Human Rights Facts (134): The Truth About the Deterrent Effect of the Death Penalty

Proponents of the death penalty usually show the following famous graph in order to “prove” that capital punishment results in fewer homicides in the U.S., and is therefore a successful deterrent:

First of all, there’s something wrong with this graph. It’s intentionally tweaked so as to highlight the recent rise in the number of executions, and to do so in a way that shows how closely correlated it is with the recent drop in the number of homicides. Compare it to this version:

homicides and executions in the U.S.

(source)

The important difference is that the second graph counts the number of executions per homicide, and not just the total number of executions. From the point of view of deterrence, this is obviously the better measure.

We can see from the second graph that the recent upswing in the number of executions is really quite small, compared to earlier periods (there was moratorium on executions in the U.S. in the early 1970s). Unless deterrence has somehow become much more effective than it was in the early parts of the 20th century – which is doubtful given the relatively low numbers and humane methods – it’s doubtful that such a relatively small increase in the number of executions during the last decades is the cause of the extraordinary decrease in the number of homicides during the same period. We have here a clear example of correlation being not equal to causation. And when we look at the whole time series, there isn’t even a clear correlation. It’s cherry picking: take that part of the time series that confirms your prejudice, and forget the rest. A common manipulation technique in statistics.

It’s not only cherry picking in terms of the period being considered, but also in terms of sidelining other possible explanatory factors. The same guys who gave us the second graph show how this works by comparing U.S. data with Canadian data, and by comparing the data for different states in the U.S.

Canada and the U.S have had and continue to have radically different capital punishment policies. Canada abolished the death penalty in the 1960s. According to those who believe in deterrence, Canada should have a completely different evolution of the number of homicides; in fact a steeper increase than the U.S. when the U.S. has an increase, or a less pronounced decrease when they both show a decrease. The fact, however, is that the graphs for both countries are very similar (although the levels are lower in Canada):

(source)

These similar movements in the rate of homicides, combined with very different capital punishment policies, indicate that the latter don’t have a real influence on the former. While the moratorium in the U.S. in the 1970s is blamed for the concurrent rise in number of homicides, we see that a similar rise occured in Canada, where the death penalty was abolished many years earlier. And, similarly, the recent decrease in homicides, said to be the result of the reinstatement of the death penalty in the U.S., also occured in Canada where there hasn’t been a reinstatement.

The same is true when we compare states within the U.S. Death-penalty states and non-death penalty states have witnessed very similar movements in homicide rates:

The drivers behind the movements in homicide rates can’t be found in capital punishment policies and hence must be found elsewhere. But then you need to be willing to look. I don’t think that those who believe in deterrence are willing to look.

More on capital punishment.