A county-level analysis of homicide rates

Earlier on twitter I shared some scatter plots comparing homicide rates by percentage of each state black.

gun_hom_by_pct_state_black

homicides_by_pct_black

Since some people believe this might somehow only be a state-level issue, I thought I’d just quickly plot the data at a county level.


Including even the smallest counties, a number with zero homicides, the correlation isn’t quite as strong (r=0.58):

county_hom_by_pct_black_all_counties

However, if I filter to only include counties with 100K plus people, it tightens up (r=0.69).

county_hom_by_pct_black_100k

@500k+ (r=0.73)county_hom_by_pct_black_500k

@ 1M+ (r=0.76)county_hom_by_pct_black_1M

Even in relatively modestly sized counties (50-200K people), the correlation coefficient is 0.58.county_hom_by_pct_black_50_to_200k

I am, of course, not the first person to take this sort of approach.  Ron Unz analyzed this in 2013 at a city level, on an annual basis, and for a variety of other crimes, but my approach is somewhat different (CDC stats vs FBI UCR; multi-year average; and scatterplots vs reporting correlation coefficients).  My results are generally consistent with his with respect to blacks and homicide rates (somewhat weaker correlation, but that’s not surprising given counties vs city differences).

However, he is wrong about hispanics though.  There is a weak correlation (r=0.05) without any controls, but their demographic footprint and relatively low homicide rate (as compared to blacks) masks the signal a bit.  Similar confounds also exist for asians.

hom_splom

I quickly modeled the percentage black, hispanic, asian, and white to suss out the relative impacts of various groups at once.

The coefficients are approximately:

Intercept: 18.8

Black: 7.4

Hispanic: -25.2

White: -34.3

Asian: -62.0

…which implies that larger hispanic populations are correlated with higher homicide rates and larger asian populations are correlated with smaller homicide rates.  This is generally consistent with criminal justice statistics in broad strokes (e.g., victims, perpetrators, imprisonment rates, etc).

If I plot the predicted values (r=0.73):

actual_vs_pred_hom_100K_black_hispanic_asian_white

No doubt I could improve this and take this analysis further, but that’s enough for now. 🙂

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4 thoughts on “A county-level analysis of homicide rates

  1. I think Ron Unz may have hit on something that might confuse the data i.e. extremely low hispanic crime rates where they are 80-90% of the population but higher rates where they are a minority or only just becoming the majority i.e. the homicide rate is a function of ethnic conflict over territory.

    If correct the hispanic homicide rate should be high when the white/black population is high increasing as the hispanic population increases as a percentage to a peak and then declining as the hispanic population becomes a majority and the black/white population has disappeared.

    Effectively homicide as a function of diversity.

  2. If the above is correct then historical data might show the effect e.g. a 100% white town that became 90% hispanic over time might go
    – small hispanic minority: high rate per capita but low total due to low population
    – medium hispanic minority: high rate per capita and medium total
    – large minority: high rate per capita and peak homicides
    – small majority: homicide rate per capita starts to drop but large population means still high total homicides
    – large majority: homicde rate per capita drops to minimum, low total homicides

  3. If correct this might also speak to the rise and fall of crime in the northern cities during the great migration: very high during the immigration phase itself dropping to the black default level (still high but lower than peak) one the white people have all gone.

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