Some numbers put all this in perspective. The F.B.I. reports there were 14,827 cases of homicide or non-negligent manslaughter in 2012, of which 11,298 occurred in jurisdictions that have the death penalty. Research indicates that around one-fifth to a quarter of these homicides were for capital-eligible crimes, suggesting there were around 2,500 capital-eligible homicides in 2012, which is both high by global standards and much lower than in previous decades.

Yet there were only 45 executions last year. When fewer than one in 50 capital-eligible homicides leads to the death chamber, it is clear that capital punishment is rare.

Capital punishment is not only rare, but it’s also an extraordinarily long and drawn-out process. In total, there were 3,033 prisoners on death row at the end of 2012. The fact that so many receive death sentences but so few are killed tells us that many prisoners are living decades on death row.

A simple thought experiment makes the point: If a death sentence puts you at the back of the queue of 3,000 prisoners to be executed, and only 50 people are executed each year, then it would take you, on average, 60 years to reach the front of the line. Not surprisingly, many die of natural causes while waiting their turn.