Operationalizing the Search for Less Discriminatory Alternatives in Fair Lending

The Less Discriminatory Alternative is a key provision of the disparate impact doctrine in the United States. In fair lending, this provision mandates that lenders must adopt models that reduce discrimination when they do not compromise their business interests. In this paper, we develop practical methods to audit for less discriminatory alternatives. Our approach is designed to verify the existence of less discriminatory machine learning models – by returning an alternative model that can reduce discrimination without compromising performance (discovery) or by certifying that an alternative model does not exist (refutation). We develop a method to fit the least discriminatory linear classification model in a specific lending task – by minimizing an exact measure of disparity (e.g., the maximum gap in group FNR) and enforcing hard performance constraints for business necessity (e.g., on FNR and FPR). We apply our method to study the prevalence of less discriminatory alternatives on real-world datasets from consumer finance applications. Our results highlight how models may inadvertently lead to unnecessary discrimination across common deployment regimes, and demonstrate how our approach can support lenders, regulators, and plaintiffs by reliably detecting less discriminatory alternatives in such instances.