What does empirical research say about sanctuary policies’ effect on local crime and policing trust?
Executive summary
A growing body of empirical research finds that sanctuary policies do not increase local crime and in some studies are associated with modest reductions in certain crimes, especially violent and property offenses [1] [2] [3]. The leading explanation in the literature is that limiting local cooperation with federal immigration enforcement can strengthen trust between immigrant communities and local police — boosting reporting, cooperation, clearance rates and informal social control — although evidence and magnitudes vary and important caveats remain [4] [5].
1. What the best empirical studies conclude about crime
Multiple quasi-experimental and panel studies conclude there is no evidence that sanctuary policies cause increases in crime, and several find reductions in specific crime categories: cross-city and county analyses report either null effects or modest declines in violent crime andproperty crime after sanctuary adoption, with one county-level study estimating declines in aggravated assaults and another estimating an aggregate drop in violent incidents per 100,000 residents [2] [1] [3]. Large reviews and research syntheses similarly summarize that the most consistent pattern is null or negative relationships between sanctuary policies and crime rates, rather than the increases claimed by some federal actors [6] [7] [5].
2. The mechanisms scholars identify — trust, reporting and police effectiveness
Researchers routinely point to a “spiral of trust” and improved informal social control as plausible mechanisms: by decoupling local policing from immigration enforcement, jurisdictions reduce the chilling effect that deportation fears impose on reporting and cooperation, which can raise clearance rates and deterrence; this mechanism is explicitly invoked across empirical papers and working papers that find sanctuary-induced declines in crime [4] [8] [5]. Conversely, some analysts note alternative pathways — for example, changes in police resources or the possible selection of more efficient jurisdictions to adopt such policies — which complicate simple causal stories and require careful empirical controls [8] [4].
3. Heterogeneity, limits of the evidence, and credible critiques
The literature is not unanimous and has notable limits: many studies focus on a subset of crime types (homicide, robbery, assault), leaving open whether effects generalize to others, and several authors caution about state- or city-level confounders and the possibility that better-run jurisdictions both adopt sanctuary policies and have improving crime trends for unrelated reasons [4] [8] [2]. Reviews emphasize that relatively few rigorous studies existed early on and that disentangling selection, timing, and reporting effects requires more research — points underscored in systematic reviews that call for attention to heterogeneity and methodological robustness [5] [7]. Critics aligned with opponents of sanctuary policies also raise normative and legal concerns — for instance, that limiting cooperation could complicate federal-state criminal immigration enforcement — and those political arguments shape both research agendas and public messaging [9].
4. Political narratives, costs and enforcement tradeoffs
Political actors have used crime claims both to pressure localities and to defend local autonomy: federal rhetoric asserting that sanctuaries “release criminals back to the street” has been a recurring justification for coercive funding threats, yet national analyses and legal briefs have repeatedly cited empirical work showing no crime uptick and sometimes economic savings from reduced violent incidents, framing sanctuary policies as public-safety enhancing as well as rights-protecting [6] [9]. Cost estimates and litigation filings presented by advocacy groups argue that honoring detainers is expensive and that sanctuary policies can save local governments money through crime reductions, but such fiscal claims depend on the underlying crime-effect estimates and on assumptions about deportation patterns [10] [6].
5. Bottom line and where research should go next
On balance, current empirical research consistently contradicts the claim that sanctuary policies increase crime and offers credible evidence that such policies can improve trust and, in some contexts, reduce certain crimes — though effect sizes are modest and contingent on local context, crime type, and study design [1] [2] [3]. Remaining research needs include broader crime-type data, better causal identification of mechanisms (reporting versus true incidence), and clearer accounting for selection bias in which jurisdictions adopt sanctuary rules; until then, policymakers should treat blanket claims of "sanctuary = more crime" as unsupported by the peer-reviewed evidence catalogued in multiple academic studies and policy reviews [5] [7] [4].