The death penalty is an emotive and complex subject. The degree to which it is accepted in some countries, especially in the USA, seems bizarre to many Europeans. An argument for the death penalty is that it acts as a deterrent. But is this supported by the evidence? One would hope that if the state is going to kill people, that at least the decision should be evidence based. The analysis reported below shows that the evidence for the deterrence effect is not that robust and indeed there is even evidence that the death penalty could actually increase the murder rate.
C F Manski , J V Pepper
Researchers have long used repeated cross sectional observations of homicide rates and sanctions to examine the deterrent effect of the adoption and implementation of death penalty statutes. The empirical literature, however, has failed to achieve consensus. A fundamental problem is that the outcomes of counterfactual policies are not observable. Hence, the data alone cannot identify the deterrent effect of capital punishment. How then should research proceed? It is tempting to impose assumptions strong enough to yield a definitive finding, but strong assumptions may be inaccurate and yield flawed conclusions. Instead, we study the identifying power of relatively weak assumptions restricting variation in treatment response across places and time. The results are findings of partial identification that bound the deterrent effect of capital punishment. By successively adding stronger identifying assumptions, we seek to make transparent how assumptions shape inference. We perform empirical analysis using state-level data in the United States in 1975 and 1977. Under the weakest restrictions, there is substantial ambiguity: we cannot rule out the possibility that having a death penalty statute substantially increases or decreases homicide. This ambiguity is reduced when we impose stronger assumptions, but inferences are sensitive to the maintained restrictions. Combining the data with some assumptions implies that the death penalty increases homicide, but other assumptions imply that the death penalty deters it.
NBER working paper W17455