When the ends don't justify the means: Learning a treatment strategy to prevent harmful indirect effects

TL;DR

This paper develops a nonparametric method to identify subgroups where treatment causes harmful indirect effects through mediators, even when the overall effect is beneficial, with application to housing voucher data.

Contribution

It introduces a novel nonparametric approach to detect individuals with harmful indirect effects, accounting for confounders and without distributional assumptions.

Findings

Identified subgroup of boys with harmful indirect effects in housing data.

Method effectively distinguishes harmful mediating effects despite overall benefits.

Applicable to various treatment and mediator scenarios.

Abstract

There is a growing literature on finding so-called optimal treatment rules, which are rules by which to assign treatment to individuals based on an individual's characteristics, such that a desired outcome is maximized. A related goal entails identifying individuals who are predicted to have a harmful indirect effect (the effect of treatment on an outcome through mediators) even in the presence of an overall beneficial effect of the treatment on the outcome. In some cases, the likelihood of a harmful indirect effect may outweigh a likely beneficial overall effect, and would be reason to caution against treatment for indicated individuals. We build on both the current mediation and optimal treatment rule literature to propose a method of identifying a subgroup for which the treatment effect through the mediator is harmful. Our approach is nonparametric, incorporates post-treatment variables that may confound the mediator-outcome relationship, and does not make restrictions on the distribution of baseline covariates, mediating variables (considered jointly), or outcomes. We apply the proposed approach to identify a subgroup of boys in the Moving to Opportunity housing voucher experiment who are predicted to have harmful indirect effects, though the average total effect is beneficial.