Historical racism and the black/white incarceration rate ratio

A couple of days ago, I came across a fascinating table of data indicating that states with a longer history of racism tend to have lower black/white incarceration rate ratios. As the person who posted the table noted, "The more stereotypically 'racist' a state is, the *lower* the ratio of black to white imprisonment". It is fairly obvious that this is true just from eyeballing the data, but I thought I'd conduct a slightly more detailed analysis anyway.

The original table can be found at the website for the Bureau of Justice Statistics. (If you click on the link that says "PDF", it is Table 14 on page 11.) Here I examine two alternative measures of historical racism: first, the number of years with anti-miscegenation laws (those that prohibit interracial marriage); and second, school segregation prior to Brown v. Board of Education. The black/white incarceration rate ratio is unavailable for New Mexico and Wyoming. School segregation is not reported for Alaska or Hawaii since both of these states joined the union after Brown v. Board of Education.

The first chart below plots the black/white incarceration rate ratio in 2005 against the number of years for which the state had anti-miscegenation laws. The relationship is negative and significant, namely r = –.53 (p < 0.001, n = 48). It becomes even stronger if Alaska and Hawaii, which joined the union after many states had already repealed their anti-miscegenation laws, are excluded: r = –.65 (p < 0.001, n = 46). 

Years with anti-miscegenation laws is negatively related to the black incarceration rate at r = –.29 (p = 0.042, n = 48), and positively related to the white incarceration rate at r = .28 (p = 0.055, n = 48). Again, both of these relationships become stronger if Alaska and Hawaii are excluded, namely r = –.40 (p = 0.006, n = 46) and r = .31 (p = 0.035, n = 46), respectively. 

The second chart plots the mean black/white incarceration rate ratio by school segregation prior to Brown v. Board of Education. Consistent with the results above, it is monotonically decreasing in the stringency of school segregation laws. States that prohibited school segregation have the highest mean black/white incarceration rate ratio, while states that required school segregation have the lowest mean black/white incarceration rate ratio. 

The mean white incarceration rate is significantly higher in states that required school segregation than in states that prohibited school segregation (difference = 167, p < 0.001). However, the mean black incarceration rate is not significantly lower in states that required school segregation (difference = –305, p = 0.2). In fact, the mean black incarceration rate is highest in states that permitted school segregation (yet there are only two states in this group: Arizona and Kansas). 

It is not clear why black/white incarceration rate ratios tend to be lower in states with a longer history of racism. One possible explanation for the finding that white incarceration rates tend to be higher in such states is simply that whites in these states commit more crime. And one possible explanation for the finding that black incarceration rates tend to be the same or lower in such states is that these states have larger black populations, meaning that there is more likely to be at least one black person on the jury when a black defendant is being tried.

An important caveat is that white and black incarceration rates correspond to inmates in state prisons and local jails, but not those in federal prisons. So it is possible that some or all of the negative association between historical racism and black/white incarceration rate ratio is attributable to a tendency for states with a longer history of racism to send relatively more of their black convicts to federal prisons.