Apparently, since 1976 – the year in which the death penalty was reinstated in the U.S. – at least 118 innocent people found themselves on death row, where they spent a total of 1.125 years (which is almost 10 years on average per person):
(source, click on the image to enlarge)
Obviously, this is only the number of innocent people who have been exonerated since 1976. It’s likely that there are still other innocent people on death row waiting for exoneration. And then there are those who have already been executed and never got a chance. Furthermore, these data are limited to the U.S., a country with a very good justice system. One doesn’t dare to estimate the number of innocent people executed elsewhere in the world. (Here‘s a map of the application of the death penalty in the world).
I wouldn’t want anyone to conclude from this that my only objection to capital punishment is the risk of irreversible harm done to innocent people. I object even to a flawless system of capital punishment. Read here why.
More on capital punishment.