Estimating Sibling Spillover Effects with Unobserved Confounding Using Gain-Scores

TL;DR

This paper introduces a gain-score regression method to identify sibling spillover effects in epidemiology, addressing unobserved confounding, and demonstrates its application with a birth cohort study on preterm birth and literacy.

Contribution

The paper presents a novel gain-score regression approach for estimating sibling spillover effects that accounts for unobserved confounding in linear models.

Findings

The method can identify certain spillover effects under specific conditions.

Applied to birth data, preterm birth modestly affects older siblings' literacy scores.

The approach shows promise but has limitations in complex spillover scenarios.

Abstract

A growing area of research in epidemiology is the identification of health-related sibling spillover effects, or the effect of one individual's exposure on their sibling's outcome. The health and health care of family members may be inextricably confounded by unobserved factors, rendering identification of spillover effects within families particularly challenging. We demonstrate a gain-score regression method for identifying exposure-to-outcome spillover effects within sibling pairs in a linear fixed effects framework. The method can identify the exposure-to-outcome spillover effect if only one sibling's exposure affects the other's outcome; and it identifies the difference between the spillover effects if both siblings' exposures affect the others' outcomes. The method fails in the presence of outcome-to-exposure spillover and outcome-to-outcome spillover. Analytic results and Monte Carlo simulations demonstrate the method and its limitations. To exercise this method, we estimate the spillover effect of a child's preterm birth on an older sibling's literacy skills, measured by the Phonological Awarenesses Literacy Screening-Kindergarten test. We analyze 20,010 sibling pairs from a population-wide, Wisconsin-based (United States) birth cohort. Without covariate adjustment, we estimate that preterm birth modestly decreases an older sibling's test score (-2.11 points; 95% confidence interval: -3.82, -0.40 points). In conclusion, gain-scores are a promising strategy for identifying exposure-to-outcome spillovers in sibling pairs while controlling for sibling-invariant unobserved confounding in linear settings.