Response to Day et al.

To the Editor: Day et al. present a striking example of how adjusting for heritable covariates correlated with the outcome in a genetic association study can bias genetic effect estimates—possibly creating a strong association where genotype has no causal effect on outcome. This bias was the focus of our recent report.1 As Day et al. point out, this phenomenon is a special case of the broader concept of collider-stratification bias2,3. By choosing an outcome that is not associated with autosomal variation (gender), but strongly associated with the covariate (height), Day et al.