Efficient semiparametric estimation of marginal treatment effects with genetic instrumental variables

TL;DR

This paper develops a semiparametric method using efficient influence functions to improve the estimation of marginal treatment effects with genetic instruments, addressing challenges of small compliers and sampling uncertainty.

Contribution

It introduces a robust estimation approach for marginal treatment effects using genetic instruments, enhancing accuracy in the presence of small compliance groups.

Findings

Genetic instruments reveal larger adverse blood pressure effects in individuals prone to heavy drinking.

Efficient influence functions improve estimation robustness against sampling uncertainty.

Reverse selection indicates those most at risk experience greater treatment effects.

Abstract

Alcohol misuse is a key target of public health strategies aimed at reducing cardiovascular risk. The effect of excessive alcohol consumption on blood pressure may vary systematically with individuals' unobserved propensity to engage in heavy drinking, complicating causal inference with observational data. The marginal treatment effects framework uses an instrumental variable for treatment choice (excessive alcohol consumption) to study how selection into treatment is linked with the treatment effect. We explore the use of a genetic instrument within this framework, which is challenging because genetic compliers (individuals for whom a change in the instrument changes their treatment choice) are likely to be a small proportion of the overall sample. This can lead to greater sampling uncertainty in the tails of the propensity score distribution, i.e., the conditional probability of choosing treatment, and in turn poor estimation of causal estimands that measure heterogeneous treatment effects. We show that the use of efficient influence functions of target estimands improves estimation in terms of robustness to sampling uncertainty in nonparametrically estimated propensity scores. We find evidence of reverse selection on gains: individuals most prone to excessive alcohol consumption experience larger adverse effects on blood pressure.