RACE ON POLICING: Results of Estimation 3
This variable would not be expected to directly affect white arrests and with one exception is not statistically significant in the odd columns. City population is negatively related to arrest rates across all eight specifications. This result is consistent with both Glaeser and Sacerdote (1997) which documents lower probabilities of arrest in big cities, and Cullen and Levitt (1998) which finds that rising crime rates are associated with urban flight. All else equal, therefore, cities with rising populations tend to have falling crime rates itat on.
Arrest rates are generally lower when a Black mayor holds office and, somewhat surprisingly, when unemployment rates are high. Income per capita appears to be positively related to drug arrests, but is not statistically significantly related to any of the other categories. State age shares do not carry a consistent sign (the omitted category is the percent of the population over age 45). This is not particularly surprising given the limited variation in these measures that remains once city and year effects have been removed.
Sensitivity of the Estimates of Differential Impact of Own-race and Cross-race Policing
Because the principal analysis of this paper is based not on the raw police coefficients themselves, but rather the differential between the coefficients on white and non-white officers, many of the standard critiques regarding bias in the estimation are not directly applicable. An omitted variable that leads to similar biases in both the police coefficients would not invalidate comparisons of the relative effects of white and non-white officers. Similarly, the likely possibility that rising crime leads to increases in the size of the police force (Levitt 1997) does not invalidate the relative comparisons, as long as rising crime does not alter the racial composition of police forces.
More generally, however, omitted variables and possible endogeneity of the racial composition of the police force could be distorting the results. If it is the case, for instance, that falling crime rates (or expectations of declining crime) lead police departments to hire more minorities, then there may be a spurious negative relationship between minority police and the number and composition of arrests. Alternatively, changes in policing strategies (e.g. towards community policing) may lead to the hiring of more minority police. It may, however, be the policing strategy, rather than the minority officers that is responsible for a change in the pattern of arrests. Since it is difficult to control for changes in policing strategy, the effect will be mistakenly attributed to the minority police.
While the biases in the preceding paragraph suggest that the effectiveness of minority officers may be exaggerated in OLS regressions, there are other cases in which those results may understate the true impact of minority police. At the beginning of our sample, minorities were greatly under-represented on police forces. Over the period of our sample, the total number of police increased 28 percent, of which nearly all (25/28) was growth in the number of non-white police. To the extent that cities with rising violent crime tend to hire more police (Levitt 1997), there may be a spurious link between rising crime rates and minority police.