We thank Blakely et al. (1) for their observations. We welcome their stance that researchers conducting instrumental variable (IV)–fixed-effects (FE) analysis should be wary of potential violations of the exclusion restriction—in our study (2), the assumption that eligibility for the free bus pass only impacts cognitive function through the effect on public transportation use. The exclusion restriction is essentially untestable and must be supported by strong arguments. We argue that Blakely et al.’s concerns do not offer convincing evidence to conclude that the difference in magnitude between our FE estimates and our IV-FE estimates is due to an exclusion restriction violation.
