The Murder Rate – A Regression with Many Variables

In this post, I want to look at the murder rate, by state. I ran a regression with the state murder rate for 2015 as the dependent variable, and literally threw the kitchen sink at it: demographics, weaponry, income, education, population density, etc. Basically, if its something some reasonable percentage of the population believes matters, and I could find data for it, I threw it into the hopper.

I also included variables relating to immigration status. The latter stems from some from some debate in the comments section to other posts in which I stated my belief that illegal immigrants drive up the crime rate. Several detractors insisted that illegal immigrants have lower, not higher crime rates than the rest of the population, and that I am racist to boot. Before presenting results, I will note – I am not too proud to admit the regression results did not fit with my preconceptions. I am also not too proud to admit the regression results did not fit with the preconceptions of my detractors. Finally, while I am always interested in whatever the data has to say, I suspect my detractors will really, really not the results.

So… without further ado, the output from R:

r output 20170402a

What does this all mean? Simply put, only two variables are statistically significant at the 5% (or even 10%) level: percent of the population made up of non-Hispanic Whites, and population density. The greater the share of the population made up of non-Hispanic Whites, the lower the murder rate. On the other hand, the greater the population density, the higher the murder rate. To those who don’t use statistics very often, remember – this is taking into account all other variables.

Now, there are a few variables that come close to being statistically significant at the 10% level. In other words, it is possible (not necessarily likely, just possible) that under other circumstances – with a better defined model, or more precise variables – these variables would prove to be statistically significant as well. These variables are:

1. Percent of the population made up foreign citizens here legally. That variable would have a negative effect on the murder rate if it were statistically significant.
2. Percent of the population that is Asian. This variable also would have a negative effect on the murder rate if it were statistically significant.
3. Percent of the population age 18 to 64. Obviously, most of the murders are committed by people within a subset of this range – probably around 18 to 30. If I had the data to separate out this cohort, I believe we would find that the more people in this cohort, the greater the murder rate.

So… what doesn’t matter? First, the percentage of the population made up of illegal immigrants. Ditto the percentage of the population made up of naturalized citizens. These did not increase the murder rate nor lower it. If the murder rate parallels the crime rate in general, then the media narrative that illegal immigrants have lower crime rates than the population as a whole is not supported and to some extent contradicted by the data.

Second, race & ethnicity don’t matter, at least once you pull out non-Hispanic Whites and maybe Asians. Holding all other variables (including education and income) constant, it doesn’t appear that the murder rate differs in a statistically significant way from one non-Hispanic White or Asian racial/ethnic group to another.

Median income doesn’t matter. Neither does the percentage of the population with an income under 20K. Or the percentage of the population with an income over 100K. Or education level. The murder rate is not affected by these variables.

Another thing that doesn’t matter is the degree to which the population happens to be armed. And Lord knows, there are all sorts of variables here. These include “destructive devices” (think grenades, rockets, missiles, mines, poison gas, explosives, or incendiary devices – apparently all these and more are registered by the ATF), machine guns, silencers, short barreled rifles, short barreled shotguns, or other. The innocuous sounding other group includes your garden variety revolvers and pistols.

So essentially, in summary – accounting for education, income, nativity. immigration status, the regression suggests that having more non-Hispanic Whites decreases the murder rate, and having a greater population density increases the murder rate. No other variables in this regression are statistically significant.

Anyway, I can babble on about the results. For example, it would be interesting to see immigrants (both legal and illegal) broken up with enough granularity to see if the results of non-Hispanic Whites and Asians apply to immigrants as well.

But enough of my prattling. What are your thoughts?

As always, if you want my spreadsheet, drop me a line. If you contact me within a month of the publication of this post, I will send it to you and possibly make some sort of witty remark. Since I am adorable, I probably will send you my spreadsheet after that date as well, but I reserve the right to have a file crash, lose my computer, acquire dementia, or die if too much has elapsed. My contact info is my first name (mike) and a dot, then my last name (kimel – only one m there) at gmail dot com.

Links and details to the data are in my spreadsheet.  But if you want to replicate it yourself (it was a pain in the butt, but who am I to stop you?) the data are listed below. Where possible (which was the case for only a few exceptions, as noted below), I tried to use 2015 data to match the murder rate.

2014 data on firearms came from Exhibit 8 from this document produced by the Bureau of Alcohol, Tobacco and Firearms.

Population from the Census. 2015 data was used for most purposes. 2014 data was used for firearms per capita data.

Population density from 2010 was obtained from the Census.

2015 median hh income came from the Census.

A number of other variables came from the Census CPS Table Creator. This was used for data on race, income, native v. naturalized citizens v. foreigner, educational attainment, age, and gender.

Pew estimates on illegal immigrants, including Mexican v. non-Mexican, were available for 2014.

Finally, the number of 2015 murders originated with the FBI, but was present in this handy dandy file compiled by the Murder Accountability Project.


Update…  April 2, 2017  4:01 PM

I forgot to mention a couple corrections to the data:

1. The Pew data on % of illegal aliens that come from Mexico included a few NAs, in each case for states with a very low percentage of the population being made up of illegal immigrants.  In those instances, I assigned the national average share (i.e., 52% of the unauthorized aliens are from Mexico).

2.  The CPS table information on race and ethnicity had a few examples where no information was given for a given combination of race & ethnicity.  In each case, it was possible to determine that the number was very small because the sum total of the other race & ethnicity combinations came close to 100%.  In those instances, I simply replaced the NA with a zero.

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