Algorithmic Decision Making and the Cost of Fairness
Sam Corbett-DaviesEmma PiersonAvi FellerSharad GoelAziz Huq
Demonstrates that standard algorithmic fairness constraints force decision-makers to apply race-specific risk thresholds, revealing a critical practical trade-off between public safety and statistical equality in criminal risk assessments.
Algorithmic decision tools are increasingly deployed across the criminal justice system to guide pretrial release and detention decisions. However, widespread concern exists that these tools exacerbate racial disparities, as evidenced by findings that black defendants are substantially more likely than white defendants to be incorrectly flagged as high risk. The article evaluates mathematical formulations of algorithmic fairness, demonstrating the fundamental trade-offs between maximizing public safety and satisfying popular criteria designed to eliminate demographic disparities.
To conduct this evaluation, the authors formulated algorithmic fairness as a constrained optimization problem, balancing public safety benefits against the costs of detention. They proved mathematical theorems regarding optimal decision rules under three common fairness definitions: statistical parity, conditional statistical parity, and predictive equality. They then empirically tested these rules on a dataset of 3,377 pretrial defendants from Broward County, Florida, comparing outcomes against an unconstrained public safety baseline.
Key findings show that the optimal unconstrained strategy requires a single, uniform risk threshold applied equally to all defendants, which inherently maximizes public safety and treats individuals equally regardless of race. In contrast, mathematically satisfying any of the three fairness criteria requires setting race-specific decision thresholds. Applying these race-specific constraints causes significant, measurable public safety losses. In the Broward County data, enforcing statistical parity requires detaining 17% low-risk defendants and results in an estimated 9% increase in violent recidivism among releasees relative to the safety-optimal benchmark. Enforcing predictive equality results in 14% of detainees being low risk and a 7% increase in violent crime, while conditional statistical parity yields 10% low-risk detainees and a 4% violent crime increase. Conversely, an unconstrained single threshold that detains 30% of defendants overall leads to stark racial disparities, detaining 40% of black defendants compared to 18% of white defendants and producing a 32% false positive rate for blacks versus 14% for whites.
These findings establish that tensions between public safety and algorithmic fairness are mathematically inevitable whenever underlying risk distributions differ across demographic groups. Furthermore, applying race-specific decision thresholds carries severe legal risks in the United States, as holding individuals of different races to different standards would likely trigger strict scrutiny under the Equal Protection Clause. The authors also show that testing for score calibration is insufficient to detect discriminatory modeling practices, because strategically adding noise can artificially depress scores for favored groups while preserving calibration.
Policymakers and system leaders should avoid treating fairness metrics as simple algorithmic fixes and instead address systemic disparities through broader policy levers. Practical next steps include improving risk model accuracy with higher-quality data to reduce overall error rates, adjusting detention thresholds upward to lower total incarceration, and investing in non-custodial interventions such as community supervision and supportive services to lower the social costs of classification errors. System designers must also subject predictive algorithms themselves to rigorous inspection rather than evaluating only output calibration.
These conclusions rest on the key assumption that two-year violent arrest data accurately reflects true recidivism risk, though systemic arrest patterns may introduce observational bias. The immediate utility framework also focuses on proximate safety and detention costs rather than long-term systemic effects. Despite these limitations, there is high confidence in the core finding that mathematical fairness constraints inherently force significant trade-offs against public safety in risk-based decision systems.
- Paper: Inherent Trade-Offs in the Fair Determination of Risk Scores, Jon Kleinberg et al. (2017). This paper establishes the mathematical impossibility of simultaneously satisfying calibration and error-rate balance across demographic groups, providing the core theoretical tension analyzed in the source.
- Paper: Fair Prediction with Disparate Impact: A Study of Bias in Recidivism Prediction Instruments, Alexandra Chouldechova (2017). This study introduces the empirical analysis of calibration versus error rate trade-offs in recidivism risk scoring using Broward County data, directly motivating the constrained optimization formulation in the source.
- Paper: Equality of Opportunity in Supervised Learning, Moritz Hardt et al. (2016). This work formalizes equalized odds and equal opportunity via group-specific threshold adjustments, which serve as the primary fairness definitions evaluated in the source.
- Paper: Fairness through awareness, Cynthia Dwork et al. (2012). This foundational paper introduces formal definitions of individual and group fairness as optimization constraints, establishing the conceptual paradigm used by the source.
- Paper: Certifying and Removing Disparate Impact, Michael Feldman et al. (2014). This paper establishes the statistical criteria and data repair methods for disparate impact, providing the foundation for demographic parity constraints analyzed in the source.
- Paper: Big Data's Disparate Impact, Solon Barocas et al. (2016). This paper details the legal and structural mechanisms through which data-driven models produce disparate impact, providing necessary legal context for fairness constraints.
- Paper: A Reductions Approach to Fair Classification, Alekh Agarwal et al. (2018). This paper generalizes the constrained optimization approach of fair classification by framing fairness constraints as a systematic reduction to cost-sensitive learning for general black-box predictors.
- Paper: Delayed Impact of Fair Machine Learning, Lydia T. Liu et al. (2018). This work extends static threshold-optimization models by examining how fairness constraints and utility-maximizing policies dynamically impact group welfare and score distributions over time.
- Paper: Fairness and Abstraction in Sociotechnical Systems, Andrew D. Selbst et al. (2019). This paper critiques the formalistic abstraction of fairness optimization in criminal justice by analyzing the sociotechnical traps of reducing justice to isolated mathematical thresholds.
- Paper: AI Fairness 360: An extensible toolkit for detecting and mitigating algorithmic bias, Rachel Bellamy et al. (2019). This work operationalizes the suite of fairness metrics and post-processing threshold adjustments explored in the source into an open-source software library.
- Paper: A Survey on Bias and Fairness in Machine Learning, Ninareh Mehrabi et al. (2019). This comprehensive survey contextualizes the trade-offs between accuracy, public safety, and mathematical fairness definitions across the broader machine learning lifecycle.
