  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.
