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Does the Death Penalty deter crime?

A short visualisation-based study on the United States crime rates from 1960 to 2019

6 min readJul 13, 2022

While browsing youtube, I came across a very enticing video title from Jubilee. Jubilee is a large youtube channel, which does different series around various social issues.

One of the series they make is called Middle Ground. They invite six people divided into two groups, who have a differing opinion on some topic: veganism, gun rights, abortion, etc. As you can guess by now, there is also an episode on thedeath penalty:

Before I start, I’d like to make it clear that this is not in any way an article to bash one side or the other, and it’s my best attempt at an objective look into the data.

Introduction

The statement that lead me to do my research and publish this article can be found at 1:39, when a pro death penalty is defending his opinion and says the following:

Michigan abolished capital punishment in 1958, look at the crime rate in Michigan right now

That immediately intrigued me for several reasons: I was pretty sure I could get the data on that, and I was wondering how accurate of a statement that was. I was also curious for myself, as I also happen to have an opinion on the topic. So I got to work. You can find the sources at the bottom of the article.

Note: the statement the person made was slightly inaccurate, the year was 1963 and not 1958. Furthermore, Michigan abolished death penalty for general crimes in 1846, and it was retained only in case of treason. So the person was either misinformed or insicere.

Important note: Furman v. Georgia

One of the tasks was to setup the intervals during which different states had the death penalty. Some states have had it forever, other have removed it centuries ago, and some have moved on and off. However, one federal event which affected all states was the case Furman v. Georgia , which escalated to the supreme court and lead to a decision making death penalty temporarily illegal, until adjustements were done by each state (a moratorium, to be accurate).

Dataset Information

The dataset includes, for each state and each year, the rates of property and violent offenses per 100'000 population. Since we’re discussing death penalty, I decided to only use the violent offenses, which include murder, rape, assault, and robbery.

Given the dataset, we have to make a choice regarding what data will be actually use: should we show all violent crimes, or only a subset? In the U.S., death penalty has been almost exclusively used for murder cases, so it makes some sense to isolate it. Someone will not be deterred by the death penalty from, let’s say, doing a burglary, because they know it simply wouldn’t apply top them if they were caught.

Let’s get visual

So, the result? Let’s look at Michigan, since it’s the state that brought us here to begin with:

So.. wow. The red area represents the time during which death penalty was legal. Looking at this plot, it seems like very strong evidence for death penalty being a deterrent.

Misinformation, visualised

Okay now wait a second, let’s look back at that Michigan image, and recall for a second: death penalty had been abolished for all crimes but treason in 1846, and it was entirely abolished in 1963, but by then it shouldn’t impact the murder rates. However, one could argue that people didn’t know these details and simply thought it was legal. In that case, the thought of it in people’s mind would be enough to act as a deterrent.

Alright then, well let’s instead look at some other states:

Well, here are three other states that kept the death penalty (except during the Fruman period), and one state which never had it. They all share that similar spike pattern early on culminating in high murder rates in the 70s to 80s.

Except for Massachusetts, the rates are similar enough. There are two obvious outliers, Hawaii which had a much smaller population (about 780k in that period) and California which had a much larger population (about 20M). Maryland and Massachusetts however were fairly similar to Michigan in terms of population (ranging between 4M to 8M at the time). Hawaii was chosen as the representative due to being one of the few states which never had death penalty.

To further emphasize how an image without context or without comparison can be used to push some interest insicerely, we could even look at Rhode Island as an example of death penalty increasing the murder rate!

So, what can we tell?

Let’s look at the general murder trend in the U.S.

This gives a global picture, and what we can tell is that murder rate got very high in the period between the 70s and the 90s. When we add the percentage of states with, we get a second plot:

So there doesn’t seem to be much evidence of any deterrence. In fact, we can calculate the linear correlation between these two.

And the correlation is: 0.38. Not great, not terrible. A correlation of 1 would mean maximal connection, and a correlation of 0 would mean no connection at all. In this case, we’re in the lower half, which gives evidence of non-existent connection.

People may comment that not all states are equal in terms of impact due to the population differences. I also computed a different statistics, using the population in the formula to re-balance, the result: correlation was 0.4, so the impact turns out to be minimal.

Concluding words

The analysis suggests a lack of connection between the presence of the capital punishment, and the murder rate. However, it is important to also state what we don’t know from this rather basic dataset.

We don’t take into account how different states that do have the penalty actually use it, and how easily do they give it. This would likely be an important factor for deterrence: if the legal systems hands them out like cupcakes, it might change the results.

Moratoriums also weren’t taken into account, but since these do not abolish the sentence and instead only make it temporarily inapplicable, I don’t believe it would have an impact.

With respect to what caused the spike in murder rate in the 70s to 90s, my best guess is that it relates to the massive drug trafficking increase, however this is just a guess, and it would require a different dataset and analysis to actually find out.

Also, the full list of the plots showing the states and their murder rate can be found here:

https://raw.githubusercontent.com/dominusmi/Death-Penalty-Study/main/figures/total_murder_rate_per_year.png

If you find any error, please let me know and I will correct the best I can!

Appendix: Correlation

The correlation is a metric which states how connected two metrics are. The idea is that high correlation may suggest causation, which is what we are really after. Causation means that the two events have a cause in common, or that one causes the other.

Keep in mind correlation is a very easily tricked metric, and it should be used wisely. If you want to have fun, see this website for example of how easily random statistics can be correlated: https://www.tylervigen.com/spurious-correlations