardly helps; how many are in prison right now, convicted based on "scientifically proven DNA evidence" botched in the lab by some podunk PD's drunken "evidence technician" who only got the job because he was the Sheriff's little brother? It all comes down to whom a jury made up of twelve people not bright enough to get out of jury duty chooses to believe, and no, I don't know of a better way.
The question, though, seems to me to be predicated on the idea that there's something unique about the death penalty and it shares no traits with other punishments. Yet if an innocent person is falsely convicted of a crime, and serves twenty years in prison instead, only to be exonerated and released, can the state give him back his health and his youth, the twenty years of his life that have been stolen from him? No.
>rape
Incest and rape victims are, what, one ten thousandth of one percent of patients at abortion clinics? Leaving their numbers aside, a more relevant question is, do we hate rapists so much that we desire to put their unborn children to death? I'd settle for executing the rapist instead, and the child can go to an orphanage or foster home when he or she is born.