Research on the relationship between police and crime, like many criminological topics, is subject to uncertain causal direction and omitted controls. We recommend procedures that mitigate these problems: the Granger causality test, proxies for missing variables, robustness checks, and making data available to other researchers. Because specification problems are common in the social sciences, this strategy has applicability beyond the issue of police and crime. We analyze yearly police data and UCR crime rates, at the state and city levels, pooled over two decades. We find Granger causation in both directions. The impact of crime on the number of police is slight, but the impact of police on most crime types is substantial. The latter result is more robust at the city level.

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We estimate the impact of determinate sentencing laws (DSLs) on prison commitments, prison populations, and Uniform Crime Report crime rates. Ten states enacted these laws between 1976 and 1984; all abolished parole and most established presumptive sentences. The research uses a multiple time‐series design that, among other benefits, controls for national trends and facilitates the use of control variables. We found that DSLs are clearly associated with prison population growth in only one state, Indiana, and with major reductions in two, Minnesota and Washington. The remaining laws show no evidence of increasing populations and may have reduced them somewhat. The estimated impacts on commitments are similarly varied. There is little or no evidence that DSLs affect crime. Earlier studies evaluating individual DSLs are often criticized for poor research designs, and our findings support the criticisms.